Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Contents
ABBREVIATIONS ..................................................................................................................................................... XVII
GLOSSARY ............................................................................................................................................................. XVIII
1
GENERAL PROVISIONS .................................................................................................................................... 1-1
1.1 SCOPE AND APPLICATION .......................................................................................................................................... 1-1
1.2 GOVERNING LAWS, LOCAL ORDINANCES, RULES & REGULATIONS, CODES AND DEPARTMENT ORDERS..................................... 1-1
1.3 REFERENCE MATERIALS ............................................................................................................................................. 1-1
1.4 GENERAL CONCEPTS ................................................................................................................................................. 1-2
1.5 HIGHWAY TYPES / CLASSIFICATION.............................................................................................................................. 1-7
1.5.1 Highway Types .............................................................................................................................................. 1-7
1.5.2 2-Lane Highways .......................................................................................................................................... 1-7
1.5.3 4-Lane Undivided Highways ......................................................................................................................... 1-8
1.5.4 Divided Highways ......................................................................................................................................... 1-8
1.5.5 Classification of Highways According to System........................................................................................... 1-9
1.5.6 Road Classification According to Primary Function .................................................................................... 1-11
2
DESIGN DATA ................................................................................................................................................. 2-1
2.1 FIELD SURVEY INFORMATION ..................................................................................................................................... 2-1
2.1.1 Highway Location ......................................................................................................................................... 2-1
2.2 FIELD INVESTIGATIONS ............................................................................................................................................ 2-11
2.2.1 Proposed Sites for Stream Crossings ........................................................................................................... 2-11
2.2.2 Road Alignment .......................................................................................................................................... 2-11
2.2.3 Existing Utility Services ............................................................................................................................... 2-12
2.3 SOIL INVESTIGATIONS ............................................................................................................................................. 2-12
2.3.1 Subsurface Investigation............................................................................................................................. 2-12
2.3.2 Subgrade Investigation ............................................................................................................................... 2-13
2.3.3 Widening of Existing Pavements ................................................................................................................ 2-13
2.3.4 Sampling and Testing ................................................................................................................................. 2-13
2.4 EXISTING PAVEMENT EVALUATION ............................................................................................................................ 2-14
2.4.1 Visual Inspection/Surface Defects .............................................................................................................. 2-14
2.4.2 Joints ........................................................................................................................................................... 2-19
2.4.3 Pavement Cracks ........................................................................................................................................ 2-22
2.4.4 Pavement Deformation .............................................................................................................................. 2-27
2.5 DRAINAGE RECOMMENDATIONS ............................................................................................................................... 2-33
2.5.1 Classification of Highway Drainage ............................................................................................................ 2-34
2.6 DESIGN CONTROLS ................................................................................................................................................. 2-36
2.6.1 Anticipated Traffic Volume ......................................................................................................................... 2-36
2.6.2 Character of Traffic ..................................................................................................................................... 2-37
2.6.3 Design Speed .............................................................................................................................................. 2-38
2.6.4 Design Traffic (Vehicles) ............................................................................................................................. 2-38
2.6.5 Highway Capacity ....................................................................................................................................... 2-38
2.6.6 Classification of Highway ............................................................................................................................ 2-39
2.6.7 Accident Information .................................................................................................................................. 2-39
2.7 REQUIREMENTS FOR SPEEDY PLAN PREPARATION......................................................................................................... 2-40
2.7.1 Plans ........................................................................................................................................................... 2-40
2.7.2 Profile.......................................................................................................................................................... 2-40
2.7.3 Detailed Cross Section ................................................................................................................................ 2-41
2.7.4 Detailed Drainage Cross Section ................................................................................................................. 2-41
2.7.5 Geotechnical Drawings ............................................................................................................................... 2-41
3
GEOMETRIC DESIGN OF ROAD LINKS .............................................................................................................. 3-1
3.1 INTRODUCTION........................................................................................................................................................ 3-1
3.1.1 Departure from Standards ............................................................................................................................ 3-1
3.2 REQUIREMENTS FOR DESIGN ANALYSIS IN OPERATING ENVIRONMENT ............................................................................... 3-1
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
3.3 GENERAL PRINCIPLES FOR GEOMETRIC DESIGN ............................................................................................................ 3-11
3.4 DESIGN SPEED ....................................................................................................................................................... 3-12
3.5 ROAD CLASSIFICATION ............................................................................................................................................ 3-13
3.6 BASIC DESIGN CONSIDERATION ................................................................................................................................ 3-13
3.6.1 Sight Distance ............................................................................................................................................. 3-14
3.6.2 Horizontal Alignment .................................................................................................................................. 3-18
3.6.3 Vertical Alignment ...................................................................................................................................... 3-39
3.6.4 Combination of Horizontal and Vertical Alignments .................................................................................. 3-51
3.6.5 Other Elements Affecting Geometric Design .............................................................................................. 3-52
3.7 CROSS SECTION ELEMENTS ...................................................................................................................................... 3-61
3.7.1 Pavement .................................................................................................................................................... 3-61
3.7.2 Lane Widths ................................................................................................................................................ 3-63
3.7.3 Shoulders .................................................................................................................................................... 3-64
3.7.4 Horizontal Clearance to Obstruction .......................................................................................................... 3-65
3.7.5 Curbs ........................................................................................................................................................... 3-67
3.7.6 Sidewalks .................................................................................................................................................... 3-68
3.7.7 Drainage Channels and Side Slopes ............................................................................................................ 3-68
3.7.8 Traffic Barriers ............................................................................................................................................ 3-71
3.7.9 Medians ...................................................................................................................................................... 3-75
3.7.10 Frontage Roads ....................................................................................................................................... 3-77
3.7.11 Noise Control ........................................................................................................................................... 3-78
3.7.12 Roadside Control ..................................................................................................................................... 3-78
3.7.13 Tunnels .................................................................................................................................................... 3-79
3.7.14 Pedestrian Crossings ................................................................................................................................ 3-81
3.7.15 Curb-Cut Ramps ....................................................................................................................................... 3-85
3.7.16 Bicycle Facilities ....................................................................................................................................... 3-86
3.7.17 Bus Turnouts ............................................................................................................................................ 3-90
3.7.18 Park-and-Ride Facilities ........................................................................................................................... 3-91
3.8 HIGHWAY CAPACITY ............................................................................................................................................... 3-92
3.8.1 General Characteristics ............................................................................................................................... 3-92
3.8.2 Capacity as a Design Control ...................................................................................................................... 3-93
3.8.3 Factors Other Than Traffic Volume That Affect Operating Conditions ....................................................... 3-95
3.8.4 Levels of Service .......................................................................................................................................... 3-98
3.8.5 Design Service Flow Rates .......................................................................................................................... 3-99
4
INTERSECTION DESIGN ................................................................................................................................... 4-1
4.1 INTERSECTION AT GRADE ........................................................................................................................................... 4-1
4.1.1 Introduction .................................................................................................................................................. 4-1
4.1.2 Factors affecting Design ............................................................................................................................... 4-2
4.1.3 Types of Intersections ................................................................................................................................... 4-4
4.1.4 Plan of Traffic Volume ................................................................................................................................ 4-10
4.1.5 Basic Principles of Intersection Design ........................................................................................................ 4-11
4.1.6 Geometric Design at Intersections .............................................................................................................. 4-11
4.1.7 Turning Roadways and Channelization ...................................................................................................... 4-27
4.1.8 Auxiliary Lanes ............................................................................................................................................ 4-46
4.1.9 Median Openings........................................................................................................................................ 4-48
4.1.10 Indirect Left-Turns and U-Turns ............................................................................................................... 4-51
4.1.11 Roundabout Design ................................................................................................................................. 4-54
4.1.12 Other Intersection Design Considerations ............................................................................................... 4-60
4.1.13 Railroad-Highway Grade Crossings ......................................................................................................... 4-63
4.1.14 Lay-By ...................................................................................................................................................... 4-64
4.2 GRADE SEPARATIONS AND INTERCHANGES .................................................................................................................. 4-65
4.2.1 Introduction and General Types of Interchanges ....................................................................................... 4-65
4.2.2 Warrants for Interchanges and Grade Separation ..................................................................................... 4-65
4.2.3 Adaptability of Highway Grade Separations and Interchanges .................................................................. 4-66
4.2.4 Access Separations and Control on the Crossroad at Interchanges ............................................................ 4-68
4.2.5 Safety .......................................................................................................................................................... 4-68
4.2.6 Staged Development .................................................................................................................................. 4-68
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
4.2.7
4.2.8
4.2.9
5
Economic Factors ........................................................................................................................................ 4-68
Grade Separation Structures ...................................................................................................................... 4-69
Interchanges ............................................................................................................................................... 4-73
HIGHWAY DRAINAGE DESIGN ........................................................................................................................ 5-1
5.1 INTRODUCTION........................................................................................................................................................ 5-1
5.2 HIGHWAY ALIGNMENT .............................................................................................................................................. 5-1
5.2.1 Horizontal Alignment .................................................................................................................................... 5-1
5.2.2 Vertical Alignment ........................................................................................................................................ 5-1
5.3 PAVEMENT DRAINAGE .............................................................................................................................................. 5-2
5.3.1 General Design Considerations ..................................................................................................................... 5-3
5.4 LOCATION OF STORM DRAINAGE FACILITIES .................................................................................................................. 5-3
5.5 BASIC DESIGN DATA ................................................................................................................................................. 5-4
5.6 HYDROLOGIC ANALYSIS FOR CULVERTS ......................................................................................................................... 5-5
5.6.1 Estimation of Discharge ................................................................................................................................ 5-5
5.6.2 Runoff Analysis ............................................................................................................................................. 5-6
5.7 DESIGN STORM FREQUENCY ...................................................................................................................................... 5-8
5.8 HYDRAULIC DESIGN OF CULVERTS ............................................................................................................................... 5-8
5.8.1 Data .............................................................................................................................................................. 5-9
5.8.2 Culvert Location .......................................................................................................................................... 5-11
5.8.3 Culvert Type ................................................................................................................................................ 5-14
5.8.4 Velocity Limits ............................................................................................................................................. 5-15
5.8.5 Minimum Sizing .......................................................................................................................................... 5-16
5.8.6 Cover ........................................................................................................................................................... 5-16
5.8.7 Shape and Cross Section ............................................................................................................................. 5-16
5.8.8 Outlet Scour Control ................................................................................................................................... 5-17
5.8.9 Materials .................................................................................................................................................... 5-20
5.8.10 End Treatments ....................................................................................................................................... 5-20
5.8.11 Additional Variables to be Considered in Hydraulic Design of Culverts ................................................... 5-22
5.8.12 Culvert Hydraulics .................................................................................................................................... 5-23
5.9 STORM DRAIN SYSTEMS .......................................................................................................................................... 5-34
5.9.1 Pavement Drainage .................................................................................................................................... 5-34
5.9.2 Hydraulics of Storm Drains ......................................................................................................................... 5-42
5.9.3 Design Process and System Planning .......................................................................................................... 5-45
5.9.4 Appurtenant Structures .............................................................................................................................. 5-46
5.9.5 Manholes .................................................................................................................................................... 5-46
5.9.6 Roadside Channels (Ditches & Gutters) ...................................................................................................... 5-48
5.9.7 Subgrade Drainage Systems ....................................................................................................................... 5-49
5.9.8 Pipe Underdrains ........................................................................................................................................ 5-49
6
PAVEMENT DESIGN ........................................................................................................................................ 6-1
6.1 INTRODUCTION........................................................................................................................................................ 6-1
6.2 TYPES OF PAVEMENT ................................................................................................................................................ 6-1
6.2.1 Flexible Pavements ....................................................................................................................................... 6-1
6.2.2 Rigid or Concrete Pavement ......................................................................................................................... 6-4
6.2.3 Gravel Pavements ......................................................................................................................................... 6-5
6.3 FAILURE OF EXISTING PAVEMENTS............................................................................................................................... 6-6
6.4 DESIGNING NEW ROAD PAVEMENTS ........................................................................................................................... 6-7
6.5 DESIGN CONSIDERATIONS .......................................................................................................................................... 6-8
6.5.1 Design Requirements or Input Data for Flexible Pavements ........................................................................ 6-8
6.5.2 Design Requirements or Input Data for Rigid Pavements .......................................................................... 6-10
6.5.3 Roadbed Soil (Subgrade)............................................................................................................................. 6-18
6.5.4 Resilient Modulus of Subgrade Reaction (MR) ............................................................................................ 6-20
6.5.5 Composite Modulus of Subgrade Reaction (K) ........................................................................................... 6-20
6.5.6 Loss of Subgrade Support (Vertical Settlement) ......................................................................................... 6-25
6.5.7 Traffic .......................................................................................................................................................... 6-27
6.5.8 Design Reliability, R .................................................................................................................................... 6-29
6.5.9 Serviceability ............................................................................................................................................... 6-31
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
6.5.10 Drainage Requirements ........................................................................................................................... 6-31
6.5.11 Load Transfer Coefficient “J” ................................................................................................................... 6-32
6.6 PAVEMENT JOINTING DESIGN ................................................................................................................................... 6-34
6.6.1 Slab Length and Related Design Factors..................................................................................................... 6-35
6.6.2 Transverse Joints ........................................................................................................................................ 6-38
6.6.3 Longitudinal Joints ...................................................................................................................................... 6-43
6.6.4 Isolation Joints and Expansion Joints .......................................................................................................... 6-45
6.6.5 Slab Reinforcement ..................................................................................................................................... 6-47
6.6.6 Irregular Panels........................................................................................................................................... 6-47
6.6.7 Contraction Joint Sealants .......................................................................................................................... 6-48
6.7 RIGID PAVEMENT JOINT DESIGN ............................................................................................................................... 6-52
6.7.1 Contraction Joints ....................................................................................................................................... 6-52
6.7.2 Expansion Joints.......................................................................................................................................... 6-53
6.7.3 Construction Joints ..................................................................................................................................... 6-53
6.7.4 Longitudinal Joints ...................................................................................................................................... 6-53
6.7.5 Joint Layout................................................................................................................................................. 6-53
6.8 MATERIAL PROPERTIES AND SPECIFICATIONS ............................................................................................................... 6-55
6.8.1 Material Properties for Flexible Pavements ................................................................................................ 6-55
6.8.2 Material Properties for Rigid Pavements .................................................................................................... 6-55
6.9 DESIGN PROCEDURE ............................................................................................................................................... 6-55
6.9.1 Design Procedure for Flexible Pavements ................................................................................................... 6-55
6.9.2 Design Procedure for Rigid Pavement ........................................................................................................ 6-59
6.9.3 Concrete Pavement..................................................................................................................................... 6-64
6.9.4 Flexible Pavement ....................................................................................................................................... 6-65
6.10 RIGID PAVEMENT REINFORCEMENT DESIGN ............................................................................................................. 6-77
6.10.1 Jointed Reinforced Concrete Pavement ................................................................................................... 6-77
6.10.2 Continuously Reinforced Concrete Pavement .......................................................................................... 6-78
6.10.3 Transverse Reinforcement ....................................................................................................................... 6-82
6.11 PAVEMENT OVERLAY ........................................................................................................................................... 6-82
6.11.1 Important Considerations in Overlay Design ........................................................................................... 6-82
6.11.2 Approaches in the Design of Overlay Projects ......................................................................................... 6-85
6.11.3 Recommended Overlay Solution to Functional Problems ........................................................................ 6-86
6.11.4 Overlay Design Methodology for Pavement with Structural Deficiency.................................................. 6-94
6.11.5 Determination of Design Subgrade MR - Design CBR ............................................................................... 6-96
6.11.6 Asphalt Concrete (AC) Overlay of AC Pavement ...................................................................................... 6-97
6.11.7 AC Overlay of Fractured PCC Slab Pavement ......................................................................................... 6-102
6.12 BONDED CONCRETE OVERLAY OF JPCP, JRCP, AND CRCP ....................................................................................... 6-103
6.12.1 Feasibility ............................................................................................................................................... 6-103
6.12.2 Pre-overlay Repair ................................................................................................................................. 6-104
6.12.3 Reflection Crack Control ........................................................................................................................ 6-105
6.12.4 Subdrainage .......................................................................................................................................... 6-105
6.12.5 Thickness Design .................................................................................................................................... 6-105
6.12.6 Shoulders ............................................................................................................................................... 6-120
6.12.7 Joints...................................................................................................................................................... 6-120
6.12.8 Bonding Procedures and Material ......................................................................................................... 6-120
6.13 UNBONDED JPCP, JRCP, AND CRCP OVERLAY OF JPCP, JRCP, CRCP AND AC/PCC .................................................. 6-120
6.13.1 Feasibility ............................................................................................................................................... 6-121
6.13.2 Pre-overlay Repair ................................................................................................................................. 6-121
6.13.3 Reflection Crack Control ........................................................................................................................ 6-121
6.13.4 Thickness Design .................................................................................................................................... 6-122
6.13.5 Shoulders ............................................................................................................................................... 6-132
6.13.6 Joints...................................................................................................................................................... 6-132
6.13.7 Reinforcement ....................................................................................................................................... 6-132
6.13.8 Separation Interlayer ............................................................................................................................. 6-133
6.13.9 JPCP, JRCP, and CRCP Overlay of AC Pavement ..................................................................................... 6-133
6.14 OVERLAY PLANNING GUIDELINES ......................................................................................................................... 6-137
6.15 OVERLAY DESIGN GUIDELINES ............................................................................................................................. 6-139
6.16 OVERLAY CONSTRUCTION GUIDELINES .................................................................................................................. 6-141
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
6.17
6.18
7
SHOULDER DESIGN............................................................................................................................................ 6-143
REFERENCES .................................................................................................................................................... 6-143
EARTHWORKS ................................................................................................................................................ 7-1
7.1 INTRODUCTION........................................................................................................................................................ 7-1
7.2 FACTORS AFFECTING DESIGN ..................................................................................................................................... 7-2
7.2.1 Height ........................................................................................................................................................... 7-2
7.2.2 Slopes ............................................................................................................................................................ 7-2
7.2.3 Foundation.................................................................................................................................................... 7-2
7.2.4 Loading ......................................................................................................................................................... 7-2
7.2.5 Selection of Embankment Materials ............................................................................................................. 7-2
7.2.6 Groundwater ................................................................................................................................................ 7-3
7.3 SURVEY ON THE STABILITY OF CUT SLOPES .................................................................................................................... 7-3
7.4 FILL SURVEY ............................................................................................................................................................ 7-4
7.4.1 Points of Survey ............................................................................................................................................ 7-4
7.4.2 Survey on the Stability of Cut and Fill Slopes ................................................................................................ 7-5
7.4.3 Survey on Fill Requiring Extra Precautions ................................................................................................... 7-6
7.4.4 Survey on Fill with Construction Loads ......................................................................................................... 7-6
7.5 SLOPE FAILURES....................................................................................................................................................... 7-6
7.5.1 Cut Slope Failures ......................................................................................................................................... 7-6
7.5.2 Fill Slope Failures .......................................................................................................................................... 7-7
7.6 BASIC STABILITY DESIGN CONSIDERATIONS ................................................................................................................... 7-8
7.6.1 Gradients of Fill Slope ................................................................................................................................... 7-8
7.6.2 Examination of Stability of Fills................................................................................................................... 7-10
7.6.3 Stability Calculations .................................................................................................................................. 7-11
7.6.4 Fills Requiring Extra Precautions ................................................................................................................ 7-18
7.6.5 Execution of Fill Slope Work........................................................................................................................ 7-20
7.7 SLOPE PROTECTION WORKS ..................................................................................................................................... 7-23
7.7.1 Selection Criteria for Slope Protection Works ............................................................................................. 7-23
7.7.2 Precautions for Applying Protection Works According to Soils and Geology ............................................. 7-29
7.7.3 Vegetation .................................................................................................................................................. 7-31
7.8 RETAINING WALLS ................................................................................................................................................. 7-36
7.8.1 Definition and Applications of Retaining Walls .......................................................................................... 7-36
7.8.2 Classifications of Retaining Walls ............................................................................................................... 7-37
7.8.3 Design of Retaining Walls ........................................................................................................................... 7-41
7.8.4 Precautions for the Design of Retaining Walls ........................................................................................... 7-42
7.8.5 Execution of Retaining Wall Works ............................................................................................................ 7-43
7.9 EROSION CONTROL AND LANDSCAPING ...................................................................................................................... 7-46
7.10 REFERENCES ...................................................................................................................................................... 7-48
8
ROAD FACILITIES ............................................................................................................................................ 8-1
8.1 ROAD SAFETY AND CLEAR ZONE REQUIREMENTS............................................................................................................ 8-1
8.2 ROAD SAFETY FACILITIES ........................................................................................................................................... 8-3
8.2.1 Safety Barrier ................................................................................................................................................ 8-3
8.2.2 Median Barriers ............................................................................................................................................ 8-5
8.2.3 Drainage ....................................................................................................................................................... 8-6
8.2.4 Gateways/Traffic Calming ............................................................................................................................ 8-7
8.2.5 Noise Barriers ............................................................................................................................................... 8-9
8.2.6 Motorcycle Facilities ..................................................................................................................................... 8-9
8.2.7 Bicycle Facilities ............................................................................................................................................ 8-9
8.2.8 Pedestrian Facilities ...................................................................................................................................... 8-9
8.2.9 Parking ........................................................................................................................................................ 8-10
8.2.10 Sign Posts and Roadside Hardware ......................................................................................................... 8-10
8.2.11 Disabled Person Facilities ........................................................................................................................ 8-10
8.3 TRAFFIC CONTROL FACILITIES / DEVICES ..................................................................................................................... 8-10
8.3.1 Functional Classification of Traffic Control Devices .................................................................................... 8-10
8.3.2 Basic Principles in the Design, Installation and Maintenance of Traffic Signs ............................................ 8-11
8.3.3 Uniformity of Traffic Control Devices .......................................................................................................... 8-11
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
8.3.4
8.3.5
8.3.6
Types of Traffic Control Devices .................................................................................................................. 8-11
Markings ..................................................................................................................................................... 8-12
Speed Humps .............................................................................................................................................. 8-13
9
ROADWAY & STREET LIGHTING DESIGN (ROAD, BRIDGE, VEHICULAR TUNNEL, UNDERPASS, WALKWAY AND
BICYCLE WAY) .......................................................................................................................................................... 9-1
9.1 INTRODUCTION........................................................................................................................................................ 9-1
9.2 FACTORS IN LIGHTNING DESIGN .................................................................................................................................. 9-1
9.3 MASTER LIGHTING PLAN ........................................................................................................................................... 9-2
9.4 TECHNIQUES OF LIGHTING DESIGN .............................................................................................................................. 9-3
9.4.1 Introduction .................................................................................................................................................. 9-3
9.4.2 Illuminance and Luminance Considerations ................................................................................................. 9-3
9.4.3 Warranting Conditions ................................................................................................................................. 9-3
9.4.4 Design Values for Expressways ..................................................................................................................... 9-5
9.4.5 Streets and Highways other than Expressways ............................................................................................ 9-5
9.4.6 Pole Placement Guidelines............................................................................................................................ 9-6
9.5 HIGH-MAST LIGHTING .............................................................................................................................................. 9-7
9.6 TUNNELS AND UNDERPASSES ..................................................................................................................................... 9-7
9.6.1 Underpasses ................................................................................................................................................. 9-7
9.6.2 Vehicular Tunnels.......................................................................................................................................... 9-7
9.6.3 Lighting of Tunnel Interiors ........................................................................................................................... 9-8
9.7 WORK ZONE LIGHTING AND TEMPORARY ROADWAY LIGHTING ......................................................................................... 9-8
9.8 ROUNDABOUTS ....................................................................................................................................................... 9-9
9.9 ELECTRICAL SYSTEM REQUIREMENTS ........................................................................................................................... 9-9
9.10 SAFETY REST AREAS .............................................................................................................................................. 9-9
9.11 ROADWAY SIGN LIGHTING ...................................................................................................................................... 9-9
9.11.1 Introduction ............................................................................................................................................... 9-9
9.11.2 Sign Lighting Recommendations ............................................................................................................. 9-10
9.12 MAINTENANCE CONSIDERATIONS IN ROADWAY LIGHTING DESIGN ............................................................................... 9-11
9.13 SKY GLOW AND LIGHT TRESPASS............................................................................................................................ 9-11
9.13.1 Overview .................................................................................................................................................. 9-11
9.13.2 Mitigating Sky Glow and Light Trespass .................................................................................................. 9-11
9.14 REFERENCE ........................................................................................................................................................ 9-11
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Volumes
Volume 1
Introduction and Overview
Volume 2A
GeoHazard Assessment
Volume 2B
Engineering Surveys
Volume 2C
Geological and Geotechnical Investigation
Volume 3
Water Engineering Projects
Volume 4
Highway Design
Volume 5
Bridge Design
Volume 6
Public Buildings and Other Related Structures
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Tables and Figures
Table 1-1
Design Vehicle Dimension ........................................................................................................................ 1-5
Table 1-2
Minimum Turning Radii of Design Vehicles ............................................................................................ 1-6
Table 2-1
General Definitions of Levels of Service ................................................................................................ 2-34
Table 3-1
Minimum Design Standards for Philippine Highways – excluding Tourism Roads ............................. 3-2
Table 3-2
Minimum Design Standards for Tourism Roads ..................................................................................... 3-3
Table 3-3
AASHTO Recommended Minimum Width of Traveled Way and Shoulders for Local Rural Roads .. 3-10
Table 3-4
AASHTO Recommended Minimum Width of Traveled Way and Shoulders for Rural Collector
Roads ........................................................................................................................................................ 3-10
Table 3-5
AASHTO Recommended Minimum Width of Traveled Way and Shoulders for Rural Arterial
Roads ........................................................................................................................................................ 3-11
Table 3-6
Minimum Recommended Design Speeds for Local Rural Roads ......................................................... 3-12
Table 3-7
Minimum Recommended Design Speeds for Rural Collector Roads ................................................... 3-13
Table 3-8
Stopping Sight Distance on Level Roadways ......................................................................................... 3-15
Table 3-9
Stopping Sight Distance on Grades ........................................................................................................ 3-15
Table 3-10 Decision Sight Distance ........................................................................................................................... 3-16
Table 3-11 Passing Sight Distance for Design of Two-Lane Highways ................................................................... 3-17
Table 3-12 Minimum Radius Using Limiting Values of e and f ............................................................................... 3-20
Table 3-13 Minimum Radii (meters) for Design Superelevation Rates, Design Speeds and emax = 4% ............... 3-26
Table 3-14 Minimum Radii (meters) for Design Superelevation Rates, Design Speeds and emax = 6% ............... 3-27
Table 3-15 Minimum Radii (meters) for Design Superelevation Rates, Design Speeds and emax = 8% ............... 3-28
Table 3-16 Minimum Radii (meters) for Design Superelevation Rates, Design Speeds and emax = 10% ............. 3-29
Table 3-17 Minimum Lengths of Circular Arcs for Different Compound Curve Radii ........................................... 3-30
Table 3-18 Maximum Relative Gradients for Superelevation Runoff ..................................................................... 3-31
Table 3-19 Typical Minimum Superelevation Runoff Lengths (meters) ................................................................ 3-32
Table 3-20 Adjustment Factor for Number of Lanes Rotated ................................................................................. 3-33
Table 3-21 Runoff Locations that Minimize the Vehicle’s Lateral Motion ............................................................. 3-33
Table 3-22 Calculated and Design Values for Travelled Way Widening on Open Highway Curves (Two-Lane
Highways, One-Way or Two-Way) ......................................................................................................... 3-34
Table 3-23 Adjustments for Travelled Way Widening Values on Open Highway Curves (Two-Lane Highways,
One-Way or Two-Way) ........................................................................................................................... 3-35
Table 3-24 Maximum Radius for Use of a Spiral Curve Transition ......................................................................... 3-37
Table 3-25 Desirable Length of Spiral Curve Transition ......................................................................................... 3-37
Table 3-26 Tangent Run-out Length (m) for Spiral Curve Transition Design ....................................................... 3-38
Table 3-27 AASHTO Recommended Maximum Grades for Local Rural Roads ...................................................... 3-40
Table 3-28 AASHTO Recommended Maximum Grades for Rural Collector Roads ................................................ 3-40
Table 3-29 AASHTO Recommended Maximum Grades for Urban Collector Roads .............................................. 3-40
Table 3-30 AASHTO Recommended Maximum Grades for Rural Arterial Roads .................................................. 3-41
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Table 3-31 AASHTO Recommended Maximum Grades for Urban Arterials .......................................................... 3-41
Table 3-32 AASHTO Recommended Maximum Grades for Rural and Urban Expressways ................................. 3-41
Table 3-33 Critical Lengths of Grade ......................................................................................................................... 3-41
Table 3-34 Minimum K Value for Terrain Types ...................................................................................................... 3-42
Table 3-35 Design Controls for Crest Vertical Curves Based on Stopping Sight Distance .................................... 3-43
Table 3-36 Local Rural Road and Rural Collector Road Design Controls for Stopping Sight Distance and for
Crest and Sag Vertical Curves ................................................................................................................. 3-44
Table 3-37 Design Controls for Crest Vertical Curves Based on Passing Sight Distance ...................................... 3-44
Table 3-38 Design Controls for Sag Vertical Curves ................................................................................................ 3-45
Table 3-39 Optimal Passing Lane Lengths for Traffic Operational Efficiency ....................................................... 3-49
Table 3-40 Recommended Lengths of Turnouts Including Taper .......................................................................... 3-49
Table 3-41 Noise-Abatement Criteria for Various Land Uses ................................................................................. 3-56
Table 3-42 Stopping Sight Distance on Level Roadways ......................................................................................... 3-57
Table 3-43 Level of Service Definitions for Signalized Intersections ..................................................................... 3-60
Table 3-44 Rate of Cross Slope Range for Surface Types ........................................................................................ 3-62
Table 3-45 Road Safety Barrier System .................................................................................................................... 3-72
Table 3-46 Concrete Vs W-Beam Advantages/Disadvantages ................................................................................ 3-73
Table 3-47 Possible and Design Capacities of Highways Constructed to High Design Standard in Terms of
Passenger Cars per Hour......................................................................................................................... 3-93
Table 3-48 Deceleration Distances Required for Cars on a Level Grade ................................................................ 3-97
Table 3-49 Correction to Deceleration Distance as a Result of Grade .................................................................... 3-97
Table 3-50 Length of Acceleration Lanes for Cars on Level Grade ......................................................................... 3-98
Table 3-51 Correction of Acceleration Distances as a Result of Grade ................................................................... 3-98
Table 3-52 General Definitions of Levels of Service ................................................................................................ 3-99
Table 3-53 Guidelines for Selection of Design Levels of Service ............................................................................. 3-99
Table 3-54 Service Flow Rates Under Ideal Conditions of a Major Weaving Section (pc/h) .............................. 3-100
Table 4-1
Key Traffic Management Considerations in Selection of At-Grade Intersection Type ....................... 4-13
Table 4-2
Case A ‘No Traffic Control’ – Length of Sight Triangle Leg ................................................................... 4-16
Table 4-3
Adjustment Factors for Sight Distance Based on Approach Grade ...................................................... 4-17
Table 4-4
Case B1 ‘Left Turn from Stop’ – Design Intersection Sight Distance ................................................... 4-18
Table 4-5
Case B2 ‘Right Turn from Stop’ and Case B3 ‘Crossing Maneuver’ – Design Intersection Sight
Distance .................................................................................................................................................... 4-19
Table 4-6
Case C1 – Crossing Maneuvers from Yield-Controlled Approaches, Length of Minor Road Leg and
Travel Times ............................................................................................................................................ 4-20
Table 4-7
Case C1 ‘Crossing Maneuver at Yield-Controlled Intersections’ – Length of Sight Triangle Leg
along Major Road ..................................................................................................................................... 4-21
Table 4-8
Case C2 ‘Left or Right Turn at Yield-Controlled Intersections’ – Design Intersection Sight
Distance .................................................................................................................................................... 4-22
Table 4-9
Case F ‘Left Turn from the Major Road’ – Design Intersection Sight Distance ................................... 4-23
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Table 4-10 Edge of Traveled Way Designs for Turns at Intersection –Three Centered Curves ........................... 4-33
Table 4-11 Typical Designs for Turning Roadways ................................................................................................. 4-43
Table 4-12 Effective Maximum Relative Gradients (%) .......................................................................................... 4-45
Table 4-13 Stopping Sight Distance at Intersections for Turning Roadway .......................................................... 4-45
Table 4-14 Desirable Full Deceleration Lane Lengths ............................................................................................. 4-47
Table 4-15 Minimum Median Opening for P Design Vehicle ................................................................................... 4-49
Table 4-16 Minimum Median Opening for SU-9 Design Vehicle ............................................................................. 4-50
Table 4-17 Minimum Median Opening for SU-12, WB-12 and WB-19 Design Vehicles ........................................ 4-50
Table 4-18 Minimum Length of Median Opening for Left-Turn ............................................................................. 4-51
Table 4-19 Basic Geometric Elements of Roundabouts ........................................................................................... 4-56
Table 4-20 Design and Operational Elements for Basic Roundabout Categories .................................................. 4-56
Table 4-21 Basic Design Details for Non-Motorized Roundabout Users ................................................................ 4-60
Table 4-22 Appropriate Value for the Design Speed of the Road ........................................................................... 4-64
Table 4-23 Guide Values for Ramp Design Speed as Related to Highway Design Speed ...................................... 4-76
Table 4-24 Minimum Radius Using Limiting Values of e and f ............................................................................... 4-78
Table 4-25 Maximum Cross-Slope Difference at Crossover Crown ........................................................................ 4-79
Table 5-1
Key Components to Rainfall Analysis ...................................................................................................... 5-6
Table 5-2
Values of “C” for Use in Rational Formula ............................................................................................... 5-8
Table 5-3
Design Flood Frequency ........................................................................................................................... 5-8
Table 6-1
Recommended Thickness of Gravel Layers to be placed on the Subgrade of Gravel Road ................. 6-6
Table 6-2
Regional Factors ........................................................................................................................................ 6-9
Table 6-3
Structural Layer Coefficient .................................................................................................................... 6-10
Table 6-4
Effect of Untreated Subbase on K-values ............................................................................................... 6-21
Table 6-5
Design K-values for Cement-Treated Subbase ...................................................................................... 6-21
Table 6-6
Characteristics Pertinent to Road and Running Foundations .............................................................. 6-22
Table 6-7
Subgrade Soil Types and Range of Approximate k Values ................................................................... 6-25
Table 6-8
Loss of Subgrade Support ....................................................................................................................... 6-26
Table 6-9
Lane Distribution Factors ....................................................................................................................... 6-29
Table 6-10 An Example of Typical Reliability Levels ............................................................................................... 6-30
Table 6-11 Standard Normal Deviate (ZR) Values Corresponding to Selected Levels of Reliability................... 6-30
Table 6-12 Values of Overall Standard Deviation .................................................................................................... 6-30
Table 6-13 Recommended mi Values for Modifying Structural Layer Coefficients of Untreated Base and
Subbase Materials in Flexible Pavements .............................................................................................. 6-32
Table 6-14 Recommended Values of Drainage Coefficient, Cd, for Rigid Pavement Design .................................. 6-32
Table 6-15 Recommended Load Transfer Coefficient for Various Pavement Types and Design Conditions ...... 6-33
Table 6-16 Tie Bar Dimension and Spacings ............................................................................................................ 6-45
Table 6-17 Joint Sealant Materials ............................................................................................................................ 6-49
Table 6-18 Classification of Soil and Soil - Aggregate Mixtures (with Suggested Subgroups) ............................. 6-75
Table 6-19 Recommended Values for Subgrade and Sub-base Materials .............................................................. 6-78
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Table 6-20 Equivalency Factors for Overlay and Existing Pavement Types .......................................................... 6-83
Table 6-21 Summary of Visual (Cv) and Structural (Cx) Condition Values ............................................................. 6-91
Table 6-22 Values of C for Calculating Design CBR .................................................................................................. 6-96
Table 6-23 Types of Distress ................................................................................................................................... 6-104
Table 6-24 Load Transfer Co-efficient .................................................................................................................... 6-108
Table 6-25 Worksheet for Determination of Df for JPCP, JRCP and CRCP ............................................................ 6-115
Table 6-26 Calculation of Deff for Bonded PCC Overlay of JRCP, and CRCP ......................................................... 6-117
Table 6-27 Repair of Reflection Cracks ................................................................................................................... 6-122
Table 6-28 Worksheet for Determination of Df for Unbonded PCC Overlay ........................................................ 6-128
Table 6-29 Calculation of Deff for Unbonded FCC Overlay of JPCP, JRCP, CRCP, and AC/PCC ............................. 6-132
Table 6-30 Overlays and Repair Methods .............................................................................................................. 6-134
Table 6-31 Worksheet for Determination of Df for PCC Overlay of AC Pavement ............................................... 6-137
Table 7-1
Stability of Cut and Fill Slopes for Different Material Types .................................................................. 7-5
Table 7-2
Classification of Slope Failure Countermeasures .................................................................................. 7-24
Table 7-3
Typical Types of Slope Protection by Vegetation .................................................................................. 7-28
Table 7- 4 Types of Structural Protection ............................................................................................................... 7-28
Table 7-5
Height and Gradient Limits for Stone and Concrete Block Masonry Walls ......................................... 7-42
Table 8-1
AASHTO Suggested Clear Zone Distances in Meters from Edge of Through Travel Lane .................... 8-2
Table 8-2
Barrier Guidelines Recommended by AASHTO for Roadside Obstacles ............................................... 8-4
Table 8-3
Factors to be Considered in the Selection of Specific Types of Safety Barriers .................................... 8-5
Table 8-4
US Safety Barriers Test Levels .................................................................................................................. 8-5
Table 8-5
Design Safety Strategies for Channelized Islands and Medians in Urban Areas ................................... 8-6
Table 8-6
Design Safety Strategies for Curbs in Urban Areas ................................................................................. 8-7
Table 8-7
Design Safety Strategies for Gateways/Traffic Calming in Urban Areas ............................................... 8-7
Table 8-8
Design Safety Strategies for Bicycles ....................................................................................................... 8-9
Table 8-9
Design Safety Strategies for Pedestrians ................................................................................................. 8-9
Table 8-10 Design Safety Strategies for On-Street Parking ..................................................................................... 8-10
Table 8-11 Design Safety Strategies for Roadside Utility Poles, Light Poles and Street signs in Urban Areas .... 8-10
Table 9-1
Warranting Conditions for Continuous Expressway Lighting (CEL) ..................................................... 9-4
Table 9-2
Warranting Conditions for Complete Interchange Lighting (CIL) ......................................................... 9-4
Table 9-3
Warranting Conditions for Partial Interchange Lighting (PIL) .............................................................. 9-5
Table 9-4
Recommended Illuminous and Luminous Lighting Levels for Illuminated Signs .............................. 9-10
Figure 2-1 Typical Roadway Section for the 20.0 m RROW in Urbanized Areas Showing the Underground
Service Utilities .......................................................................................................................................... 2-3
Figure 2-2 Typical Roadway Section for the 20.0m RROW in Rural Areas Showing the Underground Service
Utilities ....................................................................................................................................................... 2-4
Figure 2-3 Typical Roadway Section for the 30.0m RROW in Urbanized and Rural Areas Showing the
Underground Service Utilities .................................................................................................................. 2-5
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Figure 2-4 Typical Roadway Section for the 30.0m RROW (Cut & Fill) for Rural Areas Showing the
Underground Service Utilities .................................................................................................................. 2-6
Figure 2-5 Typical Roadway Section for 40.0m RROW for Urbanized and Rural Areas Showing the
Underground Service Utilities .................................................................................................................. 2-7
Figure 2-6 Typical Roadway Section for 40.0m RROW (Cut and Fill) for Rural Areas Showing the
Underground Service Utilities .................................................................................................................. 2-8
Figure 2-7 Typical Roadway Section for 60.0m RROW for Urbanized and Rural Areas Showing the
Underground Service Utilities .................................................................................................................. 2-9
Figure 2-8 Typical Roadway Section for 40.0m RROW (Cut & Fill) for Rural Areas Showing the Underground
Service Utilities ........................................................................................................................................ 2-10
Figure 2-9 Example of Worn or Polished Surface ................................................................................................... 2-15
Figure 2-10 Example of Map Cracking ....................................................................................................................... 2-15
Figure 2-11 Example of Pop-outs Surface .................................................................................................................. 2-16
Figure 2-12 Example of Scaling ................................................................................................................................... 2-17
Figure 2-13 Example of Shallow Reinforcing ............................................................................................................. 2-18
Figure 2-14 Example of Spalling ................................................................................................................................. 2-19
Figure 2-15 Example of Longitudinal Joints .............................................................................................................. 2-20
Figure 2-16 Example of Transverse Joints ................................................................................................................. 2-21
Figure 2-17 Example of Transverse Slab Cracks ....................................................................................................... 2-23
Figure 2-18 Example of D-Cracks ............................................................................................................................... 2-24
Figure 2-19 Example of Corner Cracks ....................................................................................................................... 2-25
Figure 2-20 Example of Meander Cracks ................................................................................................................... 2-26
Figure 2-21 Example of Blowups ................................................................................................................................ 2-27
Figure 2-22 Example of Faulting ................................................................................................................................. 2-28
Figure 2-23 Example of Pavement Settling or Heave ................................................................................................ 2-29
Figure 2-24 Example of Utility Repairs, Patches and Potholes ................................................................................. 2-30
Figure 2-25 Example of Manhole and Inlet Cracks .................................................................................................... 2-31
Figure 2-26 Example of Curb or Shoulder Deformation ........................................................................................... 2-32
Figure 3-1 Superelevated Sections ............................................................................................................................. 3-4
Figure 3-2 Low Type Surfacing on Gravel Road ........................................................................................................ 3-5
Figure 3-3 Farm to Market Road ................................................................................................................................ 3-6
Figure 3-4 Intermediate Type Surfacing for Plant Mix Surface Course ................................................................... 3-7
Figure 3-5 High Type Surfacing Asphalt Pavement ................................................................................................... 3-8
Figure 3-6 High Type Surfacing Concrete Pavement ................................................................................................ 3-9
Figure 3-7 Example of the ‘Clear Zone’ concept for a 100 kph operating speed ................................................... 3-14
Figure 3-8 Example of Small Radius Curves in Mountainous Topography ........................................................... 3-19
Figure 3-9 Method of Attaining Superelevation for Travelled Way Revolved about Centerline ......................... 3-23
Figure 3-10 Methods of Attaining Superelvation for a Travelled Way Revolved about Outside or Inside Edge .. 3-24
Figure 3-11 Method of Attaining Superelevation for Straight Cross Slope .............................................................. 3-25
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Figure 3-12 Elements of Passing Sight distance for Two-Lane Highways ............................................................... 3-48
Figure 3-13 Effects of Depressing the Highway ......................................................................................................... 3-58
Figure 3-14 Effects of Elevating the Highway ............................................................................................................ 3-59
Figure 3-15 Example of Road with a Good Clear Zone .............................................................................................. 3-67
Figure 3-16 Drivable Culvert End ............................................................................................................................... 3-69
Figure 3-17 Example of a High Speed Road with Wide Median but Hazardous Planters within the Clear Zone . 3-77
Figure 3-18 Typical Two Lane Tunnel Cross Section ................................................................................................ 3-80
Figure 3-19 Bike Path - Class I & II ............................................................................................................................. 3-88
Figure 3-20 Bike Route Class I & II ............................................................................................................................. 3-89
Figure 4-1 General Types of At-Grade Intersections ................................................................................................. 4-3
Figure 4-2 Three-Leg Intersections ............................................................................................................................ 4-5
Figure 4-3 Four-Leg Intersections .............................................................................................................................. 4-6
Figure 4-4 Urban Splitter Island Details: Low Speed Approach ............................................................................... 4-8
Figure 4-5 Urban Splitter Island ................................................................................................................................. 4-9
Figure 4-6 Splitter Island for High Speed Approach ................................................................................................. 4-9
Figure 4-7 Roundabouts ........................................................................................................................................... 4-10
Figure 4-8 Typical Highway Curbs ........................................................................................................................... 4-26
Figure 4-9 Minimum Turning Path for Single-Unit (SU) Truck Design Vehicle .................................................... 4-29
Figure 4-10 Minimum Turning Path for Intermediate Semitrailer (WB-12 [WB-40]) Design Vehicle ................. 4-30
Figure 4-11 Minimum Turning Path for Interstate Semitrailer (WB-20, WB-65 and WB-67) Design Vehicle ..... 4-31
Figure 4-12 Symmetrical Three-Centered Compound Curve ................................................................................... 4-36
Figure 4-13 Geometric Elements of Roundabout ...................................................................................................... 4-55
Figure 4-14 Example of an Urban Roundabout ......................................................................................................... 4-58
Figure 4-15 A Rural Roundabout ................................................................................................................................ 4-59
Figure 4-16 Railroad-Highway Grade Crossings ....................................................................................................... 4-63
Figure 5-1 Culvert Location in Natural Channel ...................................................................................................... 5-12
Figure 5-2 Methods of Culvert Location in Natural Channel .................................................................................. 5-12
Figure 5-3 Possible Culvert Profiles ......................................................................................................................... 5-14
Figure 5-4 Dry Boulder (Riprap) Outlet ................................................................................................................... 5-18
Figure 5-5 Sizing of Dry Boulder Outlet Structures for Single Pipe or Box Culverts ............................................ 5-19
Figure 5-6 Sizing of Dry Boulder Outlet Structures for Multiple Pipe or Box Culverts ........................................ 5-19
Figure 5-7 Typical Rock Pad Outlet Configuration .................................................................................................. 5-20
Figure 5-8 Headwater Depth for Concrete Pipe Culverts with Entrance Control ................................................. 5-24
Figure 5-9 Headwater Depth for Concrete Box Culverts with Entrance Control .................................................. 5-25
Figure 5-10 Head for Concrete Pipe Culverts Flowing Full, n=0.012 ....................................................................... 5-26
Figure 5-11 Head for Concrete Box Culverts Flowing Full, n=0.012 ........................................................................ 5-27
Figure 5-12 Inlet Control ............................................................................................................................................. 5-29
Figure 5-13 Outlet Control .......................................................................................................................................... 5-29
Figure 5-14 Sample Summary Worksheet for Culvert Design .................................................................................. 5-32
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Figure 5-15 Types of Curb and Gutter ........................................................................................................................ 5-35
Figure 5-16 Example of Flanking Inlet ....................................................................................................................... 5-37
Figure 5-17 Parallel Grate Inlet (Not Bicycle Safe) .................................................................................................... 5-39
Figure 5-18 Parallel and Curved Vane Grates (Bicycle Safe) .................................................................................... 5-40
Figure 5-19 Curb Opening Inlet .................................................................................................................................. 5-41
Figure 5-20 Slotted Drain Inlet ................................................................................................................................... 5-41
Figure 5-21 Hydraulic Gradeline ................................................................................................................................ 5-44
Figure 5-22 “Tee” Manhole ......................................................................................................................................... 5-47
Figure 5-23 Increaser ................................................................................................................................................. 5-47
Figure 5-24 Pipe Underdrain ...................................................................................................................................... 5-51
Figure 5-25 Typical Pavement Edge Drain ................................................................................................................ 5-52
Figure 6-1 Design Chart for Flexible Pavement Based on Using Mean Values for Each Input ............................. 6-14
Figure 6-2 Design Chart for Rigid Pavement Based on Using Values for Each Input............................................ 6-15
Figure 6-3 Chart for Solving koo in Design Example for Rigid Pavement ............................................................... 6-16
Figure 6-4 Correction of Effective Modulus of Subgrade Reaction for Potential Loss of Subbase Support ........ 6-17
Figure 6-5 Approximate Interrelationships of Soil Classifications and Bearing Values ....................................... 6-24
Figure 6-6 Effective Modulus of Subgrade Reaction ............................................................................................... 6-26
Figure 6-7 Slab Length-Pavement Thickness Relationship .................................................................................... 6-38
Figure 6-8 Transverse Contraction Joint Types....................................................................................................... 6-40
Figure 6-9 Transverse Contraction Joint with Different Types of Drilled and Epoxied Load-Transfer Devices 6-41
Figure 6-10 Planned and Emergency Construction Joints ........................................................................................ 6-42
Figure 6-11 Longitudinal Joints .................................................................................................................................. 6-42
Figure 6-12 Expansion and Isolation Joints (using a doweled bar thickened-edge joint)...................................... 6-46
Figure 6-13 Joint Sealant Reservoir ............................................................................................................................ 6-50
Figure 6-14 Examples of Different Types of PCCP Joints .......................................................................................... 6-54
Figure 6-15 Design Chart for Flexible Pavements Pt=2.5 .......................................................................................... 6-67
Figure 6-16 Wet-Mix and Dry-Bound Bituminous Roadbases: Minimum Thickness of Surfacing and Roadbase 6-68
Figure 6-17 Thickness of Subbase .............................................................................................................................. 6-69
Figure 6-18 Group Index – Chart No. 1 ....................................................................................................................... 6-70
Figure 6-19 Group Index – Chart No. 2 ....................................................................................................................... 6-72
Figure 6-20 The Lowering of the Watertable by Raising the Grade of the Embankment to at Least One Meter
from the Surface ...................................................................................................................................... 6-73
Figure 6-21 The Keeping of the Water Table at Least 1.00 m Below the Road Surface by the Installations of
Under-Drains ........................................................................................................................................... 6-73
Figure 6-22 Group Index – Chart No. 3 ....................................................................................................................... 6-76
Figure 6-23 Relation Between Serviceability - Capacity Condition Factor and Traffic .......................................... 6-87
Figure 6-24 Overlay Design Procedure ...................................................................................................................... 6-90
Figure 6-25 Remaining Life Estimate Predicted from Pavement Condition Factor ................................................ 6-92
Figure 6-26 Remaining Life Factor as a Function of Remaining Life of Existing and Overlaid Pavements ........... 6-92
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Figure 6-27 Effective Dynamic k-Value Determination from d0 and Area ............................................................. 6-109
Figure 6-28 PCC Elastic Modulus Determination from k-Value, Area and Slab Thickness................................... 6-110
Figure 6-29 Chart for Estimating Composite Modulus of Subgrade Reaction, k, Assuming a Semi-Infinite
Subgrade Depth. (for Practical Purposes, a Semi-Infinite Depth is Considered to be Greater than
10 Feet Below the Surface of the Subgrade) ....................................................................................... 6-113
Figure 6-30 Relationship Between Condition Factor and Remaining Life ............................................................ 6-118
Figure 6-31 Fjc Adjustment Factor ............................................................................................................................ 6-119
Figure 6-32 Effective Dynamic K-value Determination from d0 and Area............................................................. 6-125
Figure 6-33 Fjcu Factor for Unbonded JPCP, JRCP, and CRCP Overlays .................................................................. 6-129
Figure 6-34 Fjcu Adjustment Factor for Unbonded JRCP and CRCP Overlays ........................................................ 6-131
Figure 7-1 Covering for Fill Slopes ............................................................................................................................. 7-9
Figure 7-2 Example of Fill Using Sand with Poor Grading ...................................................................................... 7-10
Figure 7-3 Calculation for the Stability of Circular Sliding Surface ........................................................................ 7-12
Figure 7-4 Example of Results of Triaxial Un-drained Shear Test of Unsaturated Fine-Grained Soil and
Design Shear Strength Parameters, Cu and Φu ...................................................................................... 7-14
Figure 7-5 Assumption of Pore Water Pressure Due to Load of Fill ...................................................................... 7-15
Figure 7-6 Simplification of Fill Slope for Stability Calculations ............................................................................ 7-16
Figure 7-7 Example of Stability Calculations ........................................................................................................... 7-16
Figure 7-8 Example Slope Stability Sample Calculation ......................................................................................... 7-17
Figure 7-9 Groundwater Drainage Facilities and Drainage Layer for Fill on Inclined Ground ............................ 7-19
Figure 7-10 Schematic Diagram of Fill on Soft Ground ............................................................................................. 7-19
Figure 7-11 Compaction by Vibrating Roller and Bulldozer..................................................................................... 7-21
Figure 7-12 Compaction of Slope Made of Coarse-Grained Soils ............................................................................. 7-22
Figure 7-13 Disposal of Surface Water During Work ................................................................................................ 7-22
Figure 7-14 Example of Temporary Drainage in a Fill Made of Decomposed Granite ........................................... 7-23
Figure 7-15 Example of Central Drain Pipe System in a Fill Slope Made of Volcanic Ash Under Construction .... 7-23
Figure 7-16 Selection of Natural Slope Failure Countermeasures ........................................................................... 7-27
Figure 7-17 Gravity Walls of Brick, Stone Masonry or Plain Concrete .................................................................... 7-38
Figure 7-18 Semi-Gravity Retaining Wall................................................................................................................... 7-38
Figure 7-19 Crib Type Retaining Wall ........................................................................................................................ 7-39
Figure 7-20 Cantilevered Retaining Wall ................................................................................................................... 7-39
Figure 7-21 Counterfort Retaining Wall ..................................................................................................................... 7-40
Figure 7-22 Mechanically Stabilized Earth Retaining Wall ....................................................................................... 7-41
Figure 7-23 Retaining Wall on Bedrock ..................................................................................................................... 7-44
Figure 7-24 Retaining Wall on Earth Stratum ........................................................................................................... 7-44
Figure 7-25 Partially Replaced Stratum ..................................................................................................................... 7-45
Figure 7-26 Replaced Foundation in Poor Ground .................................................................................................... 7-45
Figure 7-27 Erosion Control: (left) the problem; and (right) the solution .............................................................. 7-46
Figure 8-1 Clear Zone Distance ................................................................................................................................... 8-1
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Figure 8-2 Gateway/Traffic Calming Devices ............................................................................................................ 8-8
Figure 8-3 Well Designed Hump ................................................................................................................................. 8-8
Figure 8-4 Center Island to Reduce Speed ................................................................................................................. 8-8
Figure 8-5 Plan and Section of Speed Hump ............................................................................................................ 8-14
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Abbreviations
Acronym
Definition
AADT
Annual Average Daily Traffic
AASHTO
American Association of State Highway and Transportation Officials
AC
Asphalt Concrete
ACI
American Concrete Institute
ASTM
American Society for Testing & Materials
BOD
Bureau of Design
CBR
California Bearing Ratio
cm
Centimeter
dBA
Decibels
DDHV
Directional Design Hourly Volume
DGCS
Design Guidelines, Criteria and Standards
DHV
Design Hourly Volume
DID
Department of Irrigation and Drainage, Malaysia
DPWH
Department of Public Works and Highways
ESAL
Equivalent Standard Axle Load
FHA / FHWA
Federal Highway Administration, US Department of Transport
JICA
Japan International Cooperation Agency
kph
Kilometers per hour
kN
Kilonewton
m
Meter
m
2
Square meter
mm
Millimeter
MPa
Mega Pascal
MUTCD
Manual on Uniform Traffic Control Devices
PCA
Portland Cement Association
PCC
Portland Cement Concrete
SN
Structural Number
TAMS
Territory and Municipal Services, Australian Capitol Territory, Australia
USA
United States of America
30 HV
30th Highest Hourly Volume
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Glossary
Acronym
Definition
American Association of
State Highway and
Transportation Officials
(AASHTO) Soil
Classification
A system of soil classification based on grain size, liquid limit and plasticity of soils and is usually used for
highway design and construction.
Apron
A floor or lining of concrete, timber, or other resistant material at the toe of a dam, bottom of a spillway,
chute, etc. to protect the foundation from erosion and falling water or turbulent flow.
Arterial Highway
A general term denoting a highway primarily for thorough traffic, usually on continuous route.
As-Built Plan
A scaled drawing that shows a project and infrastructure components after completion of construction
At-Grade Intersection
The crossing or junction of two or more highways at a common level; includes the whole of the pavements
provided for the accommodation of through cross and turning movements.
Backwater
The rise of water level that occurs immediately upstream from a structure (eg.dam) or obstructions in a
river to a considerable distance brought about by the presence of structure.
Berm
A horizontal strip or shelf built into an embankment or cut, to break the continuity of an otherwise long
slope.
Bioengineering
The use of mechanical elements in combination with biological elements (e.g.plants) particularly for
control of erosion and prevention of slope failures.
Borrow Materials
Filling materials acquired from a Borrow Site.
Borrow Site
An excavation source ouside the project area that is used to supply soils for earthwork construction (i.e.
gravel pit).
Bridge
A structure carrying a road over a road, waterway or other feature, with a clear span over 3.0 meters along
the centreline between the inside faces of supports. A bridge may have an independent deck supported
on separate piers and abutments, or may have a deck constructed integral with supports.
California Bearing Ratio
A laboratory test that is used to determine the suitability of a soil for use as a subbase in a pavement
section.
Catchment Area
(alias Catchment Basin,
Watershed, Drainage
Area, Drainage Basin,
River Basin)
The area from which a lake, stream or waterway receives surface water which originates as precipitation.
Channelized Intersection
An at-grade Intersection in which there is division or regulation of conflicting traffic movements into definite
paths of travel by the use of pavement markings, raised Islands or other suitable means to facilitate the
safe and orderly movement of vehicles and pedestrians.
Coarse-grained Soils
Soils with more than 50% by weight of grains retained on the number 200 sieve (0.075 mm).
Cohesionless Soils
Granular soils (sand and gravel type) with values of cohesion close to zero.
Cohesive Soils
Clay type soils with angles of internal friction close to zero.
Concrete
A mixture of cement, fine aggregate, coarse aggregate and water.
Cone Penetration Test
(CPT)
A penetration test in which a cone that has a 60º point is pushed into the ground at a continuous rate.
Resistance is measured by correlating the depth penetrated with the force applied.
Cross Section
(alias Cross Section Plan)
View generated by slicing an object at an angle perpendicular to its longer axis.
Culvert
A structure in the form of a pipe or box, below road level, for conveying storm water runoff .
Design Life
Period assumed in the design for which the infrastructure is required to perform its function without
replacement or major structural repair.
Digital Terrain Model
A topographic model of the bare earth –terrain relief - that can be manipulated by computer programs. The
data files contain the spatial elevation data of the terrain in a digital format which usually presented as a
rectangular grid.
Ditch
An artificial open channel or waterway constructed through earth or rock, for the purpose of carrying
water.
Divided Highway
A highway with separated roadways for traffic in opposite directions.
Embankment
A raised structure of soil aggregate, rock or a combination of the three.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Expressway
A divided highway for through traffic with full or partial control of access and generally with grade
separations at intersections.
Factor of Safety
The ratio of a limiting value of a quantity or quality to the design value of that quantity or quality.
Geohazard
Geologic and natural hazards, particularly those that put infrastructure at risk.
Grade Separation
A crossing of two roads at different levels, one carried over the other on a structure.
Gravity Walls
Retaining walls which depend upon their selfweight to provide stability against overturning and sliding;
usually made of a high bulk structure
Grouted Riprap
When the stones in the rip-rap are fastened together by grout of mortar.
Horizontal Alignment
The position or the layout of the highway on the ground which includes straight and curved paths.
Interchange
A system of interconnecting roadways in conjunction with one or more highway separations providing for
the movement of traffic between two or more intersecting highways.
Intersection
A general term denoting the area where two or more roads join or cross.
Longitudinal Section
View generated by slicing an object at an angle parallel to its longer axis
Manhole
An opening through which a person may enter or leave a sewer, conduit, or other closed structure for
inspection cleaning, and other maintenance operations, closed by a removable cover.
Matchline
A line on a design drawing that projects a location or distance from one portion of the drawing to another
portion of the drawing.
Median
The portion of a divided highway separating travelled ways for traffic in opposite directions.
Median Lane
A speed-change lane within the median to accommodate left-turning vehicles.
Median Opening
The area between median ends, provided for use by crossing and turning traffic.
Minimum Turning Radius
The radius of the path of the outer front wheel of a vehicle making its sharpest turn with ease and comfort.
Modulus of Subgrade
Reaction
(alias Subgrade Modulus)
The ratio between the bearing pressure of a foundation and the corresponding settlement at a given point.
The slope of the line in the loading range encountered by the soil in a plate bearing value test.
Open Channel
Any conduit in which water flows with a free surface. Channel in which the stream is not completely
enclosed by solid boundaries and therefore has a free surface subjected only to atmospheric pressure.
Parkway
An arterial highway for non-commercial traffic, with full or partial control of access, and usually located
within a park or a ribbon of park-line development.
Pavement or Surfacing
The constructed all-weather surface of a highway, including parking and auxiliary lanes but excluding
shoulders. That part of roadway having a constructed surface for the facilitation of vehicular traffic.
Prime Coat
A thin layer of light, penetrating bitumen, applied by a distributor on a base to be resurfaced with
bituminous pavement, to stop the water from rising by capillarity and to coat and bind particles, thus
promoting adhesion between the base and ne pavement.
Profile
Series of elevation along a line.
Raveling
Process by which water transports soil particles downward into cavities in the underlying strata.
Reinforced Concrete
A composite material which utilizes the concrete in resisting compression forces and some other
materials, usually steel bars or wires, to resist the tension forces.
Reinforced-soil
Soil constructed with artificial reinforcing, also known as mechanically stabilized earth or MSE
Retaining Wall
A structure usually made of stone masonry, concrete or reinforced concrete that provides lateral support
for a mass of soil.
Roadbed
The grades portion of a highway, usually considered as the area between the intersection of top and side
slopes, upon which the base course, surface course, shoulders and median are constructed.
Roadway–(General)
The portion of a highway, including shoulders, for vehicular use.
Rotary Interchange
A multi-leg interchange where the major highway is grade separated from a rotary on which all turning
movements and through movements of all other highways accommodated.
Rubble Concrete
Concrete in which large stones are added to the freshly placed concrete while it is still soft and plastic.
Runoff
Surface water of an area of land.
Seal Coat
A thin bituminous treatment of the blotter type to water-proof bituminous surfaces and for non-skid
surface.
Shoulder
The portion of the roadway contiguous with the travelled way for accommodation of stopped vehicles, for
emergency use, and for lateral support of base and surface course.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Subgrade
The portion of the roadbed prepared as a foundation for the sub-base or surface course.
Superelevation
The amount by which the outer edge of a curve or railroad is banked above the inner edge to help offset
the centripetal force developed as the vehicle goes around a curve.
Tack Coat
A very thin layer of bitumen to insure a thorough bond between the new pavement and the old asphalt
surface.
Time of Concentration
The period of time for the stormwater or rainwater to flow from the most distant point to the point under
consideration.
Topographic Plan
A graphic representation of horizontal and vertical positions of an area which uses contour lines to show
mountains, valleys, and plains.
Topographic Survey
(alias Ground Survey)
Collection of data to represent horizontal and vertical positions of an area, including features such as
roads, bridges and bodies of water with contours, elevations and coordinates.
Traffic Island
An area with a roadway or between roadways from which vehicular traffic is intended to be excluded.
Travelled Way
The portion of the roadway for the movement of vehicle, exclusive of auxiliary lanes, shoulders, bicycle
lanes, parking lanes, and gutters.
Triangular Irregular
Network
A representation of a surface as a set of contiguous, non-overlapping triangles. Within each triangle the
surface is represented by a plane, where the triangles are made from a set of points called mass points.
Tributary
A stream or other body of water, surface or underground, which contributes its water, either continuously
or intermittently, to another and larger stream or body of water.
Underpass
A passage underneath something, specially a section of road that passes under another road or railroad.
Vertical Alignment
The position or the layout of the highway on the ground which includes level and gradients.
Wearing Course
The uppermost layer of asphalt placed on a finished concrete to protect the concrete and provide a
smooth riding surface.
Weep Hole
An opening provided during construction in retaining walls, aprons, canal linings, foundation, etc., to
permit drainage of water collecting behind and beneath such structures to reduce hydrostatic head.
Wingwall
A vertical wall located at both ends of the coping of the abutment or at both extreme wall of a reinforced
concrete box culvert.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
1
General Provisions
1.1
Scope and Application
These guidelines shall apply to the design of all types of highways in the
Philippines.
They have been compiled using the previous DPWH Design Guidelines and
DPWH Department Orders, with reference to current AASHTO design
publications.
1.2
Governing Laws, Local Ordinances, Rules & Regulations, Codes and Department
Orders
 General Specifications for Roads and Bridges, 2013 / Latest DPWH
Manuals/Specifications 2013.
 AASHTO, A Policy on Geometric Design of Highways and Streets, 6th Edition
2011.
 AASHTO Guide for Design of Pavement Structures, 4th Edition, 1993.
 Road Note 29, Third Edition – A Guide to the Structural Design of Pavements
for New Roads.
 Road Note 31 – A Guide to the Structural Design of Bitumen Surfaced Roads in
Tropical and Sub-Tropical Countries.
 Executive Order No. 113, Establishing the Classification of Roads.
 Hydraulic Charts for the Selection of Highway Culverts, Circular No. 5, U.S.
Bureau of Public Roads.
 Design Charts for Open-Channel Flow, U.S. Bureau of Public Roads.
 P.D. 187 as amended by P.D. 748 and Batas Pambansa Blg. 8, An act defining
the Metric System and its Units, providing for its implementation and for other
purposes; and MPWH Memorandum Circular No. 6, dated January 6, 1983, re
Metric System (SI) Tables.
 DPWH Road Sign and Pavement Markings Manual 2012.
 DPWH Road Safety Manual 2012.
 Standing/Existing DPWH Department Orders.
 DPWH, Standard Specifications for Highways Bridges and Airports Revised
2012 Edition.
1.3
Reference Materials
Many tables and figures in Volume 4 have been reproduced from the following
references with permission from AASHTO:
 A Policy on Geometric Design of Highways and Streets (2011)
 AASHTO Guide for Design of Pavement Structures (1993)
 Roadway Lighting Design Guide
1-1
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
1.4
General Concepts
The goal in Highway Design is to provide safe roads for all road users, and not just
those in motor vehicles. The principle that ‘man is the reference standard’ implies
that roads must be adapted to the limitations of human capacity. This leads to what
is called the ‘safe systems approach’ which encourages:
 Simpler, self-explaining roads (with less reliance on traffic signs).
 Designing roads that encourage / enforce safe speeds (the safe speed being
the one that guarantees the safety of the most vulnerable road user).
 Forgiving roadsides (the ‘Clear Zone’ idea about unobstructed, traversable
space beyond the edge of the travelled way for recovery of errant vehicles).
 The World Bank’s Sustainable safe Road Design Manual also discusses.
 Functionality – developing a hierarchy of mono-functional roads (e.g. truck,
distributor and access).
 Homogeneity – avoiding differences in speed, direction of travel, and mass of
vehicles (with segregation of incompatible road users).
 Predictability – ensuring that roads are easy to understand and there are not
nasty surprises (as for ‘self-explaining’ roads).
The characteristics listed below are controls in optimizing or improving the
design of the various highway and street functional classes.
Human Factors and Driver Performance
The suitability of a design rests as much on how effectively drivers are able to use
the highway as on any other criteria. Considerations include;
 Driver tasks that include vehicle control (such as simultaneous multiple tasks
and reaction time), guidance (such as road following, lane placement, car
following, passing maneuvers and response to traffic control devices) and
navigation.
 Use of the facility by older drivers and older pedestrians.
 Errors due to driver deficiencies and situational demands.
 Speed.
Properly designed highways that provide positive guidance to drivers can
operate at a high level of efficiency and with relatively few crashes.
Vehicles
Four general classes of design vehicles are (1) passenger cars, (2) buses, (3)
trucks, and (4) recreational vehicles. In the design of any highway facility, the
designer should consider the largest design vehicle that is likely to use that
facility with considerable frequency or a design vehicle with special
characteristics appropriate to a particular location in determining the design of
such critical features as radii at intersections and radii of turning roadways (refer
to Table 1-1 and Table 1-2). As a general rule;
1-2
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
 A passenger car may be selected when the main traffic generator is a parking
lot.
 A two-axle single unit truck may be used for intersection design of residential
streets and park roads.
 A three-axle single-unit truck may be used for the design of collector streets
and other facilities where larger single-unit trucks are likely.
 A bus may be used in the design of highway intersections that are designated
bus routes and that have relatively few large trucks using them.
Traffic Characteristics
The design of a highway and its features should explicitly cover traffic volumes
and traffic characteristics. Traffic volumes obtained from field studies (such as
hourly and daily traffic volumes, type and weight of vehicles and traffic trends)
can indicate the need for improvement and directly influence the selection of
geometric design features, such as number of lanes, widths, alignments and
grades. Relevant studies include average daily traffic (ADT), peak hour traffic,
directional distribution, composition of traffic, projection of future traffic
demands, speed and traffic flow relationships characterized by the volume flow
rate in vehicles per hour, the average speed in kilometers per hour, and the traffic
density in vehicles per kilometer.
Physical Elements
These elements include highway capacity, access control and management,
pedestrians, bicycle facilities, safety and environment.
Knowledge of highway capacity is essential to properly fit a planned highway to
traffic demands.
Access control to manage interference with through traffic is achieved through
the regulation of public access rights to and from properties abutting the
highway facilities, and can comprise full control, partial control, access
management or driveway/entrance regulations.
Pedestrian facilities including sidewalks, crosswalks, traffic control features, curb
cuts, ramps, bus stops, loading areas, stairs, escalators and elevators warrant due
attention in both rural and urban areas.
Existing streets and highways provide most of the network used by bicycle travel,
making bicycle traffic an important element for consideration in highway design.
Because the number of crashes increases with the number of decisions that need
to be made by the driver, it is in the interest of safety that roadways should be
designed to reduce the need for driver decisions and to reduce unexpected
situations.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Highways can and should be located and designed to complement their
environment and serve as a catalyst to environmental improvement.
Economic Factors
Highway economics is concerned with the cost of a proposed improvement and
the benefits resulting from it.
1-4
S-BUS 11
S-BUS 12
A-BUS
Large School Bus (84 pass.)
Articulated Bus
WB-19 *
WB-20 **
WB-20D
WB-28D
WB-30T
WB-33D *
Interstate Semitrailer
Interstate Semitrailer
‘Double-Bottom’ Semitrailer/Trailer
Double-Semitrailer/Trailer
Triple-Semitrailer/Trailer
Turnpike Double-Semitrailer/Trailer
P/T
P/B
MH/B
Car and Camper Trailer
Car and Boat Trailer
Motor Home and Boat Trailer
3.66
-
3.05
3.66
4.11
4.11
4.11
4.11
4.11
4.11
4.11
3.35
3.20
3.20
3.20
2.44
2.44
2.44
2.44
2.59
2.59
2.59
2.59
2.59
2.59
2.44
2.59
2.44
2.44
2.59
2.59
2.59
2.44
2.44
2.13
Width
16.15
12.80
14.84
9.14
34.75
31.94
29.67
22.04
22.40
21.03
13.87
18.29
12.19
10.91
12.19
13.86
12.36
12.04
9.14
5.79
Length
1.22
0.91
0.91
1.22
0.71
0.71
0.71
0.71
1.22
1.22
0.91
2.62
2.13
0.79
2.13
1.89
1.93
1.22
1.22
0.91
Front
5.94
2.44
2.44
3.66
6.10
3.35
3.35
6.10
3.72
1.83
3.35
0.91
5.33
1.37 a
0.91
0.91
1.37
3.35
5.94
a
1.37
3.81
a
6.71
6.10
6.49
1.37 a
3.05
3.96
3.66
2.44
7.62
8.69
2.73
7.70
b
7.62
6.10
3.35
WB1
2.73 a
3.20
1.83
1.52
Rear
Overhang
-
-
-
-
12.19
6.86
12.19
7.01
13.87
12.50
7.77
5.91
-
-
-
-
-
-
-
-
WB2
Dimensions (m)
*
Design vehicle with 14.63 m trailer
**
Design vehicle with 16.15 m trailer
a
This is the length of the overhang from the back axle of the tandem axle assembly
b
combined dimension is 5.91 m and articulating section is 1.22 m wide
c
Combined dimension is typically 3.05 m
d
Combined dimension is typically 3.05 m
e
Combined dimension is typically 3.81 m
Source: Table 2-1a in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6th Edition. Used by Permission.
MH
Motor Home
Recreational Vehicles
WB-12
Intermediate Semitrailer
Combination Trucks
CITY-BUS
Conventional School Bus (65 pass.)
3.66
BUS-14
City Transit Bus
Intercity Bus (Motor Coaches)
3.35–4.11
3.66
SU-12
Single-Unit Truck (three-axle)
3.35-4.11
1.30
Height
Overall
Design Vehicle Dimension
BUS-12
SU-9
Single-Unit Truck
Buses
P
Symbol
Passenger Car
Design Vehicle Type
Table 1-1
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
1.83
1.52
1.52
-
1.37 e
4.57
4.57
5.39
-
3.05 e
2,13
d
0.91
2.13
d
2.13 c
-
-
-
4.02
T
1.37
0.91 c
-
-
-
1.89
S
-
-
-
-
12.19
6.86
6.86
6.86
-
-
-
-
-
-
-
-
-
-
-
-
WB3
-
-
-
-
-
6.86
-
-
-
-
-
-
-
-
-
-
-
-
-
-
WB4
1-5
-
-
-
-
12.34
7.01
12.34
7.01
13.87
12.50
7.77
-
-
-
-
-
-
-
-
-
Typical Kingpin
to Center of rear
Tandem Axle
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
S-BUS 11
S-BUS 12
A-BUS
WB-12
WB-19 *
WB-20 **
WB-20D
WB-28D
WB-30T
WB-33D *
MH
P/T
P/B
MH/B
Large School Bus (84 pass.)
Articulated Bus
Intermediate Semitrailer
Interstate Semitrailer
Interstate Semitrailer
‘Double-Bottom’ Semitrailer/Trailer
Double-Semitrailer/Trailer
Triple-Semitrailer/Trailer
Turnpike Double-Semitrailer/Trailer
Motor Home
Car and Camper Trailer
Car and Boat Trailer
Motor Home and Boat Trailer
1-6
Source: Table 2-2a in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6th Edition. Used by Permission.
The turning radius assumed by a designer when investigating possible turning paths is set at the centerline of the front axle of a vehicle. If the minimum turning path is assumed, the CTR
approximately equals the minimum design turning radius minus one-half the front width of the vehicle.
10.67
2.44
5.58
7.92
4.19
2.96
16.94
5.83
0.59
2.25
5.88
6.49
7.71
7.25
7.45
7.54
7.41
11.09
b
14.02
6.40
9.14
10.97
17.04
12.47
23.77
12.47
12.50
12.50
10.97
10.82
10.79
10.64
11.52
12.25
11.53
14.46
8.64
4.39
Minimum Inside Radius (m)
School buses are manufactured from 42-passenger to 84-passenger sizes. This corresponds to wheelbase lengths of 3.35 to 6.10 m respectively. For these different sizes, the minimum design
turning radii vary from 8.58 to 11.92 m and the minimum inside radii vary from 5.38 to 7.1 m.
**Design vehicle with 16.15 m trailer
15.19
7.26
10.03
12.11
18.25
13.67
24.98
13.67
13.66
13.66
12.16
12.00
11.92
11.75
12.80
15.60
11.58
6.40
Centerlineb Turning Radius [CTR] (m)
a
*Design vehicle with 14.63 m trailer
CITY-BUS
BUS-14
Conventional School Bus (65 pass.)
13.40
BUS-12
Intercity Bus (Motor Coaches)
City Transit Bus
12.70
SU-12
Single-Unit Truck (three-axle)
12.73
SU-9
Single-Unit Truck
7.26
Minimum Design Turning Radius (m)
P
Symbol
Minimum Turning Radii of Design Vehicles
Passenger Car
Design Vehicle Type
Table 1-2
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
1.5
Highway Types / Classification
Functional classification groups highways by the character of service they
provide, and is primarily based on motor vehicle travel characteristics and the
degree of access to adjacent properties.
The six recognizable stages in motor vehicle travel include main movement,
transition, distribution, collection, access and termination. Each of these stages is
handled by a separate facility designed specifically for its function. A prominent
cause of highway obsolescence is the failure of design to recognize and
accommodate each of the different trip levels of the movement hierarchy.
Functional classification serves as a basis for organizing geometric design
criteria, with the classification of a highway or street establishing the basic
design type to be used for the facility. The first step in the design process is to
define the function that the facility is to serve. The level of service needed to
fulfill this function for the anticipated volume and composition of traffic provides
a rational and cost-effective basis for the selection of design speed and geometric
criteria. The use of functional classification as a design type should appropriately
integrate the highway planning and design process.
1.5.1
Highway Types
The traffic characteristic which has the greatest effect on highway design is the
volume of traffic. The design element which is the most affected by the volume of
traffic is the number of traffic lanes.
In modern practice, single-lane and 3-lane highways are considered
inappropriate as parts of an improved highway system. From the standpoint of
engineering design and construction at least 2-traffic lanes should be considered
in any proposed highway no matter how low the traffic volume may be. Highway
types are therefore considered to be 2-lane, multi-lane (four or more lanes),
undivided and multilane divided highways.
1.5.2
2-Lane Highways
2-lane highways constitute the majority of the total length of highways, varying
from gravel or other loose surface roads to high type pavement.
Lane widths of 2-lane highways vary from 3.00 m to 3.65 m depending upon the
traffic volume, design speed, character of terrain and economic considerations.
From the standpoint of the driver’s convenience, ease of operation, and safety, it
is desirable to construct all 2-lane highways with 3.35 m lanes and with usable
shoulders 3.0 m wide. However, narrow shoulder widths may be used in rugged
terrain where traffic volume is low or when economic considerations govern.
Where the critical length of grade is exceeded and the design capacity is reduced
because of climbing trucks, the provision of a climbing lane is desirable where
Design Hourly Volume (DHV) exceeds the reduced capacity by 20% or more.
The climbing lane should not be less than 3.00 m and preferably 3.35 m wide. A
shoulder 1.20 m wide is considered adequate. It should be signed and marked.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
The climbing lane should begin near the foot of the grade at a point determined
by the speed of the trucks at the approach to the grade. Where practicable, the
climbing lane should end at a point beyond the crest where the truck can attain a
speed of 50 kph.
Where it is anticipated that the Design Hourly Volume (DHV) within a period of
about 10 to 20 years will exceed the design capacity of a 2-lane highway, the
initial improvement should be patterned for ultimate development of a 4-lane
divided highway. The initial 2-lane width should form one of the ultimate oneway traffic lanes.
1.5.3
4-Lane Undivided Highways
A 4-lane undivided highway is the narrowest highway on which each traffic lane
is intended to be used by traffic in one direction and passing is accomplished on
lanes not subject to use by opposing traffic. The ability to pass without travelling
in the lane of opposing traffic, results in a smoother operation and a large
increase in highway capacity over that of 2-lane highways. Speed limit should be
limited to 60 kph or less, and they should feature prominent road marking to
separate opposing streams.
Adequate shoulders which encourage all drivers in emergencies to use them are
essential on 4-lane undivided highways. Vehicles stopping in through traffic lanes
are very hazardous, probably more so than on a divided highway, because
following vehicles that maneuver to pass encourage and sometimes force other
vehicles that are behind and in the inner lane to edge beyond the centerline.
Undivided highways with four or more lanes are most applicable in urban and
suburban areas where there is concentrated development of adjacent land.
1.5.4
Divided Highways
Although highways with widely separated roadways may be particularly suitable
to certain topographic conditions, there are other advantages which may be
derived among which are easy vehicle operation, pleasing appearance and better
drainage. Where there is appreciable length of a widely divided highway, an
occasional open view between the two roadways is desirable to make evident
their one-way operation.
A divided highway is one with separated roadways for traffic in opposite
direction. It has at least two full lanes for each direction of travel and a median of
1.20 m or more in width constructed in a manner to preclude its use by vehicles
except in emergencies. Increased safety, comfort and ease of operation are the
principal advantages of dividing multilane highways. A divided highway
generally is for high volume and high speed operations. Medians 1.20 m to 1.80 m
wide are acceptable under restricted rural conditions but, wherever feasible,
medians should be made 4.50 to 18.50 m wide and preferably wider to obtain full
advantage of traffic separation and to fit intersection design at cross roads.
Divided highways need not be of constant cross section. Often a more pleasing
and less costly design is obtained by appropriate variation in the width of median
and in the pavement levels. Where construction makes it desirable to narrow the
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
median or where it is advantageous to widen the median on a tangent alignment,
the change should be effected by reverse curves of one (1) degree or less.
Climbing lanes on multilane roads may be justified where the critical length of
grade is exceeded and the reduced design capacity because of trucks climbing
long grades is exceeded by the Directional Design Hourly Volume (DDHV) 30% or
more. Geometric details are the same as for climbing lanes on 2-lane highways.
In the design of divided highways the inclusion of a median in the cross section
alters somewhat the superelevation run-off treatment. The following are the
different cases in attaining the superelevation runoff treatment:
 The whole of the travelled way, including the median, is superelevated as a
plane section.
 The median is held in a horizontal plane and the two pavements are rotated
about the median edges.
 The two pavements are separately treated for runoff, resulting in variable
difference in elevation at the median edges.
1.5.5
Classification of Highways According to System
In the Philippines roads or highways are classified as national, provincial, city,
municipal or barangay roads.
National Roads
Public roads, declared as national roads by the President of the Philippines upon
recommendation of the Secretary of Public Works and Highways satisfying the
conditions set forth under Executive Order No.113, Establishing the Classification
of Roads. National roads are classified as primary and secondary roads. The
former forms the part of the main highway trunk-line system which is continuous
in extent; the latter includes all access roads forming a secondary trunk-line
system.

Road Right of way……………………………………………….20.00 m minimum

Width of travelled way 2 lane ……………………...………..6.70 m minimum

Allowable grade …………………………………………………..…6.0% maximum
Provincial Roads
These are roads connecting one municipality to another, with the terminal to be
the public plaza; plus roads extending from one municipality or from a provincial
or national road to a public wharf or railway station. For purposes of allocating
national aid maintenance, a provincial road is designated and accepted as such by
the Secretary of the Department of Public Works and Highways, upon
recommendation of the Provincial Board (Sangguniang Panlalawigan).

Road Right of way

Width of travelled way

Allowable grade
15.00 m minimum
6.10 m minimum
6.0% maximum
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
City Roads
Roads / streets within the urban area of a city not classified as provincial or
national roads.

Road Right of way

Width of travelled way

Allowable grade
15.00 m minimum
6.10 m minimum
6.0% maximum
Municipal Roads
Roads / streets within the poblacion area of a municipality not classified as
provincial or national roads.

Road Right of way

Width of travelled way

Allowable grade
15.00 m minimum
6.10 m minimum
6.0% maximum
Barangay Roads
Roads located outside the poblacion area of a municipality or urban area of a city
and those outside industrial, commercial or residential subdivision (access roads
to subdivisions are not barangay roads), and which act as a feeder from Farm-tomarket road, and are not otherwise classified as national, provincial, city or
municipal roads. Barangay roads must meet the following criteria:

Road Right of way
10.00 m minimum

Width of travelled way
4.00 m minimum

Allowable grade
10.0% maximum
Tourism Road
Tourism road is a road which marketed as particularly suited for tourist. Tourist
road may be formed when existing road are promoted with traffic sign and
advertising material. Some tourist road such as Ternate-Nasugbu road are built
for tourism purposes. Others maybe roadways enjoyed by local citizen in areas of
unique or exceptional natural beauty. It is often developed because it promises to
generate employment, enhance community infrastructure and assist in
revitalizing the flagging economies in rural areas.

Road Right of way
2.10 m minimum

Width of travelled way
6.10 m minimum

Allowable grade
6.0 % maximum
Farm to Market Road
Farm to Market Roads refer to roads linking the agriculture and fisheries
production sites, coastal landing points and post-harvest facilities to the market
and arterial roads and highways.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
1.5.6

Road Right of way
6.00 m minimum

Width of travelled way
4.00 m minimum

Allowable grade
10.0% maximum
Road Classification According to Primary Function
Road classifications based on the primary functions are the following:
Expressways
These are divided arterial highways for through traffic, with full or partial control
of access and generally with grade separations at major intersections.
Parkways
Parkways are arterial highways for non-commercial traffic with full or partial
control of access, usually located within a park or a ribbon of park-like
development.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
2
Design Data
2.1
Field Survey Information
Topography is a major factor in determining the physical location, alignment,
gradients, sight distance, cross sections and other design elements of a highway.
Hills, valleys, steep slopes, rivers and lakes often impose limitations upon
location and design. In the case of flat-land areas, topography in itself may
exercise little if at all control on location but it may cause difficulties in some
design elements such as drainage or grade separation.
2.1.1
Highway Location
Highway location is concerned with gathering of pertinent data for more effective
highway planning, design, construction and operation. It consists mainly of
reconnaissance, topographic surveys, establishment of horizontal and vertical
controls, centerline staking, centerline profile and cross-sectional leveling, bridge
site survey, parcellary survey, and other surveys related to highway engineering.
The survey shall be under the direct supervision of a Locating Engineer.
Reconnaissance
Reconnaissance is carried out in order to plan the best possible horizontal and
vertical alignments. Rock cuts, agricultural farms, steep side slopes, slides and
other controls are identified. Bridge crossings, expensive buildings and
structures are also noted. Reconnaissance is substantiated by the study of
available maps, and stereoscopic examination of the site on foot, all of which aid
in the elimination of costly locations to limit the choice to one or two possible
routes.
Preliminary Survey
In the preliminary survey the topography of the strip or strips flagged is obtained
and from which a topographic map will be prepared to be utilized as the basic
framework for projection of the line in the office.
The required preliminary borings shall include review of available topographic
and geologic information, plus aerial photographs, in addition to site
examination.
Utility Service Records
Depending on the location of a project, the utilities involved could include (1)
sanitary sewers, (2) water supply lines, (3) oil, gas and petroleum product
pipelines, (4) overhead and underground power and communication lines
including fiber optic cables, (5) cable television lines, (6) wireless communication
towers, (7) drainage and irrigation lines, and (8) special tunnels for building
connections.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Utility service providers should be consulted and records obtained for all
services in a project area, including exact locations and depths. Obtaining Utility
service records benefits both highway agencies and the impacted utilities in the
following ways;

Unnecessary utility relocations are avoided

Unexpected conflicts with utilities are reduced

Safety is enhanced
For typical Roadway Section showing the location of service utilities, refer Figure
2-1 to Figure 2-8.
2-2
2-3
Typical Roadway Section for the 20.0 m RROW in Urbanized Areas Showing the Underground Service Utilities
Source: DPWH Department Order No. 26 Series of 2011 Policy on Digging/Excavation by Private and Public Utilities on National Road.
Figure 2-1
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Typical Roadway Section for the 20.0m RROW in Rural Areas Showing the Underground Service Utilities
Source: DPWH Department Order No. 26 Series of 2011 Policy on Digging/Excavation by Private and Public Utilities on National Road.
Figure 2-2
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
2-5
Typical Roadway Section for the 30.0m RROW in Urbanized and Rural Areas Showing the Underground Service Utilities
Source: DPWH Department Order No. 26 Series of 2011 Policy on Digging/Excavation by Private and Public Utilities on National Road.
Figure 2-3
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Typical Roadway Section for the 30.0m RROW (Cut & Fill) for Rural Areas Showing the Underground Service Utilities
Source: DPWH Department Order No. 26 Series of 2011 Policy on Digging/Excavation by Private and Public Utilities on National Road.
Figure 2-4
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
2-7
Typical Roadway Section for 40.0m RROW for Urbanized and Rural Areas Showing the Underground Service Utilities
Source: DPWH Department Order No. 26 Series of 2011 Policy on Digging/Excavation by Private and Public Utilities on National Road.
Figure 2-5
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Typical Roadway Section for 40.0m RROW (Cut and Fill) for Rural Areas Showing the Underground Service Utilities
Source: DPWH Department Order No. 26 Series of 2011 Policy on Digging/Excavation by Private and Public Utilities on National Road.
Figure 2-6
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
2-9
Typical Roadway Section for 60.0m RROW for Urbanized and Rural Areas Showing the Underground Service Utilities
Source: DPWH Department Order No. 26 Series of 2011 Policy on Digging/Excavation by Private and Public Utilities on National Road.
Figure 2-7
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Typical Roadway Section for 40.0m RROW (Cut & Fill) for Rural Areas Showing the Underground Service Utilities
Source: DPWH Department Order No. 26 Series of 2011 Policy on Digging/Excavation by Private and Public Utilities on National Road.
Figure 2-8
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Office Projection
In the office the proposed highway line is projected on the topographic map
which is fitted as close as possible into the terrain within the desired standards.
Many lines should be tried so as to obtain the most economical line without
increasing the cost of surveys. This is a trial and error process to obtaining the
best line, in consideration of constraints such as alignment, grades, sight
distances and compensation.
Final Location Survey
Final location survey is done to transfer the office projection of the best line to
the actual site in the field.
Whenever possible, video logs of a site with chainages are a useful tool to assist
the process of designing upgrading and rehabilitation schemes.
2.2
Field Investigations
2.2.1
Proposed Sites for Stream Crossings
The location of a highway when crossing a stream is important for several
reasons. For example:

Hydrologic and hydraulic considerations are different when crossing near the
confluence of two streams as compared to a single stream.

Higher backwaters may be better tolerated in rural areas than in urban
locations.

Tidal areas present a list of entirely different hydraulic considerations.

Whether the structure is a bridge or a culvert can make a difference in the
hydraulic study.
In addition environmental considerations, such as land usage upstream and
downstream, the need for energy dissipation, debris control and the need for fish
passage are all aspects that impact on the extent of field investigations required
for a specific design.
2.2.2
Road Alignment
The alignment of a highway or street can produce a major impact on the
environment, the fabric of the community, and highway users. The alignment
consists of a variety of design elements that combine with the aim of creating a
facility that serves traffic safely and efficiently, consistent with the facility’s
intended function. Each element requires due consideration and they all must
complement each other to achieve a consistent, safe and efficient design.
Horizontal Alignment
Horizontal alignment is a combination of circular curves, transition curves, and
tangents. Horizontal alignment must provide safe and continuous operation at a
uniform design speed for substantial lengths of highway. The major design
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
considerations in horizontal alignment are safety, functional classification, design
speed, topography, vertical alignment, construction cost, cultural development,
and aesthetics. These factors, when properly balanced, produce an alignment that
is safe, economical, and in harmony with the natural contour of the land.
Vertical Alignment
Vertical alignment consists of a series of gradients connected by vertical curves.
Applicable design controls include safety, topography, functional classification,
design speed, horizontal alignment, construction cost, cultural development,
drainage, vehicular characteristics, and aesthetics. The terms vertical alignment,
profile grade and grade line are interchangeable.
The topography of the land has an influence on alignment, with the three terrain
classifications commonly used internationally being:
2.2.3

Level or flat

Rolling

Mountainous
Existing Utility Services
Records obtained from utility service providers should be verified in the field,
and not simply assumed to be correct. Where discrepancies are found, the
relevant service provider should be contacted and a procedure to resolve the
discrepancy should be agreed before taking any further action.
If existing services which had not previously been expected were found during
the course of investigations, the relevant service provider should be contacted
and requested to confirm the status of the service before any activity that may
affect that service proceeds.
2.3
Soil Investigations
The Geotechnical Engineer should direct his investigations towards verification
of probable GeoHazards and obtaining design data for the construction or
improvement of the road, and to this end analyze in detail the soil types along the
road in order to decide the most suitable investigation, method, and equipment
to be used. All investigations shall be performed according to ASTM or AASHTO
standards, and soil shall be classified according to the AASHTO system.
2.3.1
Subsurface Investigation
Subsurface investigation includes investigation of the area below the subgrade
level. The required depth of exploration along the alignment of road shall be
based on the knowledge of subsurface conditions from geology, soil surveys and
previous explorations, and on the configuration of the highway at any given
point.
For areas of light cut and fill where there are no special problems, the exploration
should extend to a depth of at least 1.5 m below the proposed subgrade.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Where deep cuts are to be made, large embankments across marshland, or if
subsurface information indicates the presence of weak layers, the depth shall
depend on the existing topography and the nature of subsoil.
2.3.2
Subgrade Investigation
Subgrade investigation involves detailed investigation of the soil surface on
which the pavement is constructed. On existing roads, auger borings and test pits
should be made along the centerline of the road at suitable intervals. Auger
borings should be carried out as directed by the Geotechnical Engineer. Boring
should be located alternately at the center of the pavement and the edge of the
pavement.
Logging of the bore profile should be made to determine the existing pavement
thickness, condition and type of material in the pavement structure, and to
describe the underlying subgrade soils. Samples should be taken of the subgrade
material for soil classification in the field.
Based on the soil classifications found from the auger borings, test pits should be
located at intervals along the road to cover a representation of all subgrade soil
types. Pits should be properly logged, with small samples (3 kg) taken on all
layers encountered (namely base, sub-base and subgrade) for soil classification.
In-situ density testing as per AASHTO T 191 of the subgrade layer should be
made. Large samples (60 kg) should be taken of the subgrade material for
observing the Moisture-Density-CBR relationship, and other testing.
Where road raising is proposed or new construction required, sampling and
testing of both the in-situ material and the proposed select fill source would be
required in order to supply adequate subgrade data for pavement design.
In sections of high embankments or roadside cuts (>3 m), deep borings shall be
undertaken to characterize the subsurface and carry out appropriate
geotechnical analysis (such as slope stability analysis, settlement analysis). The
provisions of Volume 2C shall be referred to in the formulation of the
geotechnical investigation program.
2.3.3
Widening of Existing Pavements
In this case, the same method of auger boring and classifying the in-situ materials
into groups, then taking representative test pits plus in-situ testing and
laboratory testing as described in Section 2.3.1 should be followed. However
borings and test pits should be located in the area of widening usually below the
shoulder. Samples of subgrade should be taken at a level below that of the
existing pavement, as any pavement widening should have a design depth of at
least as thick as that of the existing pavement.
2.3.4
Sampling and Testing
In-Situ
All pits and boreholes should be properly logged in the form shown in the
standard sheet provided in Volume 2C. Details to be shown include the thickness
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
of each layer, the color, the type and visual description of each layer (such as
asphalt, gravel, clay-loam, brown, yellow), depth below the surface, water levels
(if any).
Small samples should be taken at each auger hole of the subgrade for ‘in the field’
soils classification as per AASHTO T 88 or T 27. Small and large samples should
be taken in the test pit, plus an in-situ density test made as per AASHTO T 191.
Laboratory Tests
The following tests should be made on the subgrade samples obtained from test
pits and boreholes:

Mechanical Analysis – AASHTO T 88 or 27

Specific Gravity – AASHTO T 100 or 84 or 85

Atterberg Limits – AASHTO T 89 or 90

Moisture-Density Relationship – AASHTO T 180 or 99

CBR% – AASHTO T 193

Natural Moisture Content
Classification of soils would be made in accordance with AASHTO M 145, and all
dry samples should be prepared in accordance with AASHTO T 87.
2.4
Existing Pavement Evaluation
Whilst test pits and borings can give all the subgrade data, only a pavement
inspection combined with some background history of the pavement can guide
the Pavement Engineer in his evaluation on the remaining life of the pavement
and the original quality of its construction.
2.4.1
Visual Inspection/Surface Defects
Wear and Polishing
A worn or polished surface may appear from traffic wearing off the surface
mortar and skid resistant texture. Extensive wear may cause slight ruts where
water can collect and cause hydroplaning. Sometimes traffic may polish
aggregates smooth, causing the surface to be slippery. An asphalt overlay or
grinding of the concrete surface can restore skid resistance and remove ruts.
Refer Figure 2-9.
Map Cracking
A pattern of fine cracks usually spaced within several inches is called map
cracking. It usually develops into square or other geometrical patterns. The
cracking can be caused by improper cure or overworking the surface during
finishing. If severe, cracks may spall or surface may scale. Repair is usually
limited to very severe conditions. An asphalt overlay or partial depth patching
may then be necessary. Refer Figure 2-10.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Figure 2-9
Example of Worn or Polished Surface
Surface mortar worn away exposing larger aggregate.
Accidents or friction testing may indicate a slippery surface
in need of improved texture.
Figure 2-10
Close-up of a polished pavement
surface.
Example of Map Cracking
Hairline surface cracks, probably shallow in depth. May not cause any long term performance problems.
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Pop-outs
Individual pieces of large aggregate may pop out of the surface. This is often
caused by chert or other absorbent aggregates that deteriorate under freezethaw conditions. Surface patching can be done temporarily with asphalt. For
severe areas, a more permanent partial depth concrete patch may be necessary.
Refer Figure 2-11.
Figure 2-11
Example of Pop-outs Surface
Extensive pop-outs of large aggregate from surface. Pop- outs alone have not affected pavement
serviceability.
Scaling
Scaling is surface deterioration that causes loss of fine aggregate and mortar.
More extensive scaling can result in loss of large aggregate. Often caused by using
concrete which has not been air-entrained, the surface becomes susceptible to
freeze-thaw damage. Scaling is also aggravated by the use of deicing chemicals.
Refer Figure 2-12.
Scaling can occur as a general condition over a large area or be isolated to
locations where poor quality concrete or improper finishing techniques caused
loss of air entrainment. In severe cases, deterioration can extend deep into the
concrete. Traffic action may accelerate scaling in the wheel paths.
Grinding may remove poor quality surface concrete. Asphalt overlays or a
bonded concrete resurfacing can prolong the life of the pavement. Partial depth
patching of isolated areas may also be used.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Figure 2-12
Example of Scaling
Moderate surface scaling. Loss of mortar and fines from the surface beginning to expose larger
aggregate.
Severe scaling. Some larger aggregate is loose.
Shallow reinforcing
If the steel reinforcing bar or mesh is placed too close to the concrete surface it
will lead to concrete spalling. Corrosion of the steel creates forces that break and
dislodge the concrete. Often you can see rust stains in the surface cracks before
spalling occurs. The spalling can be temporarily patched with asphalt. Permanent
repairs are difficult and usually involve replacing the steel and making a partial
depth or full depth concrete repair. Refer Figure 2-13.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Figure 2-13
Example of Shallow Reinforcing
Reinforcing bar exposed. Shallow concrete cover caused large
spall to develop around it.
Surface stain parallel to joint.
Indicates reinforcing steel too close
to surface.
Wire reinforcing mesh placed close to surface. Corrosion of the
reinforcing steel causes the surface mortar to spall. Very difficult to
patch and repair.
Spalling
Spalling is the loss of a piece of the concrete pavement from the surface or along
the edges of cracks and joints. Cracking or freeze-thaw action may break the
concrete loose, or spalling may be caused by poor quality materials. Spalling may
be limited to small pieces in isolated areas or be quite deep and extensive. Refer
Figure 2-14.
Repair will depend on the cause. Small spalled areas are often patched. Spalling
at joints may require full depth joint repair.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Figure 2-14
2.4.2
Example of Spalling
Small surface spalls that have been patched.
Spalling along longitudinal joints.
A severely spalled crack
Spalling over shallow reinforcing.
Joints
Longitudinal Joints
Longitudinal paving joints are constructed to be narrow in width and usually well
sealed. As pavements age and materials deteriorate, joints may open and further
deteriorate. Cracks parallel to the initial joint may develop and accelerate into
spalling or raveling of the longitudinal joint. Settlement, instability, or pumping of
the subgrade soil can cause longitudinal joints to fault. One common cause of
cracks parallel to the longitudinal joints is waiting too long after the pour to saw
the joint. Then, during initial cure the slab will crack roughly parallel (but not
exactly) to the sawn joint. Figure 2-15.
Maintaining a tight joint seal can prevent intrusion of water and reduce freezethaw damage and pumping. Severe joint deterioration may require full depth
patching and replacement of the joint.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Figure 2-15
Example of Longitudinal Joints
New, well-sealed longitudinal joint.
Joint open about ½”.
Faulted longitudinal joint (over 1⁄2”) with slight edge spalling.
Additional joint cracking, spalling, and
broken pavement. Full depth repair is
needed.
Transverse Joints
Transverse joints are constructed in concrete pavements to permit move- ment
of the concrete slabs. Some joints are constructed with load transfer dowels. If
the pavement has poor subsurface drainage, traffic may eventually create voids
under the joints due to pumping and cause the slabs to settle or fault. Freezethaw deterioration at the joint can cause spalling and create additional cracks
parallel to the joint. Load transfer bars may corrode, creating expansive forces
that further deteriorate the concrete at the joint. Refer Figure 2-16.
Occasionally, severe joint deterioration may develop from poor quality aggregate
and so-called D-cracking. Joint sealing will help, but complete replacement is
usually necessary.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Figure 2-16
Example of Transverse Joints
New, well-sealed transverse joint.
Severe spalling of a transverse joint.
Transverse joint has slight faulting and spalling.
Cracks parallel to joint. Dark color next to
transverse joint likely indicates D-cracking and
additional deterioration. Full depth required.
Severe spalling has required temporary patching. Complete joint replacement necessary.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Overall, lack of joint maintenance and rehabilitation is a common problem.
Maintaining a tight, well sealed joint can reduce water intrusion and thereby
reduce freeze-thaw damage, pumping, blow-ups, D- cracking, and spalling. Early
repair of minor defects can often reduce the need for complete joint repair or
replacement.
2.4.3
Pavement Cracks
Transverse Slab Cracks
Transverse cracks may appear parallel to joints and can be caused by thermal
stresses, poor subgrade support, or heavy loadings. They are sometimes related
to slabs having joints spaced too widely. Joints spaced more than 3.5 m apart
commonly develop mid-slab transverse cracks. Refer Figure 2-17.
As with joints, these cracks may deteriorate further if not sealed well. Slabs can
fault at cracks which can spall and develop additional parallel cracking. Severe
deterioration may require patching individual cracks. Multiple transverse cracks
in individual slabs indicate further deterioration. Extensive transverse cracking
indicates pavement failure and the need for complete replacement.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Figure 2-17
Example of Transverse Slab Cracks
Transverse open crack.
Faulted transverse crack
with slight edge spalling
Closely spaced, hairline
transverse cracks. Indicates
slab is broken and in need
of replacement.
D-Cracks
Occasionally, severe deterioration may develop from poor quality aggregate. So
called D-cracking develops when the aggregate is able to absorb moisture. This
causes the aggregate to break apart under freeze-thaw action which leads to
deterioration. Usually, it starts at the bottom of the slab and moves upward. Refer
Figure 2-18.
Fine cracking and a dark discoloration adjacent to the joint often indicate a Dcracking problem. Once this is visible on the surface the pavement material is
usually severely deteriorated and complete replacement is required.
Joint or crack sealing helps slow D-cracking deterioration. This is a serious defect
because it may indicate a material quality problem throughout the pavement.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Figure 2-18
Example of D-Cracks
Surface discoloration near joints and cracks indicates D-cracking and severe slab deterioration
Multiple crack patterns adjacent to joint. Common D-cracking pattern.
Corner Cracks
Diagonal cracks near the corner of a concrete slab may develop, forming a
triangle with a longitudinal and transverse joint. Usually these cracks are within
one foot of the corner of the slab. They are caused by insufficient soil support or
concentrated stress due to temperature related slab movement. The corner
breaks under traffic loading. They may begin as hairline cracks. Refer Figure 219.
Some corner cracks extend the full depth of the slab while others start at the
surface and angle down toward the joint. With further deterioration, more
cracking develops; eventually the entire broken area may come loose. This may
be a localized failure or may point to widespread maintenance problems.
Partial or full depth concrete patching or full depth joint replacement may be
necessary when corner cracking is extensive.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Figure 2-19
Example of Corner Cracks
Corner cracking with broken concrete pieces.
First signs of corner cracking.
Severely spalled corner crack with missing pieces and
patching.
Meander Cracks
Some pavement cracks appear to wander randomly. They may cross a slab
diagonally or meander like a serpent. Meander cracks may be caused by
settlement due to unstable subsoil or drainage problems, or by utility trench
settlement. Frost heave and spring thaw can also cause them. They are often
local in nature and may not indicate general pavement problems. Figure 2-20.
Minor cracks may benefit from sealing to minimize water intrusion. Extensive or
severe meander cracking may require replacing the slab, stabilizing the
subsurface, or improving drainage.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Figure 2-20
Example of Meander Cracks
Meander crack roughly
parallel to longitudinal
joint.
Meander crack caused
by settlement. Lack of
maintenance allows
water to intrude and
debris to collect in crack.
Faulting and spalling of a
meander crack.
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2.4.4
Pavement Deformation
Blowups
Concrete slabs may push up or be crushed at a transverse joint. The cause is
expansion of the concrete where incompressible materials (such as sand) have
infiltrated into poorly sealed joints. As a result, there is no space to accommodate
expansion. It is more common in older pavements with long joint spacing.
Pressure relief joints can be installed and blowup areas must be patched or
reconstructed. Refer Figure 2-21.
Figure 2-21
Example of Blowups
Internal pressure has partially raised slab at the joint. Complete replacement is required.
A pavement blowup in progress. Concrete is crushed and slabs buckled.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Faulting
Joints and cracks may fault or develop a step between adjacent slabs. Faulting is
caused by pumping of subgrade soils and creation of voids. Heavy truck or bus
traffic can rapidly accelerate faulting. Longitudinal joints may fault due to
settlement of an adjacent slab. Refer Figure 2-22.
Faulting creates a poor ride and may cause slab deterioration. Minor faulting can
be corrected by surface grinding. Voids can be subsealed, or slabs mud jacked
back to level position. Severe cases may need joint replacement.
Figure 2-22
Example of Faulting
Faulted longitudinal joint.
Severely faulted joint. Slab jacking is necessary.
Minor faulting of transverse joints. Aggravated by heavy traffic. Surface
grinding will improve ride
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Pavement Settling or Heave
Unstable or poorly drained subgrade soils may cause pavements to settle after
construction. Poorly compacted utility trenches may also settle. This may be a
gentle swale or a fairly severe dip. Refer Figure 2-23.
Frost-susceptible soils and high water tables can cause pavements to heave
during the winter months. Extensive pavement cracking and loss of strength
during the spring can result in severe deterioration. Improved drainage and
stabilization of subgrade soils are usually necessary, along with pavement
reconstruction.
Figure 2-23
Example of Pavement Settling or Heave
Settlement caused meander crack with faulting.
Extensive cracking and patching caused by settlement. Pavement was built on unstable subgrade soils.
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Utility Repairs, Patches and Potholes
Replacement or repair of utilities will require cuts or utility openings. When
repaired these pavement patches may show settlement, joint deterioration, or
distress under continued traffic loading. Patches from previous repairs may perform like original pavement or experience joint deterioration or settlement.
Localized failures of materials or sub- grade soil can cause individual potholes.
Surface spalling or other material defects may develop into localized potholes.
Full depth patching is usually required. Refer Figure 2-24.
Figure 2-24
Example of Utility Repairs, Patches and Potholes
Utility repair or full depth joint repair. Very good
condition.
Asphalt patches. Poor (top) and fair (bottom)
condition.
Potholes caused by severe joint deterioration. Need
repair.
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Manhole and Inlet Cracks
Normal pavement movement due to frost heaving and movements due to
changes in temperature often cannot be accommodated in the pavement adjacent
to a manhole or a storm sewer inlet. Cracks and faulting may develop and the
concrete slab may deteriorate further. These are often localized defects that may
not indicate a general pavement problem. Sealing and patching may slow the
deterioration. Eventually full depth repairs may be required. Refer Figure 2-25.
Figure 2-25
Example of Manhole and Inlet Cracks
Two spalls at manhole in a new pavement. Partial depth patching would be beneficial.
Extensive cracking and spalling at manhole requiring full depth repairs.
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Curb or Shoulder Deformation
Concrete curb and gutter, or paved concrete shoulders, may separate from or
settle along the main pavement. The longitudinal joints between the pavement
and curb or shoulder may open, fault, or deteriorate like other longitudinal joints.
When severe enough to disrupt drainage, the curb and gutter need to be
replaced. Shoulder deterioration may require patching or replacement. Refer
Figure 2-26.
Figure 2-26
Example of Curb or Shoulder Deformation
Extensive curb deterioration. Free-thaw damage to curb adjacent to inlet, and gutter is displaced.
New curb and gutter are needed.
Settled gutter and joint filled with debris. Joint maintenance is needed.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Concrete Roads
Rough surface, poor joints and scaled surfaces would indicate poor or weak
concrete, while potholes, cracking and pumping may indicate localized areas of
poor concrete or inadequate subgrade compaction and drainage.
Asphalt Roads
A rough irregular surface with a considerable amount of patching and/or
potholes indicates generally inadequate pavement strength for the traffic, and,
because of its roughness, is unsuitable for evaluation by Benkelman Beam.
Longitudinal and transverse cracking and depressions generally indicate
subgrade or sub-base failure, necessitating complete replacement of the
pavement structure, possible caused by inadequate drainage, compaction or poor
materials.
Surface reveled and edge erosion of the asphalt generally indicates more of a
drainage problem than a pavement problem.
2.5
Drainage Recommendations
Water is often the cause, whether directly or indirectly, of accelerated highway
deterioration. Drainage facilities therefore, should be given the same careful
consideration as required for pavement and other road facilities.
The prime objective of highway drainage is to maintain all parts of the highway in
a competent drainage condition. This is an essential prerequisite to the
prevention of highway deterioration caused by surface water infiltrating into
various parts of the highway from the road surface and neighboring areas or by
groundwater infiltrating from the adjoining area or rising from the ground water.
Prevention of scouring or erosion of slopes due to storm water requires
maintenance of a satisfactory drainage condition.
Another objective of highway drainage is to prevent traffic congestion and slip
accidents caused by the flooding of water on a road surface.
It is desirable that rainfall of any intensity be drained thoroughly because it is the
main source of water causing highway destruction. However, this is neither
practicable nor recommended because of the huge capital input involved. In the
determination of the drainage capacity to maintain roads in excellent condition
and assure the safety of road traffic, careful consideration should be given
according to the importance of the road as well as to the physical and socioeconomic conditions of the adjoining areas. Determination of the capacity of
individual drainage facilities should be preceded by the same careful
consideration given to the objective of the drainage plan, location of drainage
structures, severity of damage likely to be incurred when the drainage exceeds
the design flood discharge, and the economic justifiability of the whole plan.
Drainage may be composed of mechanical drainage with the use of pumps and
other mechanical means, and gravity drainage.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
2.5.1
Classification of Highway Drainage
Highway drainage is divided into four parts, namely, surface drainage, subsurface
drainage, slope drainage and drainage of the structures.
Surface Drainage
Surface drainage means the drainage of surface water produced by rainfall on a
roadway and from areas other than the right-of-way.
For the purpose of satisfactory surface drainage a larger cross slope is required if
the road surface has irregularities or the wearing surface has a large
permeability. In normal cases however, it is 1.5% or 2.5% for cement concrete or
asphalt concrete pavement and 3% to 4% for gravel roads.
The longitudinal slope not only exerts a large influence on the time of
concentration of storm water, but it also affects the percentage of inflow into
inlets used for drainage of storm water from the road surface. Increase of the
longitudinal slope results in the reduction of the time of concentration so that it
inevitably calls for a larger design rainfall intensity and augmentation of the scale
of drainage structures. If increased to excess, storm water is prone to flow on the
road surface in the longitudinal direction so that care must be taken of the size
and layout of the side ditch and inlets. The recommended minimum longitudinal
slope is 0.50%.
The HCM defines the quality of traffic service provided by specific highway
facilities under specific traffic demands by means of a level of service. The level of
service characterizes the operating conditions on the facility in terms of traffic
performance measures related to speed and travel time, freedom to maneuver,
traffic interruptions, and comfort and convenience. The levels of service range
from level-of-service A (least congested) to level-of-service F (most congested).
Table 2-1 shows general definitions of these levels of service. The specific
definitions of level of service differ by facility type.
Table 2-1
General Definitions of Levels of Service
Level of Service
General Operating Conditions
A
Free Flow
B
Reasonably Flow
C
Stable Flow
D
Approaching unstable flow
E
Unstable flow
F
Forced or breakdown flow
Source: AASHTO, A Policy on Geometric Design of Highway and Streets 2011, 6th Edition. Used by
Permission.
For urban areas the recommended minimum longitudinal slope is 0.35% for
curbed pavements. The maximum spacing for curb inlet and manhole is 20 m.
The following rule-based interval and offset are used when adding pavement
drainage features:
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Inlet
-
Manhole -
Placed only if the selected road uses a road style with a curb.
inlet are placed at low point along the road curb, spaced from
high point based on the maximum inlet spacing defined on the
Pavement Drainage Design Standard.
Placed at inlet location at offset based on the horizontal and
longitudinal placement offset defined on the Pavement Drainage
Design Standard.
For longitudinal slope greater than 4%, the corresponding side ditch/canal and
shoulder should be paved on both sides so as to prevent the scouring effect of the
increased water velocity.
Subsurface Drainage
Subsurface drainage is intended to reduce the groundwater level and to intercept
and drain water infiltrating from the adjoining areas and road surface or rising
from the subgrade.
Drainage is a much more critical item in asphalt than in concrete roads. An
asphalt mixture which has been submerged under water, although compacted to
the required degree, can loose from 50 to 75% of its dry strength. Therefore, as
much as possible, roads which are subject to inundation should not be asphalted.
Whenever practicable the water table should be prevented from rising to within
0.60 m below the bottom of the sub-base. This may be done by subsoil drainage
or by raising the embankment.
It is important to provide efficient permanent drainage to remove water from the
subgrade and sub-base, both during and after construction. Water-proofing the
subgrade or sub-base during construction by sealing the entry of water may also
be desirable.
Where high grades are impractical, subgrade drains are used to lower ground
water tables. An effective subgrade drain must be placed so that the groundwater
level is lowered beyond the capillary range since capillary water cannot be
effectively drained.
Where wet spots are encountered in the subgrade due to seepage through
permeable strata underlain by an impervious material, intercepting drains are
used. The backfill placed around and above pipe under drains should be opengraded enough to permit rapid flow, but pores should not be large enough to be
infiltrated by adjacent soils.
Slope Drainage
Slope drainage is constructed to protect slopes from erosion or stability decline
which is caused by surface water on the cuts, fills and natural slopes, or by
ground water oozing to the slope surface.
Where erodible velocities will occur along a highway embankment, particularly a
new one, the slope should be protected by either structural or non-structural
methods as appropriate – refer to Section 7.6. Particular care against scouring
should be given to slope protection where the highway embankment is along the
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
outside bend of a stream, especially if the stream originally crossed the highway
location at that point.
DPWH D.O. No. 41 – Directing all Regional Directors, District and Project
Engineer to prioritize in the design consideration the usage of coconut bioengineering products/solution in all DPWH projects and activities, especially
those projects involving slope stabilization, soil conditioning, soil erosion
prevention and a hydro seeding.
Drainage of Structures
Drainage of structures is for the purpose of removing stored water from the
backfill of structures and surface runoff on bridges caused by rainfall.
For the design of the facilities for surface or slope drainage of structures such as
retaining walls, data on groundwater level, groundwater movement, spring water
condition, location of permeable layer and its permeability coefficient, and the
depth of impermeable layer should be obtained.
2.6
Design Controls
Since topography and land use have a pronounced effect on highway location,
geometrics and determination of the type of highway, information regarding
these factors should be obtained in the early stages of planning and design. This
information together with traffic and vehicle data, plus pedestrian flow data,
form the major controls for highway design.
2.6.1
Anticipated Traffic Volume
The design of a highway or any part thereof should consider jointly all data
relating to traffic such as traffic volume, character of traffic and axle loading.
Financing, quality of foundations, availability of materials, cost of right-of-way,
and other factors have important bearing on the design. However, traffic volume
indicates the service for which the improvement is being made and directly
affects the geometric features of design such as width, alignment, grades, etc. It is
no more rational to design a highway without traffic information than it is to
design a bridge without the knowledge of weights and numbers of vehicles it is
intended to support. Traffic information serves to establish the ‘loads’ for
geometric highway design.
A road should be designed so that it will accommodate or can be readily changed
to accommodate the number of vehicles which is estimated to pass it towards the
end of its life. This number is called the design volume. In estimating the design
volume, the minimum life is commonly assumed to be 10 to 15 years for a flexible
pavement, and 20 years for a rigid pavement. Traffic volumes are usually the
annual average daily traffic (AADT), though at critical points on a road, such as
intersections, peak traffic figures are also taken into account. The number of
vehicles using a road in a given time determines the number of traffic lanes
required and indicates whether there is a need for auxiliary lanes for slow speed
traffic and or whether speed change lanes are required at intersections.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
The design hourly volume (DHV) should be representative of the future year
chosen for design. It should be predicated on current traffic (existing and
attracted) plus all traffic increases (normal traffic growth, generated traffic and
development traffic) that would occur during the period between the current and
the future year chosen for design. A period of 20 years is widely used as a basis
for design, for which the usual traffic increase on a highway improvement is in
the range of 50 to 150%. Where the highway is to be a expressway, traffic
increase is likely to be higher, in the range of 80 to 200%.
On minor, low volume roads, average daily traffic (ADT) normally is sufficient. On
most highways a DHV equal to the 30th highest hourly volume (abbreviated as ‘30
HV’) is usually used for design. On highways with unusual or highly seasonal
fluctuation in traffic flow, it may be necessary to use a design hourly volume
other than the 30 HV.
The design traffic data should include the following elements:

ADT – current average daily traffic, year specified.

ADT – future average daily traffic, year specified.

DHV – future design hourly volume, two-way unless otherwise specified
(DHV usually equals 30 HV).

K – Ratio of DHV to ADT; generally 12% to 18% for Rural and 8 to 12% for
Urban.

D – Directional distribution of DHV, one-way volume in predominant
direction of travel expressed as percentage of total. D normally varies from
about 50 to 80% of two-way DHV, with an average of 67%.

T – Trucks, exclusive of light delivery trucks, expressed as a percentage of
DHV. As an average on main rural highways, T is 7 to 9% of DHV and 13% of
ADT; where weekend peaks govern, the average may be 5% to 8% of DHV.
For important intersections, data should be obtained to show simultaneous
traffic movement during both the morning and evening peak hours.
2.6.2
Character of Traffic
All roads should be designed to accommodate trucks, buses, passenger vehicles,
handcarts, cyclists and pedestrians with safety and convenience. A thorough
knowledge of the design vehicle’s weight, dimensions, mobility and other
characteristics is essential for good design.
The vehicle which should be used in design for normal operation is the largest
one which represents a significant percentage of the traffic for the design year.
For design of most highways accommodating truck traffic, one of the design
semitrailer combinations should be used – refer to Table 1-1 and Table 1-2. A
design check should be made for the largest vehicle expected to ensure that such
a vehicle can negotiate the designated turns, particularly if pavements are
curbed. This is done using a swept path analysis using either turning circle
templates or software.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Knowing the predominant character of traffic to use the highway, the required
width of lane could be determined. The total width of a highway is the sum of the
widths of traffic lanes required, dividing islands, curbs and gutter, shoulders
and/or walkways, ditches or gutters, drains and other special features.
2.6.3
Design Speed
The design speed is the speed determined for design and correlation of the
physical features of a highway that influence vehicle operation. It is the maximum
safe speed that can be maintained over a specified section of the highway when
conditions are so favorable that that the design features of the highway govern.
The choice of design is influenced principally by the character of terrain, the
extent of man-made features and economic considerations. Once selected, it sets
the limits for curvature, sight distance and other geometric features. In the design
of a substantial length of highway it is desirable, although it may not be feasible,
to assume a constant design speed on certain sections. Changes in terrain and
other physical controls may dictate a change in design speed on certain sections.
If so, the introduction of a lower or higher design speed should not be effected
abruptly but over a sufficient distance to permit drivers to change speed
gradually before reaching the section of highway with the different design speed.
When available funds are limited, it is impractical to reduce design speed just to
save construction cost; rather the savings should be on other features.
2.6.4
Design Traffic (Vehicles)
The operating characteristics of motor vehicles should be considered in
analyzing a facility. The major considerations are vehicle types and dimensions,
turning radii and off-tracking, resistance to motion, power requirements,
acceleration performance, and deceleration performance. Motor vehicles include
passenger cars, trucks, vans, buses, recreational vehicles, and motorcycles. These
vehicles have unique weight, length, size, and operational characteristics. The
forces that must be overcome by motor vehicles if they are to move are rolling,
air, grade, curve, and inertial resistance. The weight/power ratios are useful for
indicating the overall performance in overcoming these forces.
2.6.5
Highway Capacity
Roadway conditions include geometric and other elements. In some cases, these
influence the capacity of a road; in others, they can affect a performance measure
such as speed, but not the capacity or maximum flow rate of the facility.
Roadway factors include the following:

Number of lanes

The type of facility and its development environment

Lane widths

Shoulder widths and lateral clearances

Design speed
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design

Horizontal and vertical alignments

Axle loads

Availability of exclusive turn lanes at intersections
The horizontal and vertical alignment of a highway depends on the design speed
and the topography of the land on which it is constructed.
In general, the severity of the terrain reduces capacity and service flow rates.
This is significant for two-lane rural highways, where the severity of terrain not
only can affect the operating capabilities of individual vehicles in the traffic
stream, but also can restrict opportunities for passing slow-moving vehicles.
2.6.6
Classification of Highway
Certain characteristics distinguish multilane suburban and rural highways from
expressways. For example, vehicles may enter or leave multilane highways at
intersections and driveways, and they can encounter traffic signals.
Design standards for multilane highways tend to be lower than those for
expressways, although a multilane highway approaches expressway conditions
as its access points and turning volumes approach zero. Moreover, the visual
setting and the developed frontage along multilane highways have a greater
impact on drivers than they do along expressways.
The multilane highway is similar to urban streets in many respects, although it
lacks the regularity of traffic signals and tends to have greater control on the
number of access points per kilometer. Also, its design standards are generally
higher than those for urban streets. The speed limits on multilane highways are
often 10 to 20 kph higher than speed limits on urban streets. Pedestrian activity,
as well as parking, is minimal, unlike on urban streets.
Multilane highways differ substantially from two-lane highways, principally
because a driver on a multilane highway is able to pass slower-moving vehicles
without using lanes designated for oncoming traffic. Multilane highways also
tend to be located near urban areas and often connect urban areas; they usually
have better design features than two-lane highways, including horizontal and
vertical curvature.
2.6.7
Accident Information
On all proposed projects, the accident history should be analyzed and potentially
hazardous features and locations identified to determine appropriate safety
enhancement. A study of accidents by location, type, severity, contributing
circumstances, environmental conditions, and time periods may suggest possible
safety deficiencies.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
2.7
Requirements for Speedy Plan Preparation
2.7.1
Plans
The final horizontal alignment shall be plotted on a scale of 1:1000 m, and the
following items shall be shown on the plan:
2.7.2

Plans shall show the centerline of the project road, the width of the roadway
and shoulders and the right-of-way.

Azimuth, distance, elements of curve, coordinates, superelevation and
widening of every curve, and design speed shall be specified.

Each sheet shall have a north arrow indicator and lines representing the
coordinates.

Contours shall be plotted at 1.00 m intervals, however if contour lines are too
close together an interval of 5.00 m may be used. The minimum extent of
contour line should be within the Road Right-of-Way.

Elevation of bench marks with accurate descriptions, reference points and
controlling points with azimuth and distance shall be shown.

Information and data shall be provided regarding existing roads,
intersections and railways, existing rivers and waterways, existing houses
and structures, public utilities, land classifications and others.

All existing and proposed structures, such as bridges, box culverts, pipes and
other drainage, slope protection structures, traffic signs, road markings,
safety barrier and lighting columns shall be indicated – which may involve a
number of plans for clarity.

Include typical roadway section.

Existing Road Right of Way limit.
Profile
Longitudinal profile of existing ground and finished grade lines shall be plotted
on a scale of 1:1000 m horizontal and 1:100 m vertical. For mountainous areas a
scale of 1:200 for vertical may be used. The following items shall be shown on the
longitudinal profiles:

Elements of every vertical parabolic curve.

The percent grades indicated by a plus (+) for ascending and minus (-) for
descending.

The finished grade and existing ground elevations for every full station.

The station number in kilometer including invert elevations and a description
of all existing and proposed structures, such as bridges, box culverts and
pipes.

The maximum flood elevation in flooded areas and ordinary and highest
water elevations of river, creek and canals.

Side ditch profile indicating the gradient, invert elevations and outfall.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design

2.7.3
Superelevation and widening diagrams.
Detailed Cross Section
Cross sections at every 20 m full station, at intermediate breaks on the ground
and at bridge approaches and drainage structures shall be plotted on a scale of
1:100 m horizontal and vertical. The following shall be indicated on the cross
section drawings:
2.7.4

Existing ground profile and template roadway section.

The general manner of treating slopes in cut and fills, including warping and
rounding.

The manner of superelevating and widening in curves.

Coordinates of the existing ground and template roadway section.

Finished grade and natural ground elevations of roadway centerline.

Area of cut and fill, and quantities of all other involved items of work.

Drainage structures including side ditches.

Slope protection.
Detailed Drainage Cross Section
Cross section of every pipe and box culverts and plotted on a scale of 1:100 m.
horizontal and vertical. The following shall be indicated on the cross section
drawings:
2.7.5

Existing ground profile and template roadway section.

General manner of treating slopes in fill, including warping and sounding.

Manner of superelevation and widening in curves.

Coordinates of the existing ground and template roadway.

Finished grade and natural ground and template roadway centerline.

Drainage structures including wingwall and apron.

Quantities involved for drainage and other corresponding structures.

Maximum flood level.
Geotechnical Drawings
The geotechnical data in these drawings shall include the complete soil survey
data for the project, the approved sources of borrow, aggregate, sub-base,
aggregate base, concrete aggregates and asphalt aggregates.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
3
Geometric Design of Road Links
3.1
Introduction
Standards have been developed as a guide in the design of highways and to
ensure that motorist’s convenience, environmental safety, and aesthetic
considerations are implemented in the most economical manner consistent with
highway service considerations.
3.1.1
Departure from Standards
Design policies and standards generally represent minimum values. Higher
standards may be used within reasonable economic limits. To ensure uniform
practice, lower design standards may not be used without approval from the
DPWH Office of the Secretary or the Bureau of Design.
3.2
Requirements for Design Analysis in Operating Environment
Highway design involves geometrically combining the elements which establish
the road layout, that is, horizontal and vertical alignment, width of pavement and
formation, cross slopes, etc., to ensure that the finished road will be an
appropriate component of the traffic system. The system must be satisfactory for
all relevant environmental conditions so that the highway could provide
maximum service to the public with minimum hazard at reasonable cost.
It is necessary that it be constructed to endure and to provide adequate safe
passage of vehicles. To achieve this objective the design must adopt certain
criteria or standards for strength and uniformity.
These criteria and standards are subject to modifications since roads are
intimately associated with environmental conditions, vehicles, human factors and
economic considerations which seldom conform to mathematical concepts.
Table 3-1 and Table 3-2 contain the minimum design standards for Philippine
highways. Standard cross-sections ae provided in Figure 3-1 to Figure 3-6.
Table 3-3, Table 3-4 and Table 3-5 provide AASHTO minimum recommended
widths of traveled way and shoulders for local rural roads, rural collector roads,
and rural arterials.
The width of an urban collector street will be the sum of the widths of lanes for
moving traffic as determined from a capacity analysis, plus space for parking and
bicycles, and including the median width where applicable. Lanes within the
traveled way should range in width from 3.0 to 3.6 m.
Expressways should have a minimum of two 3.6 m wide through-traffic lanes for
each direction of travel. Paved shoulders should be continuous on both the right
and left sides of all expressway facilities.
3-1
30
Mountainous Topography
40
Mountainous Topography
0.08 (max.)
1.50
8.0
6.0
5.0
GRADE (percent)
50
30
2.00
6.10
6.0
5.0
3.0
80
220
280
50
80
90
Gravel, crushed gravel or crushed
stone, bitumen preservative
treatment, single or double
bituminous surface treatment,
270
350
490
40
60
90
60
115
135
350
560
615
Bituminous dense or open graded
plant mix surface course, bituminous
concrete surface course
270
420
490
PASSING SIGHT DISTANCE (meters)
40
70
90
NON PASSING SIGHT DISTANCE (meters)
30
1.0
5.5 – 6.0
9.0
7.0
6.0
50
120
160
RADIUS (meters)
40
60
70
DESIGN SPEED (kph)
3-2
420
560
645
70
115
150
30
3.00
6.70
6.0
5.0
3.0
120
220
320
60
80
95
Desirable
Bituminous concrete surface course
360
420
560
60
70
115
0.08 (max.)
30
2.50
7.0
5.0
4.0
80
120
220
50
60
80
Minimum
1000 – 2000
Desirable
400 – 1000
Minimum
PRIMARY
SECONDARY
Source: Table 3.2 in Ministry of Public Works and Highways, 1984, Design Guidelines Criteria and Standards Volume II, Bureau of Design, Manila.
Type of Surfacing
190
40
Rolling Topography
Mountainous Topography
70
Flat Topography
270
0.08 (max.)
Superelevation (meters/meter)
Rolling Topography
20
Right of Way Width (meters)
420
0.5
Shoulder Width (meters)
Flat Topography
4.0
8.0
Rolling Topography
10.0
6.0
Flat Topography
Pavement Width (meters)
30
Mountainous Topography
85
160
40
50
70
200-400
BARANGAY
Mountainous Topography
55
Rolling Topography
120
40
Rolling Topography
Flat Topography
60
Under 200
FMR
Minimum Design Standards for Philippine Highways – excluding Tourism Roads
Flat Topography
Average Daily Traffic (ADT)
Table 3-1
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
490
615
675
90
135
160
60
3.00
7.30
5.0
4.0
3.0
160
280
350
70
90
100
Desirable
Bituminous concrete surface course,
Portland cement concrete pavement
420
490
615
70
90
135
0.08 (max.)
6.70
7.0
5.0
4.0
100
160
260
60
70
90
Minimum
More Than 2000
EXPRESSWAY
230 mm for Portland Cement Concrete Pavement (PCCP)
Pavement Thickness
1.5% for Portland Cement Concrete Pavement (PCCP)/ 2% ACP
Minimum of 50 m
Minimum length of 30m
Minimum length of 60 m
Minimum of 60 kph, 40 kph and 30 kph for flat, rolling and mountainous terrain respectively
Minimum of 0.50% on cut section and maximum of 12%
Cut slope of 1.5:1 to 1:1 for common materials
Cut slope of 0.5:1 to 1:1 for soft/rippable rock
Cut slope of 0.25:1 for hard/solid rock or natural break
Box culvert: 25-year flood with sufficient freeboard to contain the 50-year flood
Pipe culvert: 15-year flood with sufficient freeboard to contain the 25-year flood; minimum size of 910 mm in diameter
As needed
Refer to DPWH Highway Safety Design Standards, Part 2 (May 2012)
910 Ø RCP
Roadway Cross Slope
Radius of Horizontal Curve
Length of Tangent between Point of Curvature (PC) and Point of Tangency
(PT) of reverse curve
Length of Vertical Curve
Design Speed
Longitudinal Grade
Side Cut Slope Ratio (H:V)
Road Drainage
Slope protection
Road Safety Devices including Pavement Marking
Minimum lateral drainage structure
Source: Department of Public Works and Highways, Department Order No. 11 series of 2014, Department Order No. 46 dated 25 June 2012, Manila.
Minimum of 1.5 m
Minimum gravel surfacing
Shoulder
Width
Material
50 mm for Asphalt Concrete Pavement (ACP)
Minimum of 6.1 m for two lanes
Pavement Width
Requirement
Portland Cement Concrete Pavement (PCCP) or Asphalt Concrete Pavement (ACP)
Minimum Design Standards for Tourism Roads
Pavement Type
Design Element
Table 3-2
3-3
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
3-4
Figure 3-1
Superelevated Sections
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Figure 3-2
Low Type Surfacing on Gravel Road
3-5
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
3-6
Figure 3-3
Farm to Market Road
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Figure 3-4
Intermediate Type Surfacing for Plant Mix Surface Course
3-7
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
3-8
Figure 3-5
High Type Surfacing Asphalt Pavement
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Figure 3-6
High Type Surfacing Concrete Pavement
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Table 3-3
Design Speed
(kph)
20
AASHTO Recommended Minimum Width of Traveled Way and Shoulders for
Local Rural Roads
Minimum Width of Traveled Way (m) for Specified Design Volume (vehicles per day)
Under 400
5.4
c
Over 2000
6.0
6.0
6.6
a
6.6
7.2 b
5.4
6.0
40
5.4
6.0 a
6.6
7.2 b
50
5.4
6.0 a
6.6
7.2 b
60
5.4
6.0 a
6.6
7.2 b
70
6.0
6.6
6.6
7.2 b
80
6.0
6.6
6.6
7.2 b
90
6.6
6.6
7.2 b
7.2 b
100
6.6
6.6
7.2 b
7.2 b
Width of graded shoulder on each side of the road (m)
1.5 a,c
0.6
b
1500 to 2000
a
30
All speeds
a
400 to 1500
1.8
2.4
For roads in mountainous terrain with design volume of 400 to 600 vehicles per day, use 5.4 m traveled
way width and 0.6 m shoulder width.
Where the width of the traveled way is shown as 7.2 m, the width may remain at 6.6 m on reconstructed
highways where there is no crash pattern suggesting the need for widening.
May be adjusted to achieve a minimum roadway width of 9 m for design speeds greater than 60 kph.
Source: Table 5-5 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6th Edition.
Used by Permission.
Table 3-4
Design Speed
(kph)
AASHTO Recommended Minimum Width of Traveled Way and Shoulders for
Rural Collector Roads
Minimum Width of Traveled Way (m) for Specified Design Volume (vehicles per daya)
Under 400
400 to 1500
1500 to 2000
Over 2000
30
6.0 b
6.0
6.6
7.2
40
6.0
b
6.0
6.6
7.2
6.0
b
6.0
6.6
7.2
6.0
b
6.6
6.6
7.2
50
60
70
6.0
6.6
6.6
7.2
80
6.0
6.6
6.6
7.2
90
6.6
6.6
7.2
7.2
100
6.6
6.6
7.2
7.2
All speeds
Width of graded shoulder on each side of the road (m)
0.6
1.5 c
1.8
2.4
a
On roadways to be reconstructed, a 6.6 m traveled way may be retained where the alignment is
satisfactory and there is no crash pattern suggesting the need for widening.
b
A 4.5 m minimum width may be used for roadways with design volumes under 250 vehicles per day.
c
Shoulder width may be reduced for design speeds greater than 50 kph provided that a minimum roadway
width of 9 m is maintained.
Source: Table 6-5 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6th Edition.
Used by Permission.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Table 3-5
AASHTO Recommended Minimum Width of Traveled Way and Shoulders for
Rural Arterial Roads
Design
Speed (kph)
Minimum Width of Traveled Way (m)a for Specified Design Volume (vehicles per day)
Under 400
Over 2000
6.6
6.6
6.6
7.2
70
6.6
6.6
6.6
7.2
80
6.6
6.6
7.2
7.2
90
6.6
6.6
7.2
7.2
100
7.2
7.2
7.2
7.2
110
7.2
7.2
7.2
7.2
120
7.2
7.2
7.2
7.2
130
7.2
7.2
7.2
7.2
Width of usable shoulder (m)b
1.2
b
1500 to 2000
60
All speeds
a
400 to 1500
1.8
1.8
2.4
On roadways to be reconstructed, an existing 6.6 m traveled way may be retained where the alignment is
satisfactory and there is no crash pattern suggesting the need for widening.
Preferably, usable shoulder on arterials should be paved; however, where volumes are low or a narrow
section is needed to reduce construction impacts, the paved shoulder width may be a minimum of 0.6 m
provided that bicycle use is not intended to be accommodated on the shoulder.
Source: Table 7-3 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
3.3
General Principles for Geometric Design
In preparing the design of a new highway or a redesign of an old one, the
designer must give attention to the following basic considerations aimed at
functionality, homogeneity and predictability:

The design must be suitable for the traffic volume, both daily and at the
design peak hour, for the design speed and for the character of the vehicles to
use the facility.

The design must be consistent and must avoid surprise changes in alignment,
grade and sight distance.

The design must be pleasing to the user and to those who live along it.

The design must be complete. However, for the designer to be able to ensure
the effectiveness of his design to a large degree, the necessary roadside
treatment and the provision of control devices, such as lane markers and
special signs, are taken into account.

The design shall be as simple as possible from the standpoint of the builder.
Excessive changes in cross sectional design or the use of a variety of types
within a project will in many cases increase the cost and difficulty of
construction beyond the commensurate value of such ‘uniqueness’.

The design should be such that the finished road can be maintained at the
least cost.

The design must be safe for driving and should ensure confidence for
motorists.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Highway design is also sometimes required to consider staged construction,
requiring careful consideration of various aspects including safety; for example,
the safety challenges of designing a dual carriageway road with grade separated
interchanges that are to operate with only one carriageway initially.
3.4
Design Speed
With reference to Section 2.6.3, design speed is not a major factor for local urban
streets and collector streets because their typical grid layout and closely spaced
intersections usually limit vehicular speed. Design speeds ranging from 30 to 50
kph for local streets, and 50 kph or higher for collector streets, are normally used
in design, depending on available right-of-way, terrain, likely pedestrian
presence, adjacent development, and other controls.
Minimum design speeds for Local Rural Roads and Rural Collector Roads are
given in Table 3-6 and Table 3-7. In the Philippines it is important to stress that
where rural roads pass through villages the MAXIMUM design speed should be
that which will reduce the risk to pedestrians to an acceptable level – either 50
kph or 30 kph depending on the volume of pedestrian traffic.
Rural arterials other than expressways should be designed for speeds of 60 to
120 kph, depending on terrain, driver expectancy, and, in the case of
reconstruction projects, the alignment of the existing facility.
Urban arterials should be designed for speeds of 50 to 100 kph, with lower
speeds in business districts and developed areas, and higher speeds in outlying
suburban and developing areas.
Expressways should be designed for speeds not less than 80 kph.
Table 3-6
Type of Terrain
Minimum Recommended Design Speeds for Local Rural Roads
Design Speed (kph) for Specified Design Volume (vehicles per day)
Under 50
50 to 250
250 to 400
400 to
1500
1500 to
2000
2000 and
over
Level
50
50
60
80
80
80
Rolling
30
50
50
60
60
60
Mountainous
30
30
30
50
50
50
Source: Table 5-1 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Table 3-7
Minimum Recommended Design Speeds for Rural Collector Roads
Type of Terrain
Design Speed (kph) for Specified Design Volume (vehicles per day)
0 to400
400 to 2000
Over 2000
Level
60
80
100
Rolling
50
60
80
Mountainous
30
50
60
Note: Where practical, design speeds higher than those shown should be considered.
Source: Table 6-1 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
3.5
Road Classification
With reference to Section 2.6.6, local urban streets are usually designed with a
standard two-lane cross section, but a four-lane cross section may be appropriate
in certain urban areas, as governed by traffic volume, administrative policy, or
other community considerations. It is usually difficult and costly to modify the
geometric design of an existing street unless provision has been made at the time
of initial construction.
Traffic volume is a major factor for streets serving industrial or commercial
areas, and urban collector roads. In this case the projected ADT should be the
design basis. Collector roads and streets should have a sufficient number of lanes
to accommodate design traffic volumes for the desired level of service.
Rural highways usually consist of two-lane local roads, which should be designed
to accommodate the highest practical criteria compatible with traffic and
topography.
New rural arterial roads, or improvement of existing rural arterial roads, require
the determination of the design traffic volume. It is usually appropriate to design
high-volume rural arterials using the 30 HV, which is typically about 15% of the
ADT on rural roads. The number of lanes on an arterial roadway is determined
based on consideration of volume, level of service, and capacity conditions.
Multilane arterials may be undivided or divided, depending upon traffic volume
and safety considerations.
For expressways, specific capacity needs should be determined from directional
design hourly volumes (DDHV) for the appropriate design period. In large
metropolitan areas, the selection of appropriate design traffic volumes and
design periods may be influenced by system planning. Segments of expressway
may be constructed or reconstructed to be commensurate with either
intermediate traffic demands or with traffic based on the completed system,
whichever may be more appropriate. Rural expressways are generally designed
for high-volume and high-speed operation.
3.6
Basic Design Consideration
This section deals with the fundamental considerations in highway design,
including reference to the safety concept of the Clear Zone.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
The term ‘clear zone’ is used to designate the unobstructed, traversable area
provided beyond the edge of the travelled way for the recovery of errant vehicles.
The clear zone includes shoulders, bicycle lanes, and auxiliary lanes unless they
function as a through lane. It has fundamental implications for the overall design,
in that it encourages minimizing the extent of embankment sections to improve
safety, and therefore needs to be kept in mind during basic design. Refer Figure
3-7. Further specific information on clear zones can be found in Section 3.7.4 and
Section 8.1.
Figure 3-7
3.6.1
Example of the ‘Clear Zone’ concept for a 100 kph operating speed
Sight Distance
Ability to see ahead is of the utmost importance in the safe and efficient
operation of the highway. Sight distance in the road design is the distance at
which a driver of a vehicle can see an object of specified height on the road ahead,
assuming adequate sight and visual acuity and clear atmospheric conditions.
The sight distance to be provided should be as great as practicable and be not
less than the distances required for certain selected maneuvers – refer to the
warrants given in the DPWH May 2012 Highway Safety Design Standards.
Designers are encouraged to calculate and report the percentage of road length
where the sight distance is adequate for safe overtaking as a useful design safety
indicator.
Stopping (Non-Passing) Sight Distance
The design stopping sight distance is the minimum distance required for a
vehicle, travelling at the design speed, to stop before reaching an object in its
path. It is the sum of the distance travelled during perception, brake reaction
time, and the distance travelled while breaking to a stop on wet pavements. The
sight distance at every point of a highway should be as long as possible.
Minimum stopping sight distance is the sum of two distances; one, the distance
traversed by a vehicle from the instant the driver sights an object for which a
stop is necessary, to the instant the brakes are applied; and the other, the
distance required to stop the vehicle after the brake application begins.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
The calculated minimum stopping sight distance for various assumed speeds is
developed in Table 3-8, while Table 3-9 provides minimum stopping sight
distances on grades.
For purpose of design, wet conditions govern in determining stopping sight
distances due to the lower coefficients of friction on wet pavements compared to
dry pavements.
Table 3-8
Stopping Sight Distance on Level Roadways
Design Speed
(kph)
Brake Reaction
Distance (m)
Braking Distance
on Level (m)
Stopping Sight Distance
Calculated (m)
Design (m)
20
13.9
4.6
18.5
20
30
20.9
10.3
31.2
35
40
27.8
18.4
46.2
50
50
34.8
28.7
63.5
65
60
41.7
41.3
83.0
85
70
48.7
56.2
104.9
105
80
55.6
73.4
129.0
130
90
62.6
92.9
155.5
160
100
69.5
114.7
184.2
185
110
76.5
138.8
215.3
220
120
83.4
165.2
248.2
250
130
90.4
193.8
284.2
285
Note: Break reaction distance predicated on a time of 2.5 s; deceleration rate of 3.4 m/s 2 used to determine
calculated sight distance.
Source: Table 3-1 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
Table 3-9
Stopping Sight Distance on Grades
Design
Speed (kph)
Downgrades
3%
6%
Upgrades
9%
3%
6%
9%
20
20
20
20
19
18
18
30
32
35
35
31
30
29
40
50
50
53
45
44
43
50
66
70
74
61
59
58
60
87
92
97
80
77
75
70
110
116
124
100
97
93
80
136
144
154
123
118
114
90
164
174
187
148
141
136
100
194
207
223
174
167
160
110
227
243
262
203
194
186
120
263
281
304
234
223
214
130
302
323
350
267
254
243
Source: Table 3-2 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Decision Sight Distance
Decision sight distance is the distance needed for a driver to detect an
unexpected or otherwise difficult-to-perceive information source or condition in
a roadway environment that may be visually cluttered, recognize the condition or
its potential threat, select an appropriate speed and path, and initiate and
complete complex maneuvers. Because decision sight distance offers drivers
additional margin for error and affords sufficient length to maneuver at reduced
speed, its values are substantially greater than stopping sight distance.
The decision sight distances in Table 3-10 may be used to provide values for sight
distances that may be appropriate at critical locations, and serve as criteria in
evaluating the suitability of the available sight distances at these locations. If it is
not practical to provide decision sight distance because of horizontal or vertical
curvature, or if relocation of decision points is not practical, special attention
should be given to the use of suitable traffic control devices for providing
advance warning of the conditions that are likely to be encountered.
Table 3-10
Decision Sight Distance
Design Speed
(kph)
Avoidance Maneuver (m)
A
B
C
D
E
50
70
155
145
170
195
60
95
195
170
205
235
70
115
235
200
235
275
80
140
280
230
270
315
90
170
325
270
315
360
100
200
370
315
355
400
110
235
420
330
380
430
120
265
470
360
415
470
130
305
525
390
450
510
Stop on rural road – t = 3.0s
Stop on urban road – t = 9.1s
Speed/path/direction change on rural road – t varies between 10.2 and 11.2s
Speed/path/direction change on suburban road – t varies between 12.1 and
12.9s
Avoidance Maneuver E:
Speed/path/direction change on urban road – t varies between 14.0 and
14.5s
Source: Table 3-3 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6th Edition.
Used by Permission.
Avoidance Maneuver A:
Avoidance Maneuver B:
Avoidance Maneuver C:
Avoidance Maneuver D:
Passing Sight Distance
Design passing sight distance is the minimum distance required to safely make a
normal passing maneuver on 2-lane highways at passing speeds common to
nearly all drivers, commensurate with design speed.
The minimum passing sight distance for a 2-lane highway is determined as the
sum of four distances:

3-16
Initial maneuver distance is the distance traversed during perception and
reaction time and during the initial acceleration to the point of encroachment
on the left lane.
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design

Distance travelled while the passing vehicle occupies the left lane.

Distance between the passing vehicle at the end of its maneuver and the
opposing vehicle.

Distance traversed by an opposing vehicle for two-thirds of the time the
passing vehicle occupies the left lane, or 2/3 of the distance travelled while
the passing vehicle occupies the left lane.
Appreciable grades increase the sight distance required for safe passing. The
sight distance required to permit a vehicle travelling upgrade to pass with safety
is greater than that required on a level road. This is due to reduced acceleration
of the passing vehicle, which increases the time of passing, and due to the
likelihood of opposing traffic speeding up increasing the distance travelled by it.
For passing to be performed safely on upgrades, the passing sight distance
should be greater than the minimum. The designer should recognize the
desirability of increasing the minimum shown in Table 3-11, which is sufficient
for a single or isolated pass only.
Table 3-11
Passing Sight Distance for Design of Two-Lane Highways
Assumed Speeds (kph)
Design Speed (kph)
Passed
Vehicle
Passing Vehicle
Passing Sight Distance
(m)
30
11
30
120
40
21
40
140
50
31
50
160
60
41
60
180
70
51
70
210
80
61
80
245
90
71
90
280
100
81
100
320
110
91
110
355
120
101
120
395
130
111
130
440
Source: Table 3-4 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6th Edition.
Used by Permission.
Headlight Sight Distance
For night driving on highways without lighting, the length of visible roadway is
that roadway that is directly illuminated by the headlights of the vehicle. For
certain conditions the minimum stopping sight distance values used for design
can exceed the length of visible roadway.
First, vehicle headlights have limitations on the distance over which they can
project the light intensity levels needed for visibility, particularly on low beam.
Second, sight distance is limited where there are horizontal and vertical
alignment curves.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
There is some mitigating effect in that other vehicles provide warning
illumination via their headlights and taillights. Furthermore, drivers are aware
that visibility at night is less than during the day, and they may therefore be more
attentive and alert.
Criteria for Measuring Sight Distance
Sight distance along a highway is measured from the driver’s eye to some object
on the travelled way when it comes into view. Measurement criteria for stopping
sight distance differ from those for passing sight distance. Different elements are
involved when sight distance is controlled by vertical alignment and by
horizontal alignment. In all cases, the height of eye of the driver is the same,
which is 1.15 m above the road surface.
Height of Object – Stopping minimum sight distance is based on the distance
required to stop with safety from the instant a stationary object in the same lane
becomes visible. On crest vertical curves the sight distance is limited by some
point on the roadway surface. On horizontal curves it is limited by a lateral
obstruction beyond the roadway, such as a cut slope, clump of trees, bridge
abutment, etc.
Vertical Control for Stopping – A height of object of 0.15 m is assumed for
measuring stopping sight distance on crest profiles.
Horizontal Control for Stopping – For consistency the measure of stopping
sight distance is taken to be the same as that on vertical curves, i.e. from the
height of eye of 1.15 m to an object on the road surface of height 0.15 m.
Control for Passing – Since vehicles are the objects that must be seen when
passing, it is assumed that the height of object for passing sight distance is 1.40
m. Passing sight distance both on profile crest and on horizontal curves should be
measured between the height of eye of 1.15 m and a height of object of 1.40 m.
3.6.2
Horizontal Alignment
The horizontal alignment of a road is usually a series of straights (tangents)
connected by circular curves. In modern practice it is common to introduce
transition or spiral curves between the tangents and circular curves. Curvilinear
alignment is horizontal alignment in which long flat curves are connected by long
transitions, generally without straights.
Criteria for the horizontal alignment of highways are directed to providing
vehicle operation at consistent speeds with safety and consideration for aesthetic
and economic factors. This is accomplished primarily through use of design
speed as an overall control.
The major considerations in horizontal alignment design are safety, grade profile,
type of facility, design speed, topography and construction cost. In rolling or
rough terrain the alignment and grade profile for multilane highways may be of a
lower order than for 2-lane roads in similar terrain. On expressways in
metropolitan areas alternate route studies often determine that right-of-way
consideration at interchange sites influence alignment more than any other
single factor. Topography controls curve radius, design speed and sight distance
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
to a large extent. All these factors must be balanced to produce an alignment that
is safest, most economical and adequate for the design classification of the
highway. Horizontal alignment should aim to avoid curve radii where the
available passing sight distance is marginal, and must afford at least the
minimum stopping distance for the design speed at all points on the highway.
Figure 3-8
Example of Small Radius Curves in Mountainous Topography
Radius of Curve
The combination of design speed and maximum superelevation controls the
maximum degree of curvature. Flatter curves must be provided where possible.
In general the alignment and curvature should fit the country, conforming to the
natural swing or directional bend of the ground, and the best alignment possible
within reasonable cost should be obtained rather than to follow blindly the
minimum curvature allowable.
Curves should be flat enough to provide minimum passing sight distance on
undivided highways, for the design speed established for that particular highway.
To facilitate the laying of curves by deflection angles, even-degree curves or
curves which are even multiples of ten minutes should be used whenever
possible.
On long or fairly steep grades, drivers tend to travel faster in the downgrade than
in the upgrade direction. Some adjustment in superelevation rates should be
considered for grades steeper than 5%.
Table 3-12 gives the maximum radius for four cases of maximum superelevation
rates.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Table 3-12
Design
Speed (kph)
3-20
Minimum Radius Using Limiting Values of e and f
Maximum
Superelevation e
(%)
Maximum
SideFriction f
Total
(e/100 + f)
Calculated
Min Curve
Radius (m)
Rounded Min
Curve
Radius (m)
20
4.0
0.35
0.39
8.1
8
30
4.0
0.28
0.32
22.1
22
40
4.0
0.23
0.27
46.7
47
50
4.0
0.19
0.23
85.6
86
60
4.0
0.17
0.21
135.0
135
70
4.0
0.15
0.19
203.1
135
80
4.0
0.14
0.18
280.0
280
90
4.0
0.13
0.17
375.2
375
100
4.0
0.12
0.16
492.1
492
15
6.0
0.40
0.46
3.9
4
20
6.0
0.35
0.41
7.7
8
30
6.0
0.28
0.34
20.8
21
40
6.0
0.23
0.29
43.4
43
50
6.0
0.19
0.25
78.7
79
60
6.0
0.17
0.23
123.2
123
70
6.0
0.15
0.21
183.7
184
80
6.0
0.14
0.20
252.0
252
90
6.0
0.13
0.19
335.7
336
100
6.0
0.12
0.18
437.4
437
110
6.0
0.11
0.17
560.4
560
120
6.0
0.09
0.15
755.9
756
130
6.0
0.08
0.14
950.5
951
15
8.0
0.40
0.48
3.7
4
20
8.0
0.35
0.43
7.3
7
30
8.0
0.28
0.36
19.7
20
40
8.0
0.23
0.31
40.6
41
50
8.0
0.19
0.27
72.9
73
60
8.0
0.17
0.25
113.4
113
70
8.0
0.15
0.23
167.8
168
80
8.0
0.14
0.22
229.1
229
90
8.0
0.13
0.21
303.7
304
100
8.0
0.12
0.20
393.7
394
110
8.0
0.11
0.19
501.5
501
120
8.0
0.09
0.17
667.0
667
130
8.0
0.08
0.16
831.7
832
15
10.0
0.40
0.50
3.5
4
20
10.0
0.35
0.45
7.0
7
30
10.0
0.28
0.38
18.6
19
40
10.0
0.23
0.33
38.2
38
50
10.0
0.19
0.29
67.9
68
60
10.0
0.17
0.27
105.0
105
70
10.0
0.15
0.25
154.3
154
80
10.0
0.14
0.24
210.0
210
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Design
Speed
(kph)
Maximum
Superelevation
e (%)
Maximum
SideFriction f
Total
(e/100 + f)
Calculated
Min Curve
Radius
(m)
Rounded
Min Curve
Radius
(m)
90
10.0
0.13
0.23
277.3
277
100
10.0
0.12
0.22
357.9
358
110
10.0
0.11
0.21
453.7
454
120
10.0
0.09
0.19
596.8
597
130
10.0
0.08
0.18
739.3
739
Source: Table 3-7 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6th Edition.
Used by Permission.
Length of Curve
The recommended minimum length of tangent between reversed curves should
be 50 m. In no cases shall the tangent length be less than 30 m. The tangent is
necessary to effect the transition from superelevation in one direction to
superelevation in the opposite direction.
Superelevation
When a vehicle moves in a circular path it is forced radially outward by
centrifugal force which is counter-balanced by the vehicle weight component due
to the roadway tires and surfacing. For a given radius and speed, a set force must
be applied to maintain the vehicle in a circular path and in road design this force
is provided by the side friction developed between tire and pavement and by
superelevation.
The basic formula for vehicle operation on a curve is:
we +wf = wV2/gR
where:
w = weight of vehicle
e
= pavement superelevation (tangent of the angle)
This is taken as positive if the pavement falls toward the center of the
curve.
f
= coefficient of side frictional force developed between vehicle tires and road
pavement.
This is taken as positive if the frictional force on the vehicle acts toward the
center of the curve.
V
= speed of the vehicle
R = radius of curve
In the basic formula ‘wV2/gR’ is the centrifugal force; ‘we’ is the force due to
tilting; and ‘wf’ is the friction force. At maximum safe speed and provided that the
vehicle does not skid, the forces are in equilibrium.
Superelevation values are now generally computed on the assumption that all
centrifugal force resulting from a speed equal to three-fourths of the design
3-21
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
speed will be counteracted by the effects of superelevation up to the maximum
value of 0.10 meter per meter of width. For any design speed V, therefore, the
required superelevation e would be:
e = 0.007859 (0.75 V)2 / R
= 0.004 V2 / R
where:
V
= design speed in kilometers per hour
R = radius of curve in meters
e
= superelevation in meters per meter width of roadway
Maximum superelevation shall be 0.10 m per meter width of roadway, i.e. 10%.
Where traffic congestion or extensive marginal development acts to restrict
speed, it is common practice to utilize a lower maximum rate of superelevation.
Superelevation may be omitted on low-speed urban streets where severe
constraints are present.
Rotation about the centerline profile is the most widely used design method in
attaining superelevation. Other methods used include revolving the pavement
about the inside edge profile and revolving the pavement about the outside edge
profile. For elevated expressways on viaducts, two-lane pavements usually are
sloped to drain the full roadway width toward one side of the roadway. On wider
facilities, particularly in areas with heavy rainfall, a crown may be located on the
lane line at one-third or one-half the total width from one edge, thus providing
two directions for surface drainage.
Figure 3-9 shows the method of applying superelevation revolved around the
centerline, while Figure 3-10 travelled way revolved about inside edge, Figure
3-11 travelled way revolved about outside edge.
Table 3-13 to Table 3-16 show minimum values of Radius for various
combinations of superelevation and design speeds for each of four values of
maximum superelevation rate.
3-22
Method of Attaining Superelevation for Travelled Way Revolved about Centerline
Source: AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6th Edition. Used by Permission.
Figure 3-9
3-23
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Figure 3-10
Methods of Attaining Superelvation for a Travelled Way Revolved about Outside
or Inside Edge
Crowned
Traveled way Revolved About Inside Edge with Curve to the Right
-A-
Crowned
Traveled way Revolved About Outside Edge with Curve to the Right
-B-
Source: AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6th Edition. Used by
Permission.
3-24
Traveled Way Revolved About Outside Edge with Curve to the Right
Method of Attaining Superelevation for Straight Cross Slope
Source: Table 3-8 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6th Edition. Used by Permission.
Figure 3-11
3-25
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Minimum Radii (meters) for Design Superelevation Rates, Design Speeds and emax
= 4%
Table 3-13
e (%)
Design Speed (kph)
20
30
40
50
60
70
80
90
100
NC
163
371
679
951
1310
1740
2170
2640
3250
RC
102
237
441
632
877
1180
1490
1830
2260
2.2
75
187
363
534
749
1020
1290
1590
1980
2.4
51
132
273
435
626
865
1110
1390
1730
2.6
38
99
209
345
508
720
944
1200
1510
2.8
30
79
167
283
422
605
802
1030
1320
3.0
24
64
137
236
356
516
690
893
1150
3.2
20
54
114
199
303
443
597
779
1010
3.4
17
45
96
170
260
382
518
680
879
3.6
14
38
81
144
222
329
448
591
767
3.8
12
31
67
121
187
278
381
505
658
4.0
8
22
47
86
135
203
280
375
492
Notes:
NC = Normal crown
RC = Remove adverse crown
Use of emax = 4% should be limited to urban conditions
Source: Table 3-8 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6th Edition. Used by Permission.
3-26
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Minimum Radii (meters) for Design Superelevation Rates, Design Speeds and emax
= 6%
e (%)
Table 3-14
Design
Speed (kph)
20
30
40
50
60
70
80
90
NC
194
421
738
1050
1440
1910
2360
2880
3510
4060
4770
5240
RC
138
299
525
750
1030
1380
1710
2090
2560
2970
3510
3880
2.2
122
265
465
668
919
1230
1530
1880
2300
2670
3160
3500
2.4
109
236
415
599
825
1110
1380
1700
2080
2420
2870
3190
2.6
97
212
372
540
746
1000
1260
1540
1890
2210
2630
2930
2.8
87
190
334
488
676
910
1150
1410
1730
2020
2420
2700
3.0
78
170
300
443
615
831
1050
1290
1590
1870
2240
2510
3.2
70
152
269
402
561
761
959
1190
1470
1730
2080
2330
3.4
61
133
239
364
511
697
882
1100
1360
1600
1940
2180
3.6
51
113
206
329
465
640
813
1020
1260
1490
1810
2050
3.8
42
96
177
294
422
586
749
939
1170
1390
1700
1930
4.0
36
82
155
261
380
535
690
870
1090
1300
1590
1820
4.2
31
72
136
234
343
488
635
806
1010
1220
1500
1720
4.4
27
63
121
210
311
446
584
746
938
1140
1410
1630
4.6
24
56
108
190
283
408
538
692
873
1070
1330
1540
4.8
21
50
97
172
258
374
496
641
812
997
1260
1470
5.0
19
45
88
156
235
343
457
594
755
933
1190
1400
5.2
17
40
79
142
214
315
421
549
701
871
1120
1330
5.4
15
36
71
128
195
287
386
506
648
810
1060
1260
5.6
13
32
63
115
176
260
351
463
594
747
980
1190
5.8
11
28
56
102
156
232
315
416
537
679
900
1110
6.0
8
21
43
79
123
184
252
336
437
560
756
951
100
110
120
130
Source: Table 3-9 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6th Edition. Used by Permission.
3-27
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Minimum Radii (meters) for Design Superelevation Rates, Design Speeds and emax
= 8%
Table 3-15
e (%)
Design
Speed (kph)
20
30
40
50
60
70
80
90
100
110
120
130
NC
184
443
784
1090
1490
1970
2440
2970
3630
4180
4900
5360
RC
133
322
571
791
1090
1450
1790
2190
2680
3090
3640
4000
2.2
119
288
512
711
976
1300
1620
1980
2420
2790
3290
3620
2.4
107
261
463
644
885
1190
1470
1800
2200
2550
3010
3310
2.6
97
237
421
587
808
1080
1350
1650
2020
2340
2760
3050
2.8
88
216
385
539
742
992
1240
1520
1860
2160
2550
2830
3.0
81
199
354
496
684
916
1150
1410
1730
2000
2370
2630
3.2
74
183
326
458
633
849
1060
1310
1610
1870
2220
2460
3.4
68
169
302
425
588
790
988
1220
1500
1740
2080
2310
3.6
62
156
279
395
548
738
924
1140
1410
1640
1950
2180
3.8
57
144
259
368
512
690
866
1070
1320
1540
1840
2060
4.0
52
134
241
344
479
648
813
1010
1240
1450
1740
1950
4.2
48
124
224
321
449
608
766
948
1180
1380
1650
1850
4.4
43
115
208
301
421
573
722
895
1110
1300
1570
1760
4.6
38
106
192
281
395
540
682
847
1050
1240
1490
1680
4.8
33
96
178
263
371
508
645
803
996
1180
1420
1610
5.0
30
87
163
246
349
480
611
762
947
1120
1360
1540
5.2
27
78
148
229
328
454
579
724
901
1070
1300
1480
5.4
24
71
136
213
307
429
549
689
859
1020
1250
1420
5.6
22
65
125
198
288
405
521
656
819
975
1200
1360
5.8
20
59
115
185
270
382
494
625
781
933
1150
1310
6.0
19
55
106
172
253
360
469
595
746
894
1100
1260
6.2
17
50
98
161
238
340
445
567
713
857
1060
1220
6.4
16
46
91
151
224
322
422
540
681
823
1020
1180
6.6
15
43
85
141
210
304
400
514
651
789
982
1140
6.8
14
40
79
132
198
287
379
489
620
757
948
1100
7.0
13
37
73
123
185
270
358
464
591
724
914
1070
7.2
12
34
68
115
174
254
338
440
561
691
879
1040
7.4
11
31
62
107
162
237
318
415
531
657
842
998
7.6
10
29
57
99
150
221
296
389
499
621
803
962
7.8
9
26
52
90
137
202
273
359
462
579
757
919
8.0
7
20
41
73
113
168
229
304
394
501
667
832
Source: Table 3-10a in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6th Edition. Used by Permission.
3-28
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Minimum Radii (meters) for Design Superelevation Rates, Design Speeds and emax
= 10%
Table 3-16
e (%)
Design
Speed (kph)
20
30
40
50
60
70
80
90
100
110
120
130
NC
197
454
790
1110
1520
2000
2480
3010
3690
4250
4960
5410
RC
145
333
580
815
1120
1480
1840
2230
2740
3160
3700
4050
2.2
130
300
522
735
1020
1340
1660
2020
2480
2860
3360
3680
2.4
118
272
474
669
920
1220
1520
1840
2260
2620
3070
3370
2.6
108
249
434
612
844
1120
1390
1700
2080
2410
2830
3110
2.8
99
229
399
564
778
1030
1290
1570
1920
2230
2620
2880
3.0
91
211
368
522
720
952
1190
1460
1790
2070
2440
2690
3.2
85
196
342
485
670
887
1110
1360
1670
1940
2280
2520
3.4
79
182
318
453
626
829
1040
1270
1560
1820
2140
2370
3.6
73
170
297
424
586
777
974
1200
1470
1710
2020
2230
3.8
68
159
278
398
551
731
917
1130
1390
1610
1910
2120
4.0
64
149
261
374
519
690
866
1060
1310
1530
1810
2010
4.2
60
140
245
353
490
652
820
1010
1240
1450
1720
1910
4.4
56
132
231
333
464
617
777
953
1180
1380
1640
1820
4.6
53
124
218
315
439
586
738
907
1120
1310
1560
1740
4.8
50
117
206
299
417
557
703
864
1070
1250
1490
1670
5.0
47
111
194
283
396
530
670
824
1020
1200
1430
1600
5.2
44
104
184
269
377
505
640
788
975
1150
1370
1540
5.4
41
98
174
256
359
482
611
754
934
1100
1320
1480
5.6
39
93
164
243
343
461
585
723
896
1060
1270
1420
5.8
36
88
155
232
327
441
561
693
860
1020
1220
1370
6.0
33
82
146
221
312
422
538
666
827
976
1180
1330
6.2
31
77
138
210
298
404
516
640
795
941
1140
1280
6.4
28
72
130
200
285
387
496
616
766
907
1100
1240
6.6
26
67
121
191
273
372
476
593
738
876
1060
1200
6.8
24
62
114
181
261
357
458
571
712
846
1030
1170
7.0
22
58
107
172
249
342
441
551
688
819
993
1130
7.2
21
55
101
164
238
329
425
532
664
792
963
1100
7.4
20
51
95
156
228
315
409
513
642
767
934
1070
7.6
18
48
90
148
218
303
394
496
621
743
907
1040
7.8
17
45
85
141
208
291
380
479
601
721
882
1010
8.0
16
43
80
135
199
279
366
463
582
699
857
981
8.2
15
40
76
128
190
268
353
448
564
679
834
956
8.4
14
38
72
122
182
257
339
432
546
660
812
932
8.6
14
36
68
116
174
246
326
417
528
641
790
910
8.8
13
34
64
110
166
236
313
402
509
621
770
888
9.0
12
32
61
105
158
225
300
386
491
602
751
867
9.2
11
30
57
99
150
215
287
371
472
582
731
847
9.4
11
28
54
94
142
204
274
354
453
560
709
828
9.6
10
26
50
88
133
192
259
337
432
537
685
809
9.8
9
24
46
81
124
179
242
316
407
509
656
786
10.0
7
19
38
68
105
154
210
277
358
454
597
739
Source: Table 3-11a in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6th Edition. Used by Permission.
3-29
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
When the design speed of a turning roadway is 70 kph or less, compound
curvature can be used to address right-of-way impacts and driver comfort and
safety. Curves that are compounded should not be too short or their effect in
enabling a change in speed from the tangent to the sharp curve is lost. Minimum
compound curve lengths are presented in Table 3-17.
Table 3-17
Minimum Lengths of Circular Arcs for Different Compound Curve Radii
Minimum Length of Circular Arc (m)
Radius (m)
Acceptable
Desirable
30
12
20
50
15
20
60
20
30
75
25
35
100
30
45
125
35
55
150 or more
45
60
Source: Table 3-14 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
Superelevation Runoff is the term denoting the length of highway needed to
accomplish the change in cross slope from a normal crown section to the fully
superelevated section or vice-versa. To meet the requirements of comfort and
safety the superelevation runoff should be effected uniformly over a length
adequate for the likely travel speed.
There is no universally accepted empirical basis in determining the length of
superelevation runoff. However, one empirical expression with fairly wide use
gives the required length in terms of the slope of the outside edge of pavement
relative to the centerline profile. In addition, for effective drainage this
longitudinal slope should not be less than 0.30%. Previous practice had been to
limit the grade difference, or relative gradient, to a maximum longitudinal slope
of 1:200 (0.50%) at 80 kph. Recommended maximum relative gradients for
different design speeds, providing longer runoff lengths at higher speeds and
shorter lengths at lower speeds, are presented in Table 3-18. Typical minimum
superelevation runoff lengths are presented in Table 3-19.
3-30
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Table 3-18
Maximum Relative Gradients for Superelevation Runoff
Design Speed (kph)
Maximum Relative Gradient (%)
Equivalent Maximum Relative Slope
20
0.80
1:125
30
0.75
1:133
40
0.70
1:143
50
0.65
1:154
60
0.60
1:167
70
0.55
1:182
80
0.50
1:200
90
0.47
1:213
100
0.44
1:227
110
0.41
1:244
120
0.38
1:263
130
0.35
1:286
Source: Table 3-15 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
A strict application of the maximum relative gradient criterion provides runoff
lengths for four-lane undivided roadways that are double those for two-lane
roadways; those for six-lane undivided roadways would be tripled. However it is
often not practical to provide such lengths in design, and Table 3-20 presents
recommended adjustment factors for multi-lane cases.
In the tangent-to-curve design, the location of the superelevation runoff length
with respect to the Point of Curvature (PC) needs to be determined. Normal
practice is to divide the runoff length between the tangent and the curved
sections, as presented in Table 3-21.
When a spiral curve is used the superelevation runoff will be affected over the
whole of the spiral length. Where transition spirals are not provided 0.667 (i.e.
2/3) of the required superelevation runoff should be applied on the straight and
0.333 (i.e. 1/3) on the circular curve.
Superelevation is usually not provided on local streets in residential and
commercial areas where wide pavements, proximity of adjacent development,
control of cross slope, drainage profiles, frequency of cross streets, and other
urban features make its use impractical.
3-31
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Table 3-19
Typical Minimum Superelevation Runoff Lengths (meters)
Design Speed (kph)
e (%)
20
30
40
50
60
100
110
120
130
7
7
8
8
9
10
11
12
12
13
14
15
2.0
9
10
10
11
12
13
2.2
10
11
11
12
13
14
14
15
16
18
19
21
16
17
18
19
21
23
2.4
11
12
12
13
14
16
17
18
20
21
23
25
2.6
12
12
13
14
16
17
19
20
21
23
25
27
2.8
13
13
14
16
17
18
20
21
23
25
27
29
3.0
14
14
3.2
14
15
15
17
18
20
22
23
25
26
28
31
16
18
19
21
23
25
26
28
30
33
3.4
15
16
17
19
20
22
24
26
28
30
32
35
3.6
16
17
19
20
22
24
26
28
29
32
34
37
3.8
17
18
20
21
23
25
27
29
31
33
36
39
4.0
18
19
21
22
24
26
29
31
33
35
38
41
4.2
19
20
22
23
25
27
30
32
34
37
40
43
4.4
20
21
23
24
26
29
32
34
36
39
42
45
4.6
21
22
24
25
28
30
33
35
38
40
44
47
4.8
22
23
25
27
29
31
35
37
39
42
45
49
5.0
23
24
26
28
30
33
36
38
41
44
47
51
5.2
23
25
27
29
31
34
37
40
43
46
49
53
5.4
24
26
28
30
32
35
39
41
44
47
51
56
5.6
25
27
29
31
34
37
40
43
46
49
53
58
5.8
26
28
30
32
35
38
42
44
47
51
55
60
6.0
27
29
31
33
36
39
43
46
49
53
57
62
6.2
28
30
32
34
37
41
45
47
51
54
59
64
6.4
29
31
33
35
38
42
46
49
52
56
61
66
6.6
30
32
34
37
40
43
48
51
54
58
63
68
6.8
31
33
35
38
41
45
49
52
56
60
64
70
7.0
31
34
36
39
42
46
50
54
57
61
66
72
7.2
32
35
37
40
43
47
52
55
59
63
68
74
7.4
33
36
38
41
44
48
53
57
61
65
70
76
7.6
34
36
39
42
46
50
55
58
62
67
72
78
7.8
35
37
40
43
47
51
56
60
64
68
74
80
8.0
36
38
41
44
48
52
58
61
65
70
76
82
8.2
37
39
42
45
49
54
59
63
67
72
78
84
8.4
38
40
43
47
50
55
60
64
69
74
80
86
8.6
39
41
44
48
52
56
62
66
70
76
81
88
8.8
40
42
45
49
53
58
63
67
72
77
83
91
9.0
40
43
46
50
54
59
65
69
74
79
85
93
9.2
41
44
47
51
55
60
66
70
75
81
87
95
9.4
42
45
48
52
56
62
68
72
77
83
89
97
9.6
43
46
49
53
58
63
69
74
79
84
91
99
9.8
44
47
50
54
59
64
71
75
80
86
93
101
10.0
45
48
51
55
60
65
72
77
82
88
95
103
1.5
70
80
90
Source: Table 3-17a in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6th Edition.
Used by Permission.
3-32
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Table 3-20
Adjustment Factor for Number of Lanes Rotated
Number of Lanes
Rotated(n1)
Adjustment Factor* (bw)
Length Increase Relative to
One-Lane Rotated (= n1 x bw)
1
1.00
1.00
1.5
0.83
1.25
2
0.75
1.50
2.5
0.70
1.75
3
0.67
2.00
3.5
0.64
2.25
‘* bw = [1 + 0.5 (n1 – 1)] / n1
Source: Table 3-16 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
Table 3-21
Runoff Locations that Minimize the Vehicle’s Lateral Motion
Design Speed
(kph)
Portion of Runoff Located Prior to the Curve
Number of Lanes Rotated
1.0
1.5
2.0-2.5
3.0-3.5
20 – 70
0.80
0.85
0.90
0.90
80 – 130
0.70
0.75
0.80
0.85
Source: Table 3-18 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
Widening
Due to the fact that on curves the rear wheels of motor vehicles do not ordinarily
travel in the same radius as the front wheels, it is desirable to widen the roadbed
especially along sharp curves. On simple curves, widening should be applied on
the inside edge of pavement only. On curves designed with spirals, widening may
be placed on the inside or divided equally between the inside and outside of the
curve. The amount of widening required for curves of various radii is shown in
Table 3-22.
The minimum widening recommended on curves is 0.60 m and depends on the
degree of curvature, design speed and design vehicle. Adjustments for design
vehicles other than WB-19 are presented in Table 3-23.
Widening is transitioned within the length of the superelevated runoff applied at
the inside edge or on both edges and should be attained linearly.
3-33
0.0
0.0
0.0
0.1
0.2
0.2
0.3
0.3
0.4
0.5
0.7
0.9
1.1
1.5
1.6
1.8
1.9
2.1
2.3
2.5
2.8
3.2
2500
2000
1500
1000
900
800
700
600
500
400
300
250
200
150
140
130
120
110
100
90
80
70
2.4
2.2
2.0
1.8
1.7
1.6
1.2
1.0
0.8
0.6
0.4
0.4
0.3
0.2
0.2
0.2
0.1
0.0
0.0
0.0
60
1.7
1.3
1.0
0.8
0.6
0.5
0.4
0.3
0.3
0.2
0.2
0.1
0.0
0.0
0.0
1.8
1.3
1.1
0.9
0.7
0.5
0.4
0.4
0.3
0.3
0.2
0.1
0.1
0.0
0.0
80
1.1
1.0
0.7
0.6
0.5
0.4
0.3
0.3
0.3
0.1
0.1
0.0
0.0
90
1.0
0.8
0.6
0.5
0.4
0.4
0.3
0.3
0.2
0.1
0.1
0.0
100
3.5
3.1
2.8
2.6
2.4
2.2
2.1
1.9
1.8
1.4
1.2
1.0
0.8
0.7
0.6
0.6
0.5
0.5
0.4
0.3
0.3
0.3
0.2
50
2.7
2.5
2.3
2.1
2.0
1.9
1.5
1.3
1.1
0.9
0.7
0.7
0.6
0.5
0.5
0.5
0.4
0.3
0.3
0.3
60
2.0
1.6
1.3
1.1
0.9
0.8
0.7
0.6
0.6
0.5
0.5
0.4
0.3
0.3
0.3
70
2.1
1.6
1.4
1.2
1.0
0.8
0.7
0.7
0.6
0.6
0.5
0.4
0.4
0.3
0.3
80
Design Speed (kph)
Design Speed (kph)
70
Roadway Width = 6.6 m
Roadway Width = 7.2 m
1.4
1.3
1.0
0.9
0.8
0.7
0.6
0.6
0.6
0.4
0.4
0.3
0.3
90
1.3
1.1
0.9
0.8
0.7
0.7
0.6
0.6
0.5
0.4
0.4
0.3
100
3-34
3.8
3.4
3.1
2.9
2.7
2.5
2.4
2.2
2.1
1.7
1.5
1.3
1.1
1.0
0.9
0.9
0.8
0.8
0.7
0.6
0.6
0.6
0.5
50
3.0
2.8
2.6
2.4
2.3
2.2
1.8
1.6
1.4
1.2
1.0
1.0
0.9
0.8
0.8
0.8
0.7
0.6
0.6
0.6
60
2.3
1.9
1.6
1.4
1.2
1.1
1.0
0.9
0.9
0.8
0.8
0.7
0.6
0.6
0.6
70
2.4
1.9
1.7
1.5
1.3
1.1
1.0
1.0
0.9
0.9
0.8
0.7
0.7
0.6
0.6
80
Design Speed (kph)
Roadway Width = 6.0 m
90
1.7
1.6
1.3
1.2
1.1
1.0
0.9
0.9
0.9
0.7
0.7
0.6
0.6
Calculated and Design Values for Travelled Way Widening on Open Highway Curves (Two-Lane Highways, One-Way or Two-Way)
Notes: Values shown are for WB-19 design vehicle and represent widening in meters; for other design vehicles use adjustments in Table 3-23
Values less than 0.6 m may be disregarded.
For 3-lane roadways, multiply above values by 1.5.
For 4-lane roadways, multiply above values by 2.
Source: Table 3-26a in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6th Edition. Used by Permission.
0.0
50
3000
Radius
of Curve
(m)
Table 3-22
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
1.6
1.4
1.2
1.1
1.0
1.0
0.9
0.9
0.8
0.7
0.7
0.6
100
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Table 3-23
Adjustments for Travelled Way Widening Values on Open Highway Curves (TwoLane Highways, One-Way or Two-Way)
Radius of
Curve (m)
Design Vehicle
SU-9
SU-12
3000
-0.4
-0.3
2500
-0.4
2000
WB-12
WB-20
WB-20D
WB-28D
WB-30T
WB-33D
-0.3
0.0
0.0
0.0
0.0
0.0
-0.4
-0.3
0.0
0.0
0.0
0.0
0.1
-0.4
-0.4
-0.4
0.0
0.0
0.0
0.0
0.1
1500
-0.4
-0.4
-0.4
0.0
-0.1
0.0
0.0
0.1
1000
-0.5
-0.4
-0.4
0.0
-0.1
0.0
0.0
0.1
900
-0.5
-0.4
-0.4
0.0
-0.1
0.0
0.0
0.1
800
-0.5
-0.5
-0.4
0.0
-0.1
0.0
0.0
0.2
700
-0.5
-0.5
-0.5
0.1
-0.1
0.1
0.0
0.2
600
-0.6
-0.5
-0.5
0.1
-0.1
0.1
-0.1
0.2
500
-0.6
-0.6
-0.5
0.1
-0.2
0.1
-0.1
0.3
400
-0.7
-0.6
-0.6
0.1
-0.2
0.1
-0.1
0.3
300
-0.8
-0.7
-0.7
0.1
-0.3
0.1
-0.1
0.4
250
-0.9
-0.8
-0.8
0.1
-0.3
0.2
-0.1
0.5
200
-1.1
-1.0
-0.9
0.2
-0.4
0.2
-0.2
0.6
150
-1.3
-1.2
-1.1
0.2
-0.6
0.3
-0.2
0.8
140
-1.4
-1.2
-1.2
0.3
-0.6
0.3
-0.2
0.9
130
-1.5
-1.3
-1.2
0.3
-0.6
0.3
-0.2
1.0
120
-1.6
-1.4
-1.3
0.3
-0.7
0.3
-0.3
1.1
110
-1.7
-1.5
-1.4
0.3
-0.8
0.4
-0.3
1.2
100
-1.8
-1.6
-1.5
0.4
-0.8
0.4
-0.3
1.3
90
-2.0
-1.8
-1.6
0.4
-0.9
0.4
-0.4
1.4
80
-2.2
-1.9
-1.8
0.5
-1.0
0.5
-0.4
1.6
70
-2.5
-2.2
-2.0
0.5
-1.2
0.6
-0.5
1.9
Notes:
Adjustments are applied by adding to or subtracting from the values in Table 3-22.
Adjustments depend only on radius and design vehicle; they are independent of roadway width and design
speed.
For 3-lane roadways, multiply above values by 1.5.
For 4-lane roadways, multiply above values by 2.
Source: Table 3-27 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
Spiral Curve Transition
If a vehicle is running at high speed on a carefully restricted path made up of
tangents connected by sharp circular curves, riding is extremely uncomfortable.
As the car approaches a curve, superelevation begins and the vehicle is titled
inward, but the passenger must remain vertical since there is no centrifugal force
requiring compensation. When the vehicle reaches the curve full centrifugal force
develops at once, and pulls the rider outward from his position. This process is
repeated in reverse order as the vehicle leaves the curve. When spiral curves are
introduced, the change in radius from infinity on the tangent to that of the
circular curve is effected gradually so that centrifugal force also develops
similarly.
3-35
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Generally the ‘Euler’ spiral, which in mathematical terminology is the ‘Clothoid’,
is used. The clothoid curve has been used not only as a transition curve but also
as the third element of a horizontal alignment (besides straight tangent and
circle). Clothoid is a rational curve adopted for smooth driving of vehicles. The
radius varies from infinity at the tangent end of the spiral to the radius of the
circular arc at the end that adjoins that circular arc. By definition, the radius of
curvature at any point on an Euler spiral varies inversely with the distance
measured along the spiral. In the case of a spiral transition that connects two
circular curves having different radii, there is an initial radius rather than an
infinite value. When the steering wheel of vehicle running at constant speed is
turned at the same speed, the path travelled by the vehicle traces exactly a
clothoid curve.
The curves on roads with low volume of traffic are normally designed as simple
curve, however highways with volume of traffic that justify a higher Level of
Service should be designed with geometric characteristics more appropriate to
their role. To this end, the curves connecting the tangent legs of the alignment,
should be provided with transition segments. In other words, between the
tangent and the circular curve a length at variable curvature (transition curve to
smooth out the passage from the straight line to the curved line should be
inserted.
The principal advantages of transition curves in horizontal alignment:
1. When a vehicle travelling at a constant speed, passes from a straight course
to a curved course, an angle of steering must be applied to the wheels for the
negotiation of the curve. For the vehicle to remain in the intended path
(middle of travelled lane) the angle of steering and steering speed must
match the degree of the curve and the travelling speed. Since there is the
reaction time of the driver, for appreciation of, and adjustment to, the
changed conditions, this takes a while to be achieved.
In this conditions, a vehicle, is also subjected to a sudden application of
centrifugal force. The effect of the combination of these two factors is
obviously, felt more in the case of sustained speed combined with a sharp
curve, resulting in a possible hazard because of discomfort for the operators.
A transition curve (of the spiral family) provides the travelling vehicle a path
of gradual passage from the straight course condition to the full curved
course condition, which eliminates said problem.
2. The superelevation run-off is developed is developed within the transition
length and therefore may be achieved with the desirable equilibrium
between radius of curvature and amount of superelevation. It may be pointed
out here that, although commonly used (in simple circular curves), the
distribution of superelevation run-off 1/3 inside the curve and 2/3 on
tangent do not fully satisfy the law of physics because, as intuitively one can
see, in this manner a force that is not yet in existence is being counteracted
on the tangent portion whilst the force applied on the curve is not fully
counteracted for 1/3 of its length.
3-36
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
3. A spiral transition curve also facilitate the transition in width where the
travelled way is widened on a circular curve use of spiral transitions provides
flexibility in accomplishing the widening of sharp curves.
Maximum radius recommended for use of a spiral are presented in Table 3-24
and Table 3-25 presents the desirable length of spiral curve transitions. Tangent
super elevation run-out lengths are presented in Table 3-26; however the lengths
in this table may be longer than desirable for combinations of low superelevation
rate and high speed where there is insufficient profile grade for adequate surface
drainage. Runoff is affected over the whole of the transition curve.
Table 3-24
Maximum Radius for Use of a Spiral Curve Transition
Design Speed (kph)
Maximum Radius (m)
20
24
30
54
40
95
50
148
60
213
70
290
80
379
90
480
100
592
110
716
120
852
130
1000
Note: The effect of spiral curve transitions on lateral acceleration is likely to be negligible for larger radii.
Source: Table 3-20 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
Table 3-25
Desirable Length of Spiral Curve Transition
Design Speed (kph)
Spiral Length (m)
20
11
30
17
40
22
50
28
60
33
70
39
80
44
90
50
100
56
110
61
120
67
130
72
Source: Table 3-21 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
3-37
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Table 3-26
Tangent Run-out Length (m) for Spiral Curve Transition Design
Design Speed
(kph)
Superelevation Rate (%)
2
4
6
8
10
20
11
-
-
-
-
30
17
8
-
-
-
40
22
11
7
-
-
50
28
14
9
-
-
60
33
17
11
8
-
70
39
19
13
10
-
80
44
22
15
11
-
90
50
25
17
13
10
100
56
28
19
14
11
110
61
31
20
15
12
120
67
33
22
17
13
130
72
36
24
18
14
Note: Based on 2.0% normal cross slope.
Source: Table 3-23 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
General Controls
3-38

Alignment should be as directional as possible, consistent with topography. A
flowing line that conforms generally to the natural contours is preferable to
one with tangents that slash through the terrain.

Generally, use flat curves and avoid curves of maximum degree except for
critical conditions.

Alignment should be consistent. Sharp curves at ends of long tangents and
sudden changes from easy to sharply curving alignment should be avoided.

For small deflection angles, curves should be sufficiently long to avoid the
appearance of a kink.

Tangents or flat curvature should be used on high long fills.

Caution should be exercised in the use of compound circular curves. Where
compounding is necessary the radius of the flatter circular arc should be not
more than 50% greater than the radius of the sharper circular arc. Where this
is not feasible, an intermediate curve or spiral should be used to provide the
necessary transition.

Abrupt reversal in alignment should be avoided by the use of sufficient length
of tangent or spiral between the two curves.

Avoid broken back curvature, that is, two curves in the same direction
separated by a short tangent length. The term ‘broken back’ is not applied
when the connecting tangent is of considerable length, say 250 m or more.
Use of spiral transitions, compound curves or a single longer curve is
preferable for such conditions.

Horizontal alignment should be coordinated with the profile.
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design

3.6.3
Ending a curve on a bridge is undesirable, unsightly and adds needless
complications to design and construction. Likewise curves beginning or
ending near a bridge should be placed such that no part of the superelevation
transition extends on to the bridge. Compound curves on a bridge are equally
undesirable. If curvature is unavoidable, the bridge should be entirely on a
simple curve as flat as physical conditions permit.
Vertical Alignment
Highways should be designed to encourage uniform operation throughout. Use of
a selected design speed is a means toward this end by the correlation of various
geometric features of the highway. Design values have been determined and
agreed upon for many highway features but few conclusions have been reached
on roadway grades in relation to design speed. Vehicle operating characteristics
on grades and established relationships of grades and their lengths to design
speed are discussed in this section.
Gradient
For economy of vehicle operation, particularly truck operation, grades should be
as flat as possible. However, flat grades in rolling or hilly country require very
costly earthwork, or much greater distance, or both. For Philippine national
roads, the maximum grade is reduced or ‘compensated’ on sharp curves
according to the following rule: ‘For every degree of curvature over 6 degrees,
the ruling grade shall be reduced by 0.1%’. On tangents or curves of less than 1
degree, 7% grades are permitted for distances not exceeding one kilometer.
Grades exceeding the above limits shall not be used in any national road without
permission from the Secretary of the Department of Public Works & Highways
and the Director of the Bureau of Design (BOD), DPWH.
The rolling of grades to avoid heavy earthwork is permissible if proper sight
distances are preserved. Secondary dips in the profile in which a vehicle may be
hidden from view should be avoided. On a long climb it is well to avoid ‘adverse
grades’, i.e. grades which cause a loss of elevation, although there are times when
such grades are justified to secure better alignment and reduce excavation when
crossing a small gully. If possible place grades so that excavation balances the
embankment. In areas subject to inundation grades should be established 0.50 m
above maximum water level. Grades should at least be above pipe culverts by
0.60 m. Grades of bridges should allow 1.50 m free board above the maximum
flood water elevations to the bottom of girders for streams carrying debris and
1.00 m for others.
Maximum Grades
Maximum grades of 5% are considered appropriate for a design speed of 110
kph. For a design speed of 50 kph maximum grades are generally in the range of
7 to 12%, depending on topography. In the Philippines the maximum grade
widely used is 6%.
3-39
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
AASHTO maximum recommended grades for local rural roads, rural collector
roads, urban collector roads, rural and urban arterials, and rural and urban
expressways of varying design speed are provided in Table 3-27 to Table 3-32.
Grades for local residential streets should be as level as practical, consistent with
the surrounding terrain, with a minimum grade for streets with outer curbs of
0.30%. Streets in commercial and industrial areas should have grades less than
8%.
Minimum Grades
On through cut sections grades should be at least 0.50% to provide longitudinal
drainage. On curved pavements a minimum of 0.30% may be used on high type
pavements and accurately crowned to facilitate drainage. Flat or level grades may
be used on uncurbed highways which have adequate crown for lateral drainage
on high fills.
Table 3-27
AASHTO Recommended Maximum Grades for Local Rural Roads
Type of Terrain
Maximum Grade (%) for Specific Design Speed (kph)
20
30
40
50
60
70
80
90
100
Level
9
8
7
7
7
7
6
6
5
Rolling
12
11
11
10
10
9
8
7
6
Mountainous
17
16
15
14
13
12
10
10
-
Source: Table 5-2 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
Table 3-28
AASHTO Recommended Maximum Grades for Rural Collector Roads
Type of Terrain
Maximum Grade (%) for Specific Design Speed (kph)
30
40
50
60
70
80
90
100
Level
7
7
7
7
7
6
6
5
Rolling
10
10
9
8
8
7
7
6
Mountainous
12
11
10
10
10
9
9
8
Note: Short lengths of grade in rural areas, such as grades less than 150 m in length, one-way downgrades, and grades on low-volume rural collectors may be up to 2% steeper than the grades shown above.
Source: Table 6-2 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
Table 3-29
AASHTO Recommended Maximum Grades for Urban Collector Roads
Type of Terrain
Maximum Grade (%) for Specific Design Speed (kph)
30
40
50
60
70
80
90
100
9
9
9
9
8
7
7
6
Rolling
12
12
11
10
9
8
8
7
Mountainous
14
13
12
12
11
10
10
9
Level
Note: Short lengths of grade in urban areas, such as grades less than 150 m in length, one-way downgrades, and grades on low-volume rural collectors may be up to 2% steeper than the grades shown above.
Source: Table 6-8 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Table 3-30
AASHTO Recommended Maximum Grades for Rural Arterial Roads
Type of Terrain
Maximum Grade (%) for Specific Design Speed (kph)
60
70
80
90
100
110
120
130
Level
5
5
4
4
3
3
3
3
Rolling
6
6
5
5
4
4
4
4
Mountainous
8
7
7
6
6
5
5
5
Source: Table 7-2 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6th Edition.
Used by Permission.
Table 3-31
AASHTO Recommended Maximum Grades for Urban Arterials
Type of Terrain
Maximum Grade (%) for Specific Design Speed (kph)
Level
50
60
70
80
90
100
8
7
6
6
5
5
Rolling
9
8
7
7
6
6
Mountainous
11
10
9
9
8
8
Source: Table 7-4 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
Table 3-32
AASHTO Recommended Maximum Grades for Rural and Urban Expressways
Type of Terrain
Maximum Grade (%) for Specific Design Speed (kph)
80
90
100
110
120
130
Level
4
4
3
3
3
3
Rolling
5
5
4
4
4
4
Mountainous
6
6
6
5
-
-
Source: Table 8-1 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6th Edition.
Used by Permission.
Critical Lengths of Grade for Design
This is the maximum length upgrade wherein a loaded truck can negotiate
without unreasonable reduction in speed. This critical grade length is determined
from a loaded truck which will effect a reduction in speed of 25 kph below the
average running speed. The critical lengths of upgrades in Table 3-33 when
approached by a level section should not be used as a control but should be
referred to as a guide.
Table 3-33
Critical Lengths of Grade
Critical Length of Upgrade (m)
Upgrade (%)
500
3
340
4
240
5
200
6
170
7
150
8
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Vertical Curves
All intersections of grade tangents shall be connected by parabolic vertical curve.
These parabolic vertical curves may either be symmetrical or unsymmetrical
curves, although the latter should be avoided whenever possible. Vertical
parabolic curves should provide adequate sight distance, safety, comfort, good
drainage and pleasing appearance.
The required length of vertical curve to satisfy the requirements of minimum
stopping sight distance, comfort and appearance, should not be shorter than
L = KA,
where:
L = length of vertical curve in meters
A = algebraic difference of grades in percent
K = corresponding constant for varying design speed, refer Table 3-34
Table 3-34
Minimum K Value for Terrain Types
Terrain
Flat
Design Speed (kph)
Minimum K Value
Rolling
70
30
desirable
60
15
absolute
25
desirable
Mountainous
40
12
absolute
30 desirable
10 absolute
The minimum requirement of vertical curve length without considering a K value
is 60 m.
Crest Vertical Curves
The major design control for crest vertical curves is the provision of ample sight
distances for the design speed – refer to Section 3.6.1.1. Wherever practical,
longer stopping sight distances should be used, particularly at decision points.
The basic equations for length of a crest vertical curve in terms of algebraic
difference in grade and sight distance follow:
When S is less than L,
L = A S2 / (100 ( (2h1)1/2 + (2h2)1/2)2
When S is greater than L,
L = 2 S – (200 (h11/2 + h21/2)2 / A)
where:
L
= length of vertical curve, m
A
= algebraic difference in grade, %
S
= sight distance, m
h1 = height of eye above roadway surface, m
h2 = height of object above roadway surface, m
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
When the height of eye and the height of object are 1.08 m and 0.60 m
respectively, as used for stopping sight distance, the above equations become:
When S is less than L,
L = A S2 / 658
When S is greater than L,
L = 2 S – (658 / A)
Table 3-35 shows the computed K values for lengths of vertical curves
corresponding to various stopping sight distances for each design speed.
Rounded values of K are shown in the right column for use in design.
Table 3-35
Design Controls for Crest Vertical Curves Based on Stopping Sight Distance
Design Speed (kph)
Stopping Sight
Distance (m)
Rate of Vertical Curvature K
Calculated
Design
20
20
0.6
1
30
35
1.9
2
40
50
3.8
4
50
65
6.4
7
60
85
11.0
11
70
105
16.8
17
80
130
25.7
26
90
160
38.9
39
100
185
52.0
52
110
220
73.6
74
120
250
95.0
95
130
285
123.4
124
K is the length of curve per percent algebraic difference in intersecting grades (A), K = L / A.
Source: Table 3-34 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
Where S is greater than L, minimum lengths of vertical curves in meters are
expressed as 0.6 times the design speed in kph.
Design values of crest vertical curves for passing sight distance differ from those
for stopping sight distance because of the different sight distance and object
height criteria. Using 1.08 m for the height of object results in the following
specific formulas:
When S is less than L,
L = A S2 / 864
When S is greater than L,
L = 2 S – (864 / A)
For the specific cases of local rural roads and rural collector roads, Table 3-36
presents K values for stopping sight distance on both crest and sag vertical
curves.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Table 3-36
Local Rural Road and Rural Collector Road Design Controls for Stopping Sight
Distance and for Crest and Sag Vertical Curves
Initial Speed (kph)
Design Stopping Sight
Distance (m)
Rate of Vertical Curvature K (m/%)
Crest
Sag
20
20
1
3
30
35
2
6
40
50
4
9
50
65
7
13
60
85
11
18
70
105
17
23
80
130
26
30
90
160
39
38
100
185
52
45
Rate of vertical curvature, K, is the length of curve per percent algebraic difference in intersecting grades
(A), K = L / A.
Source: Tables 5-3 and 6-3 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6th
Edition. Used by Permission.
Table 3-37 shows the computed K values for passing sight distance control.
Generally it is impractical to design crest vertical curves that provide sight
distance because of the difficulty of fitting the resulting long vertical curves to the
terrain and high cost where crest cuts are involved.
Table 3-37
Design Controls for Crest Vertical Curves Based on Passing Sight Distance
Design Speed (kph)
Passing Sight Distance (m)
Rate of Vertical Curvature, K, Design
30
120
17
40
140
23
50
160
30
60
180
38
70
210
51
80
245
69
90
280
91
100
320
119
110
355
146
120
395
181
130
440
224
Rate of vertical curvature K is the length of curve per percent algebraic difference in intersecting grades (A),
K = L / A.
Source: Table 3-35 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
Sag Vertical Curves
For sag vertical curves, the following criteria can be used for establishing the
lengths of sag vertical curves; (1) headlight sight distance, (2) passenger comfort,
(3) drainage control, and (4) general appearance. While each of these criteria are
relevant, headlight sight distance is the rational basis recommended.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
When a vehicle traverses a sag vertical curve at night, the portion of highway
lighted ahead is dependent on the position of the headlights and the direction of
the light beam. A headlight height of 0.60 m and a 1-degree upward divergence of
the light beam from the longitudinal axis of the vehicle are commonly assumed.
The following equations show the relationships between S, L and A:
When S is less than L,
L = A S2 / (120 + 3.5S)
When S is greater than L,
L = 2 S – ((120 + 3.5S) / A)
where:
L
= length of sag of vertical curve, m
A
= algebraic difference in grades, %
S
= light beam distance, m
For drivers to see the roadway ahead, a sag vertical curve should be long enough
that the light beam distance is approximately the same as the stopping sight
distance. Table 3-38 shows the range of computed values and the rounded values
of K selected as design controls. These lengths are minimum values based on the
design speed; longer curves are desirable wherever practical, but special
attention to drainage should be exercised where values of K in excess of 51 m per
percent change in grade are used.
Table 3-38
Design Speed
(kph)
Design Controls for Sag Vertical Curves
Stopping Sight
Distance (m)
Rate of Vertical Curvature K
Calculated
Design
20
20
2.1
3
30
35
5.1
6
40
50
8.5
9
50
65
12.2
13
60
85
17.3
18
70
105
22.6
23
80
130
29.4
30
90
160
37.6
38
100
185
44.6
45
110
220
54.4
55
120
250
62.8
63
130
285
72.7
73
Source: Table 3-36 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
Climbing Lane
For Two-Lane Highways
Freedom and safety of operation on two-lane highways, besides being influenced
by the extent and frequency of passing sections, are adversely affected by heavily
loaded vehicle traffic operating on grades of sufficient length to result in speeds
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
that could impede following traffic. In such cases the addition of climbing lanes
improves operations on upgrades and reduces crash potential.
The following three criteria, reflecting economic considerations, should be
satisfied to justify a climbing lane:
1. Upgrade traffic flow rate in excess of 200 vehicles per hour,
2. Upgrade truck flow rate in excess of 20 vehicles per hour, and
3. One of the following conditions exists:

A 15 kph or greater speed reduction is expected for a typical heavy truck,

Level of service E or F exists on the grade (refer to Table 2-1), or

A reduction of two or more levels of service is experienced when moving
from the approach segment to the grade.
In addition, high crash frequencies may justify the addition of a climbing lane
regardless of grade or traffic volumes. However, climbing lanes cannot be used in
the vicinity of road junctions and villages.
The location where an added lane should begin depends on the speeds at which
trucks approach the grade and on the extent of sight distance restrictions on the
approach. The beginning of the added lane should be preceded by a tapered
section with a desirable taper ratio of 25:1 that should be a least 90 m long.
The ideal design is to extend a climbing lane to a point beyond the crest, where a
typical truck could attain a speed that is within 15 kph of the speed of the other
vehicles. However this may not be practical in many instances, and instead a
practical point to end the added lane is where trucks can return to the normal
lane without undue interference with other traffic. An appropriate taper length
should be provided with a desirable taper ratio of 50:1 that should be at least 180
m long.
Climbing lanes must be laid out in accordance with the DPWH May 2012
Highway Safety Design Standards Manual.
A climbing lane should desirably be as wide as the through lane, and so
constructed that it can immediately be recognized as an added lane for one
direction of travel. Appropriate signs and markings should be provided.
The cross slope of a climbing lane is usually handled in the same manner as the
addition of a lane to a multilane highway. On a superelevated section, the crossslope is generally a continuation of the slope used on the through lane.
Desirably, the shoulder on the outer edge of a climbing lane should be as wide as
the shoulder on the normal two-lane cross section, particularly where there is
bicycle traffic.
For Multi-Lane Highways
Climbing lanes are generally not as easily justified on multilane facilities as on
two-way highways, because on two-lane facilities vehicles following other slower
moving vehicles on upgrades are frequently prevented from passing in the
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
adjacent traffic lane by opposing traffic. On multilane facilities there is no such
impediment to passing.
Because highways are normally designed for 20 years or more in the future,
there is less likelihood that climbing lanes will be justified on multilane facilities
than on two-lane roads for several years after construction, even though they are
deemed desirable for the peak hours of the design year. Where this is the case,
there is economic advantage in designing for, but deferring construction of,
climbing lanes on multilane facilities. In this situation, grading for the future
climbing lane should be provided initially.
Climbing lanes are particularly important for freedom of operation on urban
expressways where traffic volumes are high in relation to capacity.
Climbing lanes on multilane roads are usually placed on the outer or right-hand
side of the roadway. The principles for cross slopes, for locating terminal points,
and for designing terminal areas or tapers for climbing lanes are as discussed
above for ‘Climbing Lanes for Two-Lane Highways’.
Operational Lane Improvement on Two-Lane Highway
Passing Lane Section
An added lane can be provided in one or both directions of travel to improve
traffic operations in sections of lower capacity to at least the same quality of
service as adjacent road sections. Passing lanes can also be provided to improve
overall traffic operations on two-lane highways by reducing delays caused by
inadequate passing opportunities over significant lengths of highways, typically
10 to 100 km. Where passing lanes are used to improve traffic operations over a
length of road, they frequently are provided systematically at regular intervals.
The location of a passing lane should recognize the need for adequate sight
distance at both the lane addition and lane drop tapers. A minimum sight
distance of 300 m on the approach to each taper is recommended. The selection
of an appropriate location also needs to consider the location of intersections and
high-volume driveways in order to minimize the volume of turning movements
on a road section where passing is encouraged. Furthermore, other physical
constraints such as bridges and culverts should be avoided if they restrict
provision of a continuous shoulder.
A summary of the design procedure to be followed in providing passing sections
on two-lane highways is:
1. Horizontal and vertical alignment should be designed to provide as much of
the highway as practical with passing sight distance.
2. Where the design volume approaches capacity, the effect of lack of passing
opportunities in reducing the level of service should be recognized.
3. Where the critical length of grade is less than the physical length of an
upgrade, consideration should be given to constructing added climbing lanes.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
4. Where the extent and frequency of passing opportunities made available by
application of Criteria 1 and 3 are still too few, consideration should be given
to the construction of passing-lane sections.
Excluding tapers, a minimum length of 300 m is needed, however Table 3-39
presents optimal passing lane lengths for traffic operational efficiency.
The transition tapers at each end should be:

Where the posted speed limit is 70 kph or greater L = 0.62 WS

Where the posted speed limit is less than 70 kph
L = WS2/155
where:
L
= length of taper (m)
W = width (m)
S
= speed (kph)
The minimum passing sight distance for two-lane highways is determined as the
sum of the following four distances (refer Figure 3-12):

d1 - Distance traversed during perception and reaction time and during the
initial acceleration to the point of encroachment on the left lane.

d2 - Distance traveled while the passing vehicle occupies the left lane

d3 - Distance between the passing vehicle at the end of its maneuver and the
opposing vehicle.

d4 - Distance traversed by an opposing vehicle for two-thirds of the time the
passing vehicle occupies the left lane, or 2/3 of d2 above.
Figure 3-12
:
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Elements of Passing Sight distance for Two-Lane Highways
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Table 3-39
Optimal Passing Lane Lengths for Traffic Operational Efficiency
One-Way Flow Rate (vehicles per hour)
Passing Lane Length (km)
100 to 200
0.8
201 to 400
0.8 to 1.2
401 to 700
1.2 to 1.6
701 to 1200
1.6 to 3.2
Source: Table 3-31 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
Turnouts
A turnout is a widened, unobstructed shoulder area that allows slow-moving
vehicles to pull out of the through lane to give passing opportunities to following
vehicles. Turnouts are most frequently used on lower volume roads and in
difficult terrain with steep grades where more than 10% of the vehicle volumes
are large trucks and recreational vehicles, and construction of an additional lane
may not be cost-effective.
The recommended length of turnouts including taper is shown in Table 3-40. The
recommended lengths are based on the assumption that slow-moving vehicles
enter the turnout at 8 kph slower than the mean speed of the through traffic,
allowing the entering vehicle to coast to the mid-point without braking, and then
brake if necessary or merge back into the through lane.
The minimum width of a turnout is 3.6 m, with 5.0 m desirable. The available
sight distance should be at least 300 m on the approach to the turnout. Proper
signing and pavement marking are also needed to maximize turnout usage and
reduce crashes.
Table 3-40
Recommended Lengths of Turnouts Including Taper
Approach Speed (km per hour)
Minimum Length (m)*
30
60
40
60
50
65
60
85
70
105
80
135
90
170
100
185
‘* Maximum length should be 185 m to avoid use of the turnout as a passing lane.
Source: Table 3-32 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
Shoulder Driving
Where permitted by law and adequate paved shoulders exist, shoulders may
function as continuous turnouts to allow slow-moving vehicles to temporarily
move out of the path of another vehicle approaching from the rear.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Significant disadvantages with shoulder driving are that:

Shoulder widths of at least 3.0 m and preferably 3.6 m are needed.

Driver may be encouraged to behave similarly anywhere paved shoulders are
provided.

The effect of shoulder driving on bicyclists must be considered.

No specific signing for unrestricted shoulder driving has been developed.
Shoulder Use Sections
Specific signing at designated sites where the shoulder is adequate can be used to
permit slow-moving vehicles to use paved shoulders as an extended turnout to
provide additional passing opportunities.
Shoulder-use sections generally range in length from 0.3 to 0.5 km, with special
signs at both the beginning and end of such sections. Adequate structural
strength and good surface conditions are required.
General Controls
In addition to specific controls for vertical alignment, there are several general
controls to be considered in the design:
3-50

A smooth grade line with gradual changes, consistent with the type or class of
highway, road or street, and the character of terrain is preferred to a line with
numerous breaks and short lengths of grade. Specific design criteria are the
maximum grade and critical length of grade, but the manner in which they
are applied and fitted to the terrain on a continuous line determines the
suitability and appearance of the end result.

The ‘roller coaster’ or ‘hidden dip’ type of profile should be avoided by using
gradual grades made possible by heavier cuts and fills, or by introducing
some horizontal curvature on relatively straight sections.

Undulating grade lines, involving substantial lengths of momentum grades,
should be appraised for their effect upon traffic operation since they may
result in undesirably high downgrade speeds of trucks.

A broken-back grade line (two vertical curves in the same direction separated
by a short section of tangent grade) generally should be avoided, particularly
on sags.

On long grades, it is preferable to place the steepest grades at the bottom and
to flatten the grades near the ascent, or to break a sustained grade by short
intervals of flatter grade instead of providing a uniform sustained grade that
is only slightly below the recommended maximum. This is particularly
applicable to low design speed highways.

Where at-grade intersections occur on highway sections with moderate to
steep grades, it is desirable to reduce the gradient through the intersection.

Sag vertical curves should be avoided in cuts unless adequate drainage can be
provided.
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design

3.6.4
Climbing lanes should be considered where the critical length of grade is
exceeded and the Design Hourly Volume (DHV) exceeds the design capacity
on the grade by 20% in the case of 2-lane roads or by 30% in the case of
multilane roads.
Combination of Horizontal and Vertical Alignments
Horizontal and vertical alignment should complement each other. Both traffic
operation and overall appearance of the facility should be carefully considered in
design.
Vertical curvature superimposed on horizontal curvature, or vice-versa,
generally results in a pleasing facility. Design speed is considered in determining
the general roadway location, but as design proceeds to the development of more
detailed alignment and profile, it assumes greater importance. Design speed
determines limiting values for many elements such as curvature and sight
distance, and influences many other elements such as width, clearance and
maximum gradient.
Appropriate combinations of horizontal alignment and profile should consider
the following general guidelines:

Curvature and grades should be in proper balance. Tangent alignment or flat
curvature at the expense of steep or long grades and excessive curvature with
flat grades both represent poor design.

Vertical curvature superimposed on horizontal curvature, or vice versa,
generally results in a more pleasing facility, but such combinations should be
analyzed for their effect on traffic.

Sharp horizontal curvature should not be introduced at or near the top of a
pronounced crest vertical curve. Such an arrangement can be avoided if the
horizontal curvature leads the vertical curvature, and by using design values
well above the appropriate minimum values for the design speed.

Only flat horizontal curvature should be introduced near the bottom of a
steep grade approaching or near the low point of a pronounced sag vertical
curve.

On 2-lane highways and streets, the need for safe passing sections at frequent
intervals and for an appreciable percentage of the highway length often
supersedes the general desirability for coordination of horizontal and vertical
alignment.

Both horizontal curvature and profile should be made as flat as feasible at
highway intersections.

On divided highways and streets, variation in the width of median and the use
of separate profiles and horizontal alignments should be considered to derive
design and operational advantages of one-way roadways.

In residential areas the alignment should be designed to minimize nuisance to
the neighborhood. Generally a depressed facility makes a highway less visible
and less noisy to adjacent residents. Minor horizontal adjustments can
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
sometimes be made to increase the buffer zone between the highway and
adjacent properties.

The alignment should be designed to enhance attractive scenic views of the
natural and manmade environment, such as rivers, rock formations, parks
and notable structures.
Coordination of horizontal alignment and profile should begin with the
preliminary design. The designer should use working drawings of a size, scale
and arrangement that will allow the study of long, continuous stretches of
highway in both plan and profile, to aid visualization of the whole in three
dimensions.
After study of the horizontal alignment and profile in preliminary form,
adjustments can be made to obtain the desired alignment coordination. For the
selected design speed, the values for controlling curvature, gradient, sight
distance and superelevation should be obtained and checked. Design speed may
require adjustment during this process. All aspects of terrain, traffic operation
and appearance should be considered and the horizontal and vertical lines
should be adjusted and coordinated before the costly and time-consuming
calculations and preparation of construction plans to large scale are commenced.
For highways with gutters, the effects of superelevation transitions on gutter-line
profiles should be examined, particularly where flat grades are involved.
Crossroad or street intersections and locations of driveways are dominant
controls, but they should not override the above broader desirable features.
3.6.5
Other Elements Affecting Geometric Design
Drainage
Modern highway drainage design should incorporate safety, good appearance,
control of pollutants and economy in maintenance through the use of flat sideslopes, broad drainage channels, and liberal warping and rounding of the crosssection.
Erosion Control and Landscape Development
Effect on erosion should be considered in the location and design stages. Erosion
and maintenance are minimized by the use of flat side slopes, rounded and
blended with natural terrain; drainage channels designed with due regard to
width, depth, slopes, alignment and protective treatment; serrated cut slopes;
interceptors located and spaced to control erosion; prevention of erosion at
culvert outlets; proper facilities for groundwater interception; dikes, berms and
other protective devices; protective devices to trap sediment at strategic
locations; and protective ground covers and planting. To the extent practical,
these features should be designed and located to minimize the potential crash
severity for motorists who unintentionally run off the roadway.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Safety Rest Areas, Information Centers and Scenic Overlooks
Turnouts and rest areas along the roadside are functional and desirable elements
on heavily travelled roads and on those carrying recreation traffic. Turnouts are
areas, usually surfaced, outside the normal continuous shoulder, designed to
provide space for parking of one or more vehicles for purposes of bus loading,
mail delivery, observing the scenery, or semi-emergency stopping. Rest areas are
forms of turn-outs used for longer periods of time.
The design and location of roadside rest areas depend much on the character and
volume of traffic, type of highway, and adjacent land use.
Site selection for rest areas, information centers, and scenic overlooks should
consider the scenic quality of the areas of accessibility and adoptability to
development. Other essential consideration includes an adequate source of water
and a mean to treat and/or properly dispose sewage.
Usually in rural areas, the rest area is constructed at 5 km intervals.
Lighting
The design procedure for road lighting shall be in accordance with Roadway
Lighting Guidelines published by Department of Energy December 2008.
In urban and suburban areas, where there are concentrations of pedestrians and
roadside intersections, fixed source lighting tends to reduce accidents.
On expressways where there are no pedestrians, roadside entrances, or other
intersections at grade, and where rights-of-way are relatively wide, the
justification for lighting differs from that of non-controlled streets and highways.
Lighting of rural highways seldom is justified except on certain critical portions,
such as intersections and long bridges, depending on layout and traffic volumes,
and on interchanges and areas where roadside interference is a factor, including
railroad-highway grade crossings. Tunnels, toll plazas and movable bridges are
nearly always lighted.
Where limited section of a highway are provided with fixed source lighting, it is
desirable that the intensity of light diminish gradually as the distance from the
lighted area increases. This gives the eyes of the drivers leaving the intersection,
built-up area, etc., time to adjust themselves to the darkness beyond and may
eliminate the blind interval experienced upon leaving a comparatively brightly
lighted area. Since eye accommodation for change in lighting requires long
distances which may be impractical it may be desirable to use low light
intensities for short sections of highway.
To minimize the effect of glare, luminaires normally are mounted at heights 8 to
10 m. Lighting poles should be placed clear of shoulders, normally not closer than
3 m from the edge of through traffic lane, or not closer than 2 m where a barrier
curb is at the pavement edge. Lighting poles should not be located on the median,
unless it is at least 6 m and preferably 10 m wide. Lighting columns within the
Clear Zone must be passively safe, otherwise they will need to be shielded by
safety barrier.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Where highway lighting may be considered for future installation, considerable
savings can be achieved through design and installation of necessary conduits
under roadways and curbs as part of the initial construction.
Highway lighting for expressways is directly associated with the type and
location of highway signs, and these two aspects should be designed jointly.
Utilities
Utilities in the nature of power lines and water, gas and sewage mains, which
occupy or cross the highway right-of-way, should be considered in location and
design of the highway. Normally on new construction, no utility should be
situated under any part of the pavement, except where it must cross the highway.
Preferably underground utilities should be located outside the roadway to avoid
any disturbance to traffic during utility maintenance operations. Where an
underground utility crosses the highway, generally, it should be placed within a
conduit or pipe of sufficient size so that repairs of the utility can be made without
disturbing the travelled way.
Poles of overhead utilities should be located clear of shoulders, preferably 4.50 m
or more from the edge of pavement. Normally, no poles should be located on the
median. In general overhead utility line paralleling the highway should be
situated as far as practicable from the roadway, recognizing that accessibility for
maintenance may be a factor in their location depending on terrain and character
of highway grading.
To the extent practical, utilities along expressways should be constructed so they
can be serviced from outside the controlled access lines. On new installations or
adjustments to existing utility lines, provision should be made for known or
planned expansion, particularly those located underground. All utility
installations on, over, or under highway or street right-of-way and attached
structures should be of durable materials designed for long service-life
expectancy, relatively free from routine servicing and maintenance, and meet or
exceed the applicable industry codes or specifications.
D.O. No. 26 Series of 2011 Policy on Digging/Excavation by Private and Public
Utilities on National Road show the typical cross section and location of various
utilities in Urban and Rural areas with 20.0 m to 60.0 m RROW. (see Figure 2-1 to
2-8).
Signing and Marking
Traffic signs, pavement markings and traffic signals are directly related to the
design of the highway and are features of traffic control and operation which the
designer must consider in the geometric layout of the highway. Traffic control
devices should be designed concurrently with the geometrics. The extent to
which signs and markings are used depends upon the traffic volume, the type of
highway, and the frequency of use by unfamiliar drivers.
Although safety and efficiency of operation depend to a considerable degree on
the geometric design of the highway, the physical layout must also be
supplemented by effective signing as a means of informing, warning and
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
controlling drivers. Signing plans coordinated with horizontal and vertical
alignment, sight distance obstructions such as abutments, operating speeds and
maneuvers and other applicable items should be worked out prior to completion
of design. Highway signs are of three general types: regulatory signs used to
indicate the required method of traffic movement; warning signs used to indicate
conditions that may be hazardous to highway users; and guide signs used to
direct traffic along a route or towards a destination. Reflectorization and lighting
of signs are important considerations.
Marking, like signs, have the function of controlling traffic to encourage safe and
expeditious operation. Markings either supplement the regulations or warnings
of traffic signs or serve independently to indicate certain regulations or
hazardous conditions. For rural highways there are three general types of
markings in use – pavement markings, object markings and delineators which
are utilized to guide traffic, particularly at night.
Traffic control signals for vehicles, pedestrians and bicycles are devices that
control crossing or merging traffic by assigning right-of-way to various
movements for certain intervals of time. They are one of the key elements in the
function of many urban streets and some rural intersections.
Because supports for highway signs and signals have the potential of being struck
by motorists, they should be placed on structures outside the desired clear zone
or behind traffic barriers placed for other reasons. If these measures are not
practical, the supports should be breakaway, or for overhead sign and signal
supports, shielded by appropriate traffic barriers. Supports should not be placed
in such a way that they restrict pedestrian access on adjacent sidewalks.
Noise Barriers
The designer should aim to minimize the radiation of noise into noise-sensitive
areas along the highway.
Noise is measured on a logarithmic scale, so a noise of 70 decibels (dBA) sound
only one-half as loud as 80 dBA. If a single vehicle produces a noise level of 60
dBA and a certain distance from the receiver, two of these vehicles at a common
point of origin with produce 63 dBA, four vehicles will produce 66 dBA, eight
vehicles will produce 69 dBA, and so forth.
Noise decreases with distance at a rate of approximately 3 to 4.5 dBA for each
doubling of distance. The higher the pitch or more pronounced the intermittency,
the greater the degree of annoyance.
Public annoyance at traffic noise also depends on the environment in which the
noise is heard; high traffic noise levels are usually more tolerable in industrial
than in residential areas; and the reaction is usually less if the noise source is
hidden from view.
Noise impacts are particularly important to noise-sensitive areas such as
residential areas, schools, churches, hospitals, libraries, nursing homes, parks,
hotels and motels.
Table 3-41 provides USA FHWA noise-abatement criteria for various land uses.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
The existing noise level generated by the traffic is then determine by one of the
noise prediction methods, that presently available. Pertinent factors are traffic
characteristic (speed, volume and composition), topography (vegetation, barriers
and distance) and roadway characteristics (configuration, pavement type, grades
and type of facility. The prediction is normally based on the highway traffic that
will yield the worst hourly traffic noise on a regular basis for the design year.
Table 3-41
Noise-Abatement Criteria for Various Land Uses
Activity
Category
Category Description
A
Design Noise
Levels (dBA)a
Leq(h)b
L10(h)
Tracts of land in which serenity and quiet are of extraordinary
significance and serve an important public needs and where the
preservation of those qualities is essential if the area is to continue to
serve its intended purpose. Such areas could include amphitheatres,
particular parks or portions of parks, open spaces, or historic districts
which are dedicated or recognized by appropriate local officials for
activities requiring special qualities of serenity and quiet.
57
60
(Exterior)
B
Picnic areas, recreation areas, playgrounds, active sports areas, and
parks not included in Category A and residences, motels, hotels,
public meeting rooms, schools, churches, libraries, and hospitals.
67
70
(Exterior)
C
Developed lands, properties, or activities not included in categories A
or B above.
72
75c
(Exterior)
D
Undeveloped lands which do not contain improvements or activities
devoted to frequent human habitation or use and for which such
improvements or activities are unplanned and not programmed.
-
-
E
Residencies, motels, hotels, public meeting rooms, schools,
churches, libraries, hospitals, and auditoriums.
52
55d
(interior)
a
Source: Federal Aid Highway Program Manual, Vol. 7, Ch. 7, Sec. 3 Transmittal 348, August 9, 1982.
b
Either L10(h) or Leq(h) (but not both) may be used for a specific project.
c
Noise-abated criteria have not been established for these lands. They may be treated as developed lands
if the probability for development is high. Provisions for noise abatement would be based on the need,
expected benefits, and costs of such measures.
d
Interior noise abatement criteria in this category apply to (1) indoor activities where no extreme noisesensitive land use or activity is identified, and (2) exterior activities that are either remote from the highway
or shielded so that they will not be significantly affected by the noise, but the interior activities will.
Careful consideration should be exercised so that the construction and placement
of these noise barriers will not increase the severity of crashes that may occur.
Every effort should be made to locate noise barriers to allow for sign placement
and to provide lateral offsets to obstructions outside the edge of the traveled
way. It is recognized, however, that such a setback may sometimes be
impractical. In such situations, the largest practical width commensurate with
cost-effectiveness considerations should be provided. Stopping sight distance is
another design consideration. Therefore, horizontal clearances should be
checked for adequate sight distances. Construction of a noise barrier should be
avoided at a given location if it would limit stopping sight distance below the
minimum values shown in Table 3-42. This situation could be particularly critical
where the location of a noise barrier is along the inside of a curve. Some designs
use a concrete safety shape either as an integral part of the noise barrier or as a
separate roadside barrier between the edge of the roadway and the noise barrier.
On non-tangent alignments, a separate concrete barrier may obstruct sight
distance even though the noise barrier does not. In such instances, it may be
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
appropriate to install metal rather than concrete roadside barriers in order to
retain adequate sight distance.
Care should be exercised in the location of noise barriers near gore areas.
Barriers at these locations should begin or terminate, as the case may be, at least
60 m (200 ft) from the theoretical nose.
Potential noise problems should be identified early in the design process. Line,
grade, earthwork balance, and right-of-way should all be worked out with noise
in mind. Noise attenuation may be inexpensive and practical if built into the
design and expensive if not considered until the end of the design process. An
effective method of reducing traffic noise from adjacent areas is to design the
highway so that some form of solid material blocks the line of sight between the
noise source and the receptors. Advantage should be taken of the terrain in
forming a natural barrier so that the appearance remains aesthetically pleasing.
In terms of noise considerations, a depressed highway section is the most
desirable. Depressing the roadway below ground level has the same general
effect as erecting barriers (i.e., a shadow zone is created where noise levels are
reduced (Refer Figure 3-13). Where a highway is constructed on an embankment,
the embankment beyond the shoulders will sometimes block the line of sight to
receptors near the highway, thus reducing the potential noise impacts (Refer
Figure 3-14).
Table 3-42
Stopping Sight Distance on Level Roadways
Metric
Design
Speed
(km/h)
Brake
Reaction
Distance
(m)
Breaking
Distance
on Level
(m)
20
13.9
30
U.S. Customary
Stopping Sight
Distance
Design
Speed
(mph)
Brake
Reaction
Distance
(ft)
Braking
Distance on
Level (ft)
Calculate
d (m)
Design
(m)
4.6
18.5
20
15
55.1
20.9
10.3
31.2
35
20
40
27.8
18.4
46.2
50
50
34.8
28.7
63.5
60
41.7
41.3
70
48.7
80
Stopping Sight
Distance
Calculated
(ft)
Design
(ft)
21.6
76.7
80
73.5
38.4
111.9
115
25
91.9
60.0
151.9
155
65
30
110.3
86.4
196.7
200
83.0
85
35
128.6
117.6
246.2
250
56.2
104.9
105
40
147.0
153.6
300.6
305
55.6
73.4
129.0
130
45
165.4
194.4
359.8
360
90
62.6
92.9
155.5
160
50
183.8
240.0
423.8
425
100
69.5
114.7
184.2
185
55
202.1
290.3
492.4
495
110
76.5
138.8
215.3
220
60
220.5
345.5
566.0
570
120
83.4
165.2
248.6
250
65
238.9
405.5
644.4
645
130
90.4
193.8
284.2
285
70
257.3
470.3
727.6
730
75
275.6
539.9
815.5
820
80
294.0
614.3
908.3
910
Note: Brake reaction distance predicated on a time of 2.5 s; deceleration rate of 3.4 m/s2 (11.2 ft/s2) used to determine calculated sight
distance.
Source: Table 3-1 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6th Edition. Used by Permission.
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Figure 3-13
Effects of Depressing the Highway
Source: AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6th Edition. Used by
Permission.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Figure 3-14
Effects of Elevating the Highway
Source: AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6th Edition. Used by
Permission.
Signals
Intersections are commonly designed for the safe movement of automobiles,
trucks, pedestrians, bicyclists, and buses, which all have different characteristics.
Traffic signals are traffic control devices to avoid conflict in the movement of
traffic, particularly at intersections but also at pedestrian crossings. Their use
often delays users traveling along roads, potentially resulting in driver
discomfort, frustration, additional fuel consumption, and increased travel time.
The capacity of signalized intersections along a roadway can determine the
capacity of that roadway.
Available green time at signalized intersections is substantially less than the total
time available for free flow. For these reasons, capacity and level of service
analysis is of major importance in designing signals. Intersection levels of service
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
defined to represent ranges in control delay and intersection conditions for
signalized intersections are provided in Table 3-43.
Table 3-43
Level of Service Definitions for Signalized Intersections
Level of Service
Intersection Conditions
A
Very short delay and most vehicles do not stop as a result of favorable progression,
arrival of most vehicles during green phase, and short cycle length.
B
Short delay and many vehicles do not stop or stop for a short time as a result of short
cycle lengths and good progression.
C
Moderate delay, many vehicles have to stop, and occasional individual cycle failures
as a result of longer cycle lengths and fair progression.
D
Longer delays; many vehicles have to stop; and a noticeable number of individual
cycle failures as a result of some combination of long cycle lengths, high volume to
capacity ratios, and unfavorable progression.
E
Long delays and frequent individual cycle failures result from one or both of the
following: long cycle lengths or high volume to capacity ratios, which, in turn, result in
poor progression.
F
Delays considered unacceptable to most drivers occur when the vehicle arrival rate is
greater than the capacity of the intersection for extended periods of time.
Source: Table 9-1 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
Intersections – Grade Separation & Interchange
The three general types of highway crossings are at-grade (refer Section
3.6.5.10), grade separations without ramps, and interchanges.
An interchange is a system of interconnecting roadways in conjunction with one
or more grade separations that provides for the movement of traffic between two
or more roadways or highways on different levels. They provide the greatest
efficiency, safety and capacity for accommodating intersecting traveled ways.
Refer to Section 4.2 for information on design of Grade Separated Intersections
and Interchanges.
Intersection At-Grade Design – Left & Right Turn Lanes, Channelization, Median, Island,
Roundabout, Railway Crossing & Traffic Control
At grade intersections are among the most complicated elements of a street or
highway. Intersections usually have less capacity than other parts of the roadway
and are where most traffic conflicts occur. Design criteria should be selected that
will result in balanced and cost-effective design that provides efficient operations
and low crash frequencies, and considers the needs of all user groups. Design
criteria should also meet mobility, environmental, scenic, aesthetic, cultural,
natural resource, and community needs.
Physical design elements include alignment and profile, sight distance, medians
and median openings, provision for right and left-turn lanes, islands, and other
physical elements. The functional area of an intersection extends both upstream
and downstream from the physical intersection area, and includes any auxiliary
lanes and their associated channelization.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
A roundabout is an intersection with a central island around which traffic must
travel counter-clockwise and in which entering traffic must yield to circulating
traffic. They commonly feature channelized approaches: splitter islands on each
leg to separate entering and exiting traffic, deflect and slow entering traffic, and
provide a pedestrian refuge; and appropriate curvature so that travel speeds on
the circulatory roadway are typically less than 50 kph.
Refer to Section 4.1 for information on design of At-Grade Intersections.
For location of underground utilities refer to Figure 2-1 to Figure 2-8.
3.7
Cross Section Elements
These comprise the types of surfaces, the width of pavement and the shoulders,
the cross slopes, medians, sidewalks, and drainage channels and side slopes.
3.7.1
Pavement
For the purpose of defining the width of pavement, the pavement is regarded as
the running surface, excluding shoulders, regardless of the width of the pavement
courses which support the running surface.
Pavements may be classified as single lane, two-lane or multilane. A traffic lane is
a portion of the pavement allotted for the use of a single line of vehicles.
Surface Type
The selection of surface type is determined based on the traffic volume and
composition, soil characteristics, climate, performance of pavements in the area,
availability of materials, energy conservation, initial cost, and the overall annual
maintenance and service-life cost. Important pavement characteristics that are
related to geometric design are the effect on driver behavior and the ability of a
surface to retain its shape and dimensions, to drain, and to retain adequate skid
resistance.
For national roads, all surfaces or pavements shall have a minimum width of 6.1
m sufficient crown slope for drainage shall be provided. Straight slopes of
pavement are favored instead of the use of parabolic crowns. Also the slope of the
subgrade section immediately beneath any surfacing and shoulder shall be
appropriate for lateral drainage.
Types of surfaces broadly are referred to as high, intermediate and low in
consideration of the effect on geometric design. A low design speed should not be
assumed solely because of an initial low type surface.
High type pavements are justified for high volume traffic, which requires that the
road surface be smooth, possess non-skid qualities, and could adequately support
the expected volume and weights of vehicles without fatigue. Intermediate type
surfaces are slightly less in cost and somewhat less in strength than customary
high type pavements. Low type surfaces range from surface treated earth, such as
earth, shell and gravel. The important characteristics of surface type in relation to
geometric design are the ability of a surface to retain its shape, the ability to
drain, and the effect on driver behavior. Smooth surfaces encourage higher
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
operating speeds than do poor surfaces. The choice is dependent on so many
other more important factors such that the effect of type of surface is small.
Skidding crashes are a major concern in highway safety. Other than ‘driver error’,
the main causes of poor skid resistance on wet pavements are rutting, polishing,
bleeding and dirty pavements. Hydroplaning occurs when the drainage capacity
of the tire tread pattern and the pavement surface is exceeded, and water begins
to build up in front of the tire. As the water builds up, a water wedge is created
and this wedge produces a hydrodynamic force which may provide lift to the
rolling tire in some situations. To reduce the potential for both skidding and
hydroplaning, consideration should be given to pavement roughness
characteristics and transverse slopes.
Cross Slope
Cross slope or crossfall is the slope of the surface of a pavement measured at
right angles to the horizontal alignment. Two-lane and wider undivided
pavements on tangents or on flat curves usually have a crown or high point in the
middle and slope downward toward both edges. With plane cross slopes, there is
a cross slope break at the crown line and a uniform slope on each side. The
purpose of the cross slope is to drain the pavement on tangents and on curves
and to provide superelevation on horizontal curves.
Pavement cross slopes on tangents should be as flat as drainage needs permit
and these in turn are conditioned by the type of pavement and nature of surface.
For a given slope the smoother the surface the more efficient it is in shedding
water, but hazardous conditions likely to result from a thin film of water on the
surface should not be overlooked. On sections other than those with
superelevation, surface cross slopes normally conform to the ranges provided in
Table 3-44.
Table 3-44
Surface Type
Rate of Cross Slope Range for Surface Types
Range in rate of Cross Slope (meter per meter)
High
0.01 – 0.02
Intermediate
0.015 – 0.03
Low
0.02 – 0.04
Where two or more lanes are inclined in the same direction on multilane
pavements, each successive lane outward from the crown line preferably should
have an increased slope. The lane adjacent to the crown line should be pitched at
the normal minimum slope, and on each successive lane outward the rate should
be increased by about 0.5% (0.005 meters per meter). Cross slopes greater than
2% (0.02 meters per meter) should be avoided on high type surfaces. Multilane
divided roadways with unidirectional cross slopes tend to provide more comfort
to drivers when they change lanes and may either drain away from or toward the
median. Where curbed medians are present, this has the disadvantage of
concentrating drainage next to or on higher speed lanes.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
On intermediate type surfaces, the running speeds may not be less than those on
high type pavements because of the generally lower volumes and fewer freight
vehicles. A somewhat greater slope rate for intermediate type surfaces is used
because of the likelihood of less accuracy in construction procedures and greater
proneness to settlement and warping of the cross-section. On these surfaces the
longitudinal grade will assist drainage in the event of ‘rutting’ type of settlement;
otherwise a greater rate than normal cross slope should be used. Also the
intermediate type surfaces frequently are of coarser texture, which tends to
retard the runoff of water.
Low type surfaces, e.g. loose earth, broken stones or gravel, require a greater
cross slope on tangents to prevent the absorption of water into the surface, and
due to greater surface irregularities. On highways with these surfaces, vehicle
speeds generally are not as high as on better type surfaces so that, in an
operational sense, no sacrifice is made.
Small cross slopes are needed on uncurbed pavements to assist drainage in the
event of uneven settlement. Curbed pavements require greater slopes to reduce
water seeping on the traffic lane adjacent to the curb.
3.7.2
Lane Widths
The width of pavement is determined by the lane width, which depends on the
width and size of vehicles, speed of travel, the annual average daily traffic and the
width of shoulders.
The desirable lane width is 3.65 m which allows large vehicles to pass without
either vehicle having to move sideways toward the edge of the pavement. On
grounds of economy, lane widths as low as 2.75 m may be used in low-volume
rural and residential areas. Roads with pavement widths less than 5.50 m should
be regarded as single lane roads. Pavement width greater than 7.32 m for twodirection movement is not recommended for two-lane roads as some drivers will
attempt to travel three vehicles abreast on a wide pavement.
Commercial vehicles are commonly of the full legal width of 2.50 m. Even if such
vehicles are steered perfectly on straight road with a smooth pavement, the rear
wheels of the vehicle track several centimeters lower down the pavement crossslope than the front wheels. Lateral wind forces too can cause a large deviation in
tracking. Vehicle, pavement and driving imperfections may result in other
variations in the vehicle path.
Clearance between vehicles passing at high speed needs to be greater than
clearance between slow moving vehicles. Drivers unfamiliar with a road may
slow down when meeting an oncoming wide vehicle on a narrow pavement, but
in general drivers maintain speed. At night time a reduction in speed can be
expected on narrow pavements as the driver cannot gauge the size of the
oncoming vehicle owing to its outline being masked by bright headlights.
Roads carrying large volumes of traffic require wider pavements than those
carrying only small volumes. Heavy traffic on a road means frequent passing and
overtaking maneuvers, and as a result the path of vehicles using the road is
farther from the centerline.
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Where unequal-width lanes are used, locating the wider lane on the outside
(right) provides more space for large vehicles that usually occupy that lane,
provides more space for bicycles, and allows drivers to keep their vehicles at a
greater distance from the right edge. Where a curb is adjacent to only one edge,
the wider lane should be placed adjacent to that curb.
Auxiliary lanes at intersections and interchanges should be as wide as the
through-traffic lanes, but not less than 3.0 m. Where continuous two-way leftturn lanes are provided, the optimum lane width design is 3.0 to 4.8 m.
Shoulders 1.22 m wide or wider do not influence the position of a vehicle
travelling on the adjacent pavement and accordingly do not affect the lane width.
Obstructions within the width, such as retaining walls, bridge trusses or
headwalls reduce the effective width of travelled way in a capacity sense and
wider lane widths may be required to compensate for this.
In residential areas a parallel parking lane at least 2.1 m wide may be provided
on one or both sides of the street, as appropriate to the conditions of lot size and
intensity of development. In commercial and industrial areas, parking lane
widths should be at least 2.4 m wide, usually on both sides of the street.
3.7.3
Shoulders
The term ‘shoulder’ is variously used with modifying adjectives to describe
certain functional or physical characteristics. The ‘graded’ width of shoulder is
measured from the edge of the through traffic lane to the intersection of shoulder
slope and the side slope planes. The ‘surfaced’ width of shoulder is that part
constructed to provide a better all-weather load support than afforded by the
native soils. The ‘usable’ width of shoulder is the actual width that can be used
when a driver makes an emergency or parking stop.
Shoulder width commonly varies from 0.6 m on minor rural roads to 3.6 m on
major roads. Shoulder width is measured from the edge of the pavement to edge
of usable formation and excludes any berm, rounding or extra widths required to
accommodate guide posts, guard fencing, etc. Shoulders must be sufficiently
stable to support occasional vehicle loads in all kinds of weather. Desirably,
shoulder surface should contrast in color and texture with that of through traffic
lanes.
Adequate shoulders should be continuous along the full length of the highway,
but where this is not economically feasible, consideration should be given to the
use of intermittent sections of wide shoulder that can be placed at favorable
locations along the highway with little additional cost.
Well designed and properly maintained shoulders are needed on highways with
an appreciable volume of traffic, on expressways, and on some urban highways to
provide the advantages of:
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
Structural support to the pavement.

Space for pedestrian and bicycle use, for bus stops, for mail delivery vehicles,
and for the detouring of traffic during construction.
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design

Space away from the travelled way for vehicles to stop because of mechanical
difficulties, flat tires, or other emergencies.

Space for motorists to stop occasionally to consult road maps or for other
reasons.

Space for evasive maneuvers to avoid potential crashes or reduce their
severity.

Improved sight distance in cut sections, thereby improving safety.

Improved highway capacity because uniform speed is encouraged.

Lateral clearance for signs and guardrails.

Stormwater discharge further from the travelled way.

Space for temporary maintenance operations such as repair work and
storage.

The sense of openness created by shoulders of adequate width contributing
to driving ease and reduced stress.

Some types of shoulders enhance highway aesthetics.
All shoulders should have a minimum width of 1.0 m, but preferably should be
wider. If the shoulders are to be used by animal-drawn vehicles, pack animals or
pedestrians, even greater widths should be considered.
Paving of shoulders with a minimum width of 1.5 m shall be considered when
AADT is greater than 1,250 vehicles, when closely spaced driveways and/or
frequent turning movements affect maintenance, on high embankment sections,
on curved alignment with more than 7% superelevation, where pedestrians are
normally concentrated, and in areas with steep (>6%) and long (>100 m)
gradients. Surfacing of shoulder shall be either 230 mm thick concrete or 75 mm
thick asphalt for arterial national roads; 150 mm thick or 50 mm thick asphalt for
secondary national roads; both with edge line pavement markings. Shoulders
should be so constructed that superelevated pavements will not be soiled with
loose material during heavy rains. Shoulders should also be sufficiently porous to
permit lateral drainage of the subgrade.
There must be no difference in level between the surface of the shoulder and the
surface of the adjoining travel lane – i.e. no edge drop. Also note that pedestrians
and cyclists are unlikely to use a shoulder that has a rougher surface than the
adjacent travel lane.
3.7.4
Horizontal Clearance to Obstruction
Right-of-way should be of sufficient width to include all the cross section
elements with good balance throughout. Although it may be convenient to utilize
a uniform width of right-of-way, there should be no inhibition in the use of rightof-way greater than the minimum required for construction of locations where
operation, safety and appearance of the highway may be improved. This is
particularly pertinent at intersections with other highways and in areas where
roadsides are apt to become developed.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Right-of-way frequently is a major item of cost in highway development. In some
cases, a slight alteration in alignment or cross section elements may produce
substantial savings in cost of right-of-way without materially impairing the
effectiveness of the highway. Therefore, the economic as well as the engineering
factors affecting the design should be analyzed, with overall balance obtained
through close cooperation between the designer and the right-of-way engineer.
While stated here in reference to 2-lane highways, this point applies to any type
of highway.
Where there is likelihood of a greater number of lanes in the future, the initial
right-of-way should be sufficient to provide the wider highway section. In no
event shall less than 30 m width be taken anywhere for any national road.
Where existing right-of-way are widened through developed places it is best to
do all the widening on one side of the roadway in order to minimize property
damage, to improve the design of the new road, and not be controlled by the
existing facility.
In undeveloped areas, the minimum width of right-of-way shall be 60 m.
Consideration of the lateral offset to obstructions is needed to help:

Avoid adverse impacts on vehicle lane position and encroachments into
opposing or adjacent lanes.

Improve driveway and horizontal sight distances.

Reduce the travel lane encroachments from occasional parked and disabled
vehicles.

Improve travel lane capacity.

Minimize contact between obstructions and vehicle mirrors, car doors, and
trucks that overhang the edge when turning.
Where a curb is present, the lateral offset is measured from the face of the curb.
Appropriate clear zone widths, related to speed, traffic volume and embankment
shape, should be provided. Clear zone areas should be appropriately graded with
relatively flat slopes and rounded cross-sections, clear of all unyielding objects
such as trees, sign supports, utility poles, light poles, and any other fixed objects
that could increase crash severity. Minimum clear zone widths from the edge of
the traveled way are recommended for the following road categories:
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
Local rural roads and rural collector roads should have a minimum clear zone
of 2 to 3 m except where the less desirable option of guardrail is provided.

Urban collector roads and arterials are usually within a limited right-of-way,
with on-street parking, sidewalks, numerous fixed objects (including utility
services, drainage, and traffic signs), frequent traffic stops, and lower
operating speeds; as many clear zone concepts should be incorporated as
possible under the prevailing circumstances.
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Figure 3-15
3.7.5
Example of Road with a Good Clear Zone
Curbs
A curb, by definition, incorporates some raised or vertical element.
Configurations include both vertical and sloping curbs, designed as a separate
unit or integral with the pavement. Vertical and sloping curb designs may include
a gutter, forming a combination curb and gutter section.
Curbs serve any or all of the following purposes: drainage control, roadway edge
delineation, right-of-way reduction, aesthetics, delineation of pedestrian
walkways, reduction of maintenance operations, and assistance in orderly
roadside development.
Curbs are used extensively on all types of low-speed urban highways. Although
they are not considered fixed objects in the context of a clear zone, they may have
an effect on the trajectory of an impacting vehicle and a driver’s ability to control
a vehicle that strikes or overrides one. Sloping curbs with heights up to 100 mm
located at the outside edge of shoulder may be considered for use on high-speed
facilities when necessary for drainage considerations, restricted right-of-way, or
where there is a need for access control. Sloping curbs with 150 mm heights may
be considered for use on urban/suburban facilities with frequent access points
and intersecting streets. Vertical curbs should not be used along expressways or
other high-speed roadways because an out-of-control vehicle may overturn or
become airborne as a result of impact.
The visibility of channelizing islands with curbs and of continuous curbs along
the edges of the traveled way may be improved through the use of reflectorized
markers that are attached to the top of the curb.
When using curbs in conjunction with traffic barriers, such as on bridges,
consideration should be given to the type and height of barrier. Curbs placed in
front of traffic barriers can result in unpredictable impact trajectories, and curbs
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placed closer than 3m in front of traffic barriers may cause an impacting vehicle
to vault over the barrier.
3.7.6
Sidewalks
Justification for the construction of sidewalks depends on the potential for
vehicle-pedestrian conflicts. In general, wherever roadside and land development
conditions affect regular pedestrian movement along a highway, a sidewalk or
path suitable to the conditions should be provided. Ordinarily, little or no
provision is made for pedestrian use of highways. Justification of sidewalks in
rural areas depends upon the volume of pedestrian and vehicular traffic. Likely
sidewalk locations are at points of community development, such as schools,
local businesses and industrial plants.
When two urban communities are in proximity to one another, consideration
should be given to connecting the two communities with sidewalks, even though
pedestrian traffic may be light, to avoid driver-pedestrian conflicts along the
roadway between these communities.
Sidewalk widths in lower speed residential areas may vary from 1.2 to 2.4 m.
Additional width should be considered for higher volume sidewalks. A good
minimum width for a sidewalk that allows two people to pass is 1.8m. Sidewalks
less than 1.5 m wide require the addition of a passing section every 60 m for
accessibility. If provided, the width of a planted strip between the sidewalk and
traveled-way curb should be at least 0.6 m to allow for maintenance activity.
Where sidewalks are placed adjacent to the curb, the widths should be 0.6m
wider than the minimum required width to provide space for roadside hardware
and maintenance, and to allow for the proximity of moving traffic, the opening of
parked car doors, and bumper overhang on angled parking.
Sidewalks used for pedestrian access to schools, parks, shopping areas, and
transit stops, and sidewalks in commercial areas should be provided along both
sides of the street.
For higher speed roadways, a barrier-type rail of adequate height may be used to
separate the walkway from the traveled way.
Sidewalks should have all weather surfaces to serve their intended use and
discourage pedestrians from walking on the traveled way. Cross slope should not
exceed 2%, and they must be designed to accommodate persons with disabilities.
Consideration should be given to the relative locations of inlets and sidewalks or
crosswalks to ensure that neither grates nor ponded water are encountered by
pedestrians.
3.7.7
Drainage Channels and Side Slopes
On expressways and other arterials with relatively wide roadsides, drainage
channels and side slopes should be designed to provide a reasonable opportunity
for a driver to recover control of an errant vehicle. A Drivable Culvert End is
shown in Figure 3-16.
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Roads and streets should also be designed with a similar clear roadside, however
because of generally lower speeds and narrower side clearances along streets,
the clear roadside area concept may only be partially used.
Desirably, clear zone slope combinations should be selected so that unrestrained
vehicle occupants could be expected to sustain only minor or no injury, and the
vehicle would not incur major damage. Where right-of-way or cost constraints
make recovery distance impractical, the need for roadside barrier should be
considered.
Figure 3-16
Drivable Culvert End
Drainage Channels
Drainage channels perform the important function of collecting and conveying
surface water from the highway right-of-way. Therefore, drainage channels
should have adequate capacity for the design runoff, provide for unusual
stormwater with minimum damage to the highway, and be located and shaped to
provide a smooth transition from the roadway to the back-slope. Channels should
be protected from erosion with the least expensive protective lining that will
withstand the expected flow velocities.
The most economical method of constructing a roadside channel usually entails
the formation of open-channel ditches by cutting into the natural roadside
terrain. A channel with steep sides is the most hydraulically efficient
arrangement, but slope stability usually needs flatter slopes. Construction and
maintenance factors are also relevant, as is the offset available within the rightof-way. Potential trajectories of vehicles that run off the road are also an
important consideration in designing the roadside.
Where possible, the use of flat fore-slopes of 1V:5H provide recovery distance for
an errant vehicle and permit greater flexibility in the selection of back-slopes that
permit safe travel. The depth of channel should be sufficient to remove surface
water without saturation of the subgrade, which in turn depends on the subgrade
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soil characteristics. A broad, flat, rounded drainage channel in combination with
a 3.0 m shoulder visible to the driver provides a sense of openness. This
increases driver comfort and enhances driver’s willingness to use the shoulder in
an emergency.
The minimum desirable grade for channels should be based upon the drainage
velocities needed to avoid sedimentation. The maximum desirable grade for
unpaved channels should be based upon a tolerable velocity for vegetation and
shear on soil types. The channel grade does not have to follow that of the
roadbed. Depth and width of the channel can be varied to meet different amounts
of runoff, as well as slopes of channel, types of lining, distances between
discharge points, and lateral distance between the channel and the edge of the
travelled way.
The type of linings used in roadside channel depends upon the velocity of flow,
type of soil, and grade and geometry of the channel. Where grass will not provide
adequate protection, alternatives include concrete, asphalt, stone and nylon. If
erosive velocities are developed, a special channel design or energy dissipater
may be needed.
Median drainage channels are generally shallow depressed areas, or swales,
located at or near the center of the median and formed by the flat side-slopes of
the divided road. The swale is sloped longitudinally for drainage and water is
intercepted at intervals by inlets or transverse channels and discharged from the
roadway in storm drains or culverts.
Flumes are sometimes used to carry water collected by intercepting channels
down cut slopes and to discharge the water collected by shoulder curbs. Flumes
can either be open channels or pipes. Usually high velocities preclude sharp turns
in open flumes, and a means of dissipating energy at the outlet of the flume is
necessary.
Side Slopes
Side slopes should be designed to enhance roadway stability and to provide a
reasonable opportunity for recovery for an out-of-control vehicle.
Three regions of the roadside are important to reducing the potential for loss of
control for vehicles that run off the road: the top of the slope (hinge point), the
fore-slope, and the toe of the slope (intersection of the fore-slope with level
ground or with a back-slope forming a ditch).
The hinge point contributes to loss of steering control because vehicles tend to
become airborne in crossing this point. The fore-slope region is important in the
design of high slopes where a driver could attempt a recovery maneuver or
reduce speed before impacting the ditch area. The toe of the slope is often within
the roadside clear zone and therefore, the probability that an out-of-control
vehicle will reach the ditch is high. In this case, a smooth transition between foreand back-slopes should be provided.
Rounding at the hinge point can increase the general safety of the roadside. Foreslopes steeper than 1V:5H are not desirable because they are non-recoverable
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and their use severely limits the choice of back-slopes; when steeper slopes are
used consideration should be given to the addition of a roadside barrier.
Earth cut and fill slopes should be flattened and rounded to fit with the
topography, consistent with the type of highway. Effective erosion control, lowcost maintenance, and adequate drainage of the subgrade are largely dependent
upon proper shaping of the side slopes and slope stability. Overall economy
depends not only on the initial construction cost but also on the cost of
maintenance.
Normally, back-slopes should be 1V:3H or flatter. When space is limited backslopes steeper than 1V:3H should be evaluated for stability and crash severity.
Retaining walls set back as far as possible from the travelled way should be
considered where slopes are steeper than 1V:2H or are necessary for stability.
Cut slopes used for design should be flat enough to be stable. The slope ratio will
depend upon the nature of the material in the cut and the height of cut or fill.
Generally earth cuts will require slopes of at least 1V:1H, and 1V:2H is better if
vegetation is to be established. Rock slopes may vary from6V:1H to 1V:1H,
depending upon the nature of the rock, direction of stratification and jointing.
Desirably, the toe of the rock-cut slope should be located beyond the minimum
lateral distance from the edge of the traveled way needed by the driver of an
errant vehicle to either regain control or to slow down the vehicle. Wide shelves
at the bottom of rock cuts also have advantages in providing space for falling
boulders.
Embankment slopes outside of clear zones, likewise, will depend upon the
embankment materials. Earth slopes shall be 2V:3H or flatter. Slopes composed
of large fragments of coarse, hard rock may stand as steep as 1V:1H. Slopes
steeper than 1V:1H shall be of hand-placed materials. The use of cribbing or
retaining walls should be avoided wherever possible, as these are expensive to
build and maintain.
Earth slopes in cut or fill which are one meter high or less shall, in general, have
1V:5H slopes to provide clear zone capability.
The intersection of the back slope line with the adjacent original ground surface
shall be rounded to provide a pleasing transition from man-made to natural
conditions, to promote the growth of vegetation and to avoid slides. The original
design should provide for covering raw slopes with protective mulch of rice
straw, grass cuttings, or similar materials to protect them until voluntary
vegetation covers can establish themselves.
3.7.8
Traffic Barriers
Traffic barriers are used to prevent vehicles that leave the traveled way from
colliding with objects that have greater crash severity potential and the barrier
itself. They include both longitudinal roadside barriers and crash cushions.
Six options are available for the treatment of roadside obstacles: (1) remove or
redesign the obstacle so it can be safely traversed, (2) relocate the obstacle to a
point where it is less likely to be struck, (3) reduce impact severity by using an
appropriate breakaway device, (4) redirect a vehicle by shielding the obstacle
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with a traffic barrier, (5) delineate the obstacle if the above alternatives are not
appropriate, or (6) take no action.
Longitudinal roadside barriers are located along the roadside and in medians,
and are generally denoted as one of three types: flexible, semi-rigid or rigid. The
main difference between each type is the amount of barrier deflection that takes
place when the barrier is struck. The function and description of each roadside
barrier is provided in Table 3-45.
Flexible barrier systems undergo considerable dynamic deflection upon impact
and generally impose lower impact forces on the vehicle than semi-rigid and
rigid systems. They are designed primarily to contain rather than redirect the
vehicle and need more lateral clearance from fixed objects due to the deflection
during impact.
Semi-rigid systems achieve resistance through combined flexure and tensile
strength of the rail, with posts near the point of impact being designed to break
away. Posts outside the impact zone provide sufficient resistance to control the
deflection of the longitudinal rail to an acceptable limit and redirect the errant
vehicle along the path of traffic flow.
A rigid system (a concrete or block barrier) does not deflect substantially upon
impact, and energy is dissipated by raising and lowering of the vehicle and
deformation of the vehicle body. As the angle of impact increases barrier
deflection forces increase because of the absence of barrier deflection.
Installation of a rigid system is most appropriate where shallow impact angles
are expected such as along narrow medians or shoulders, where deflection
cannot be tolerated such as at a work zone, and where heavy traffic volumes
hamper replacement of damaged rail. Rigid systems are generally able to provide
higher containment than flexible or semi-rigid systems, and so are more
appropriate where containment is critical – such as on expressway medians and
bridges. Also they often do not need to be repaired after an impact.
Table 3-45
Types of Barrier
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Road Safety Barrier System
Samples
Function and Description
Flexible Barrier System
Four wire ropes
Wire Rope safety system work through high tension cables. An
errant vehicle deflects the wire ropes, the supporting posts bend
and the vehicle is re-directed back toward the direction travel.
Semi-Rigid System
W-Beam Steel Barrier
Beam Steel Barrier
Hollow Box Steel Barrier
Steel W-beam barriers are perhaps the most common barrier,
and are used extensively in urban and rural areas. The
effectiveness of W-beam is dependent on its length and offset
from the travelled way.
Rigid System
Stone Masonry-Parkway
T-Shape Concrete Barrer
Concrete Single Slope Barrier
Vertical Face Concrete Barrier
High Containment Conc. Barrier
Concrete barrier system maybe considered on high volume
roads as it return full functionality after impact, provide excellent
whole of life time cost and minimize the risk to workers on
roadwork sites. F-shape concrete block barrier system are
adequately and physically connected back to each other to form
a continuous system of units.
Road Work System
F-Shape Concrete Barrier
Plastic Water filled Barrier
Truck Mounted Alternator
Concrete barrier are best suited to situation where there is
limited space between the barrier and the hazard. Typically, this
occur in narrow median or in areas of restricted road cross
section
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Factors to consider in the selection of a longitudinal system include barrier
performance, lateral deflection characteristics, the space available to
accommodate barrier deflection, operational transitions and end treatments, and
initial construction and future maintenance costs.
Roadway cross section significantly affects traffic barrier performance. Curbs,
dikes, sloped shoulders, and stepped medians can cause errant vehicles to vault
or submarine a barrier or to strike a barrier so that the vehicle overturns.
Optimum barrier system performance is provided by a relatively level surface in
front of the barrier and, for semi-rigid and flexible barriers, beneath and behind
the barrier. Where curbs and dikes are used to control drainage, they should be
located either flush with the face of the barrier or slightly behind it.
Safety barrier types and their use are:

Rigid Barrier – Prevention of vehicles crossing over into incoming traffic

Semi-Rigid Barrier – Protection for vehicle traffic from large drop or from
embankment along split level street and provide barrier for vehicle reversing
from properties on high side of street.

Flexible Barrier – Restraint and redirection of errant vehicles.
Roadside Barrier System Selection
Once it has been determined that a longitudinal barrier will be installed, the
decision to use W-Beam or F-shaped concrete barrier must be made. Both
systems have passed the required testing, but each has different characteristics
that may enhance or subtract from their desirable performance under specific
circumstances. Criteria that should be considered in barrier selection include:
performance capability, deflection, site conditions (section cross-slope),
compatibility with available end treatments and adjacent barrier systems, cost,
and maintenance. The general principles of proper barrier placement must be
addressed. Some of the advantages and disadvantages of the basic barrier
systems are listed in Table 3-46.
Table 3-46
Type
Concrete Vs W-Beam Advantages/Disadvantages
Advantages
Disadvantages
W-Beam
Barrier
Relatively flexible placement criteria
Softer impact to occupants
Higher maintenance costs.
Generally damaged on impact, incurring
maintenance
costs
and
exposing
maintenance personnel to traffic
Must accommodate deflection
Less vehicle damage at impact
Concrete
Barrier
Low maintenance costs
Minimal damage on impact, lowering life
cycle cost and minimizing exposure of
maintenance personnel
No deflection
Less (or none) vehicle damage on
shallow angle impacts
Higher initial costs
High impacts to occupants
Strict placement criteria
May require installation of storm drainage
system
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Longitudinal Roadside Barriers
The primary function of longitudinal roadside barriers is to redirect errant
vehicles. They may be used to protect pedestrians, bystanders, cyclists,
playgrounds, roadside obstacles, and embankment obstacles from vehicular
traffic. They may also enhance safety at points of hazard, such as fixed objects
along the pavement edge, along long through fills, at sharp curvature, adjacent to
water courses, deep ditches and escarpments, in cuts and similar locations.
The height and slope of an embankment are key factors in determining barrier
need through a fill section. Road alignment, traffic speed and traffic volume are
also relevant in determining whether a barrier will be cost-effective for the level
of risk in an embankment situation.
Short gaps in roadside barriers should be avoided due to the need to develop
barrier strength and the need to treat each terminal end.
Barriers should be located beyond the edge of the shoulder so that the full
shoulder width may be used. The fill supporting the barrier should be sufficiently
wide to provide lateral support. Proper treatment of the exposed end of the
barrier is also important. Ends may be buried, flared back, fitted with an impactattenuating terminal piece, or protected with a crash cushion.
Options for longitudinal roadside barrier are steel beam guardrail, wire rope
guardrail and concrete barrier.
An alternative to longitudinal barriers at low volume, less hazardous locations is
the use of guide posts which outline the roadway. Guide posts are used primarily
to delineate curves, but also to mark abrupt changes in shoulder width, at
approaches to structures, at drop inlets, and at cut sections to provide warning.
They must be fitted with retro-reflective elements, on a white post. Their greatest
value is at night when their visibility is needed most. Guide posts are also
desirable in areas subject to fog. It is essential that they are crashworthy, and so
are often made of plastic.
Median Barrier
A median barrier is a longitudinal system used to minimize the possibility of an
errant vehicle crossing into the path of traffic travelling in the opposite direction.
Although cross-median collisions may be reduced by median barriers, total crash
frequency will generally increase because the space available for return-to-theroad maneuvers is decreased.
For all divided highways regardless of median width and traffic volume, the
median roadside should be examined for obstacles and lateral drop-offs that may
indicate the use of a barrier is appropriate.
As for longitudinal barriers, proper treatment of the exposed end of median
barriers is important.
An evaluation of the number of median openings, crash history, alignment, sight
distance, design speed, traffic volume and median width should be conducted
prior to installing median barriers on non-expressway facilities. Median barriers
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are always provided on expressways unless the median is so wide that an errant
vehicle is unlikely to cross it.
Types of median barrier include double-faced blocked-out steel W-beam installed
on strong posts, box-beam barrier installed on weak posts, concrete barrier, and
cable barrier installed on light steel posts.
The dynamic lateral deflection characteristics of a median barrier should be
matched to the site. The median barrier should be designed to redirect the
colliding vehicle in the same direction as the traffic flow. Maximum deflection
should be less than one-half the median width to prevent penetration into the
opposing lanes of traffic.
A concrete barrier with a sloping face has advantages on heavily traveled
facilities, in particular on highways with narrow medians, because of its high
containment capability and its rigidity and lack of deflection. Precast concrete
median barrier can be used for temporary protection of work areas and for
guiding traffic during construction, and it can be incorporated permanently as
part of the completed facility.
Crash Cushions
The primary function of crash cushions is to decelerate errant vehicles to a safe
stop. A common application of a crash cushion is to shield roadside and median
barrier terminals, especially at expressway gores.
Crash cushions should also be considered as an alternative to a roadside barrier
to shield rigid objects such as bridge piers, overhead sign supports, abutments,
and retaining wall ends.
Crash cushions should be located on a level area free from curbs or other
physical obstacles.
3.7.9
Medians
A median is the portion of a highway separating opposing directions of the
traveled way, and are highly desirable on arterials carrying four or more lanes.
Median width is the dimension between the edges of the traveled way for the
roadways in the opposing directions of travel, including the width of the left
shoulders, if any. In addition to separating opposing traffic, they provide a
recovery area for out-of-control vehicles, a stopping area in cases of emergencies,
allow space for speed changes and for storage of left-turning and U-turning
vehicles, diminish headlight glare, and provide width for future lanes. They may
also offer an open green space, provide a refuge area for pedestrians, and may
control the location of intersection traffic conflicts.
Medians should be highly visible day and night, in contrast with the through
traffic lanes, and as wide as feasible but in balance with other components of the
highway cross section. The width of the median should be great enough to
prevent most of the vehicles from reaching the opposing traffic lanes. The general
range of median widths is from a minimum of 1.5 m to a desirable dimension of
24 m for a large tractor-trailer trucks without encroaching on the through lanes
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of a major road. They may be depressed, raised, or flush with the traveled way
surface.
A depressed median is generally preferred on expressways for efficient drainage.
Median side-slopes should preferably be no steeper than 5H:IV. Drainage inlets in
the median should be designed either with the top of the inlet flush with the
ground or with culvert ends provided with traversable safety grates.
Raised medians are commonly used on arterial streets where it is desirable to
regulate left-turn movements.
Flush medians are normally used on urban arterials, but when used on
expressways a median barrier is included. They are also used on urban streets
where two-way left-turn lanes are required.
Un-signalized intersections on rural divided highways are high risk, and no
longer used in some countries in preference to roundabouts or signals. Where
un-signalized intersections are used on rural divided highways, the median
should generally be as wide as practical that will allow all selected design
vehicles to safely execute left, right and U-turn maneuvers. Wide medians also
facilitate storage area for vehicles crossing the highway at un-signalized
intersections, and at median opening serving commercial and private driveways.
However, wide medians are not suitable for signalized intersections.
Narrower medians operate better at un-signalized intersections in urban and
suburban areas, except where wider medians are needed to accommodate
turning and crossing maneuvers by larger vehicles.
Medians about 4.5 m or more in width usually are constructed without curbs.
Medians of lesser width may be curbed to provide a more positive separation.
Curbs on the median may be either mountable or barrier types. Barrier curbs are
sometimes fitting on narrow medians in built-up areas where it is necessary to
prevent drivers from turning left or making U-turns across the median. Where
barrier curbs are used, an offset is desirable.
Sometimes mountable curbs are used on narrow medians where there is little or
no reason for drivers to make left turns or U-turns, and on medians of
intermediate width. A paved flush median is also used for median widths of 2 to
4.5 m, sometimes in conjunction with median guardrail.
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Figure 3-17
3.7.10
Example of a High Speed Road with Wide Median but Hazardous Planters within
the Clear Zone
Frontage Roads
Frontage roads are used to control access to nearby arterial highways, while also
functioning as a street facility serving adjoining properties, and maintaining
traffic circulation on each side of the arterial. They segregate local traffic from
higher speed through traffic, and intercept driveways of residences and
commercial properties along the highway. In doing so, they provide more
favorable access for commercial and residential developments, while preserving
arterial highway capacity and reduced crash potential.
Frontage roads are usually parallel to arterial highways, may be on one or both
sides of the arterial, and may or may not be continuous. Where frontage roads
are continuous, they provide a backup system in case of any accident or other
expressway disruption.
Frontage roads may be one-way or two-way, depending on the service they are
intended to provide, the circumstances that apply, and the available right-of-way.
Connections between arterial and frontage roads are an important element of
design. On expressways and other arterials with high operating speeds,
connecting one-way slip ramps and their terminals should be designed to
provide for speed changes and storage.
The area between the through-traffic roadway and a frontage road or street is
referred to as the ‘outer separation’. It functions as a buffer between the through
traffic on the arterial and the local traffic on the frontage road, while providing
space for a shoulder for the through roadway plus ramp connections to or from
the through facility. Wide separations provide space for drainage and landscape
treatments, and reduce potential confusion and distraction between opposing
traffic movement.
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3.7.11
Noise Control
Noise barriers are being used to an increasing extent in recognition of the
adverse effect that noise can have on people living on, working on, or otherwise
using land adjacent to highways.
The placement and construction of noise barriers must take into consideration
their effect on:

Sign placement and the provision of lateral offsets to obstructions outside the
edge of the travelled way.

Horizontal clearance affecting stopping sight distance.
Potential noise problems should be identified early in the design process as noise
attenuation is likely to less expensive and more practical if built in to the original
design. Where possible, advantage should be taken of the terrain in forming a
natural barrier of solid material that blocks the line of sight between the noise
source and receptors while also providing an aesthetically pleasing appearance.
3.7.12
Roadside Control
Abutting property owners have right of access, but it is desirable that the
highway authority keep the full width of right-of-way unaffected for public
highway purposes by being empowered to regulate and control the location,
design and operation of access driveways in order to minimize interference to
through traffic movement. Interference resulting from indiscriminate roadside
development and uncontrolled driveway connections results in lowered capacity,
increased hazard, and early obsolescence of the highway.
To the extent practical, driveway designs should consider: (1) maintaining the
operations and efficiency on the intersecting roadway; (2) providing reasonable
access to property; (3) providing sight distance between vehicles and
pedestrians as well as efficient travel for sidewalk users; (4) incorporating
requirements for pedestrians with disabilities; (5) accommodating bicycle lanes
or paths, where present; and (6) maintaining public transportation locations,
where present.
Driveway regulations generally control right-of-way encroachment, driveway
locations, driveway design, sight distance, drainage, use of curbs, parking,
setback, lighting and signing. No advertising signs should be permitted in the
highway right-of-way. Billboards or other distracting elements outside the rightof-way which obstruct sight distance should be controlled by purchase of
easements.
For roadways without access control but with concentrated business
development along the roadside, consideration should be given to the use of
frontage roads.
Fencing is often used to delineate the control of access acquired for a highway
and to reduce the likelihood of encroachment onto the highway right-of-way.
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3.7.13
Tunnels
Tunnels may be constructed to carry highways under or through natural
obstacles or to minimize the impact of a highway on the community. General
conditions under which tunnels may be constructed include:

Long, narrow terrain ridges where a cut section may be either costly or have
environmental consequences.

Narrow rights-of-way where all of the surface area is needed for street
purposes.

Large intersection areas or a series of intersections on an irregular or
diagonal pattern.

Railroad yards, airport runways, or similar facilities.

Existing or planned parks or similar land uses.

Locations where right-of-way acquisition costs exceed the cost of tunnel
construction and operation.
Tunnel construction can be carried out by either cut-and-cover or mining
methods. A typical two lane tunnel cross section is provided in Figure 3-18.
Cut-and-cover tunnels are constructed from the surface as either trench or cutand-cover methods. In the trench method, prefabricated tunnel sections are
constructed, floated to site, sunk into a dredged trench, joined together
underwater, and then the trench is backfilled. The cut-and-cover method is the
most common type of tunnel construction for shallow tunnels, often in urban
areas. This method consists of excavating an open cut, building the tunnel within
the cut, and backfilling over the completed structure. Surface disruption and
management of utilities add to expense and difficulty for this method.
Mining methods involve constructing tunnels, without removing overlying rock
or soil, through either hard rock or soft ground. A tunnel constructed through
solid, intact, and homogeneous rock normally involves the least structural
demands and lowest construction costs. However a tunnel located below water in
material that needs immediate and heavy support may involve expensive softground tunneling techniques such as shield and compressed air methods.
The shape of the structural cross section of a tunnel varies with the type and
magnitude of loadings. In those cases where the structure will be subjected to
roof loads with little or no side pressures, a horseshoe-shaped cross section is
used. As side pressures increase, curvature is introduced into the sidewalls and
invert struts added. When loadings approach a distribution similar to hydrostatic
pressures, a full circular section is usually more efficient and economical. All
cross sections are dimensioned to provide adequate space for ventilation ducts.
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Figure 3-18
Typical Two Lane Tunnel Cross Section
Source: AASHTO, 2010, Technical Manual for Design & Construction of Road Tunnels – Civil Elements,
September. Used by Permission.
Tunnels should be as short as practical, with their length on tangent as much as
possible to minimize length and maximize stopping sight distance. Vertical
alignment needs to consider driver comfort against construction, operation and
maintenance costs. Lighting and ventilation are also important design
considerations.
Overall roadway design should avoid the need for guide signs within tunnels,
with entry and exit ramps located a sufficient distance upstream and
downstream from tunnel portals to allow for any guide signs that may need to be
placed between the tunnel and the points of entry and exit. It is also undesirable
to expect traffic to merge, diverge, or weave within a tunnel.
Design criteria used for tunnels should not differ from that used for grade
separation structures, except that minimum values are typically used because of
high cost and restricted right-of-way. Ideally, full left and right shoulder widths of
the approach expressway should be carried through the tunnel. Where it is not
practical to provide shoulders in a tunnel, arrangements must be made for
around-the-clock emergency service vehicles that can promptly remove any
stalled vehicles.
For two-lane tunnels:
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
Minimum roadway width between curbs for two-lane tunnels should be at
least 0.6m more than the approach traveled way, but not less than 7.2 m.

The curb or sidewalk on each side should be a minimum of 0.5 m wide.

The total clearance between walls should be a minimum of 9 m.

The roadway width and curb or sidewalk widths can be varied as needed
within the 9 m minimum wall clearance, as long as the above minimum
widths are maintained.
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design

Minimum vertical clearance is 5.0 m for all roads, to provide allowance for
future paving of the roadway. Any vertical clearance below 5.0 m shall be
subject to the approval of the Director of Bureau of Design.
Normally, pedestrians are not permitted in expressway tunnels, however space is
required for emergency walking and for access by maintenance personnel.
Raised sidewalks 0.7 m wide beyond the shoulder areas are desirable to both
provide a safety walk and prevent the overhang of vehicles from damaging the
tunnel wall finish or tunnel fixtures.
Directional traffic should be separated, to reduce the potential for crashes and
overcome the adverse effects of two-way traffic in a confined space, by providing
twin openings, multilevel sections, or terraced structures.
Road tunnels require drainage to deal with surface water as well as water
leakage. A drainage system can comprise of pipes, channels, sump/pumps, oil
water separators and control systems for the safe and effective collection,
storage, separation and disposal of liquids from tunnels. The drainage system
should accommodate water intrusion and firefighting requirements.
Combustable materials such as PVC and fibreglass pipe should not be used to
prevent fire from spreading through the drainage system.
The ventilation system in a tunnel maintain acceptable air quality levels for short
term exposure within the tunnel. Ventilation requirements consider two primary
criteria: handling of emissions from vehicles using the tunnel and handling of
smoke during a fire. The two main ventilation systems used in tunnels are
longitudinal ventilation and transverse ventilation.
For more information on Tunnel Design, refer to Volume 5.
Detailed information on design and construction of road tunnels, tunnel drainage
requirements and tunnel ventilation requirements may be obtained from the
AASHTO Technical Manual for Design & Construction of Road Tunnels – Civil
Elements, September 2010.
3.7.14
Pedestrian Crossings
Marked pedestrian crosswalk is one approach to get pedestrians safely across the
street, though they are often best used in combination with other treatments.
Crossings may be at an intersection or midblock, and both cases should be
considered in assessing the frequency of crossing opportunities. In general,
marked crossings alone should not be installed within an uncontrolled
environment when speeds are greater than 60 kph.
Pedestrian crossings are regularly marked in urban areas but are less common
on rural highways. Where there are pedestrian concentrations, appropriate
traffic-control devices should be used.
Pedestrian crossings or crossing facilities on arterial streets are not likely to be
used unless it is obvious to the pedestrian that it is easier to use such a facility
than to cross the traveled way. Pedestrians tend to weigh the perceived safety of
using facilities against the extra effort and time needed to cross the roadway.
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Key issues in the designing of pedestrian crossings are:

It is assumed that pedestrians want and need safe access to all destinations
that are accessible to motorists, plus they will want access to other
destinations that are not accessible to motorists such as trails and parks.

Typical pedestrian generators and destinations include residential
neighborhoods, schools, parks, shopping areas, employment centers, and
public transport stops.

All controlled intersections that have signals, stop signs, or yield signs to
facilitate motor vehicle crossing of streets and arterials must also be designed
to accommodate pedestrians.

Pedestrians need safe access at uncontrolled locations, including both
intersection and midblock locations.

Pedestrians must be able to cross streets and highways at regular intervals.
Unlike motor vehicles, pedestrians cannot be expected to go far out of their
way to take advantage of a controlled intersection.
Marked crossings are not only used to advise pedestrians where to cross the
street, but also send a message to motorists that they are in, or approaching, a
pedestrian area and can expect to encounter pedestrians crossing the street. To
effectively send this message, the design of the crosswalk must be easily
understood, clearly visible, and incorporate realistic crossing opportunities for
all pedestrians.
The width for marked crosswalks in the Philippines is 4 m.
An additional design treatment used in traffic-calming situations is the raised
crosswalk. Raised crosswalks are typically used at midblock locations to serve
not only as a visual element for motorists, but also to slow traffic speeds. They
are typically used on two-lane streets with posted speeds of less than 55 kph.
Where raised crosswalks are used, visible pavement markings are required on
the roadway approach slopes.
Refer to the DPWH May 2012 Highway Safety Design Standards, Road Safety
Design Manual, Part 2B, Section 11 to 14 for required line marking details.
Pedestrian Refuge
A median or crossing island is a raised area separating two main directions of
traffic movement. Medians tend to be long and continuous, while crossing islands
are shorter.
The primary advantage of a median or crossing island is that it separates conflicts
in time and place. The pedestrian faced with two or more lanes of traffic in each
direction must determine a safe gap for two, four, or even six lanes at a time. A
refuge allows pedestrians to cross one direction of traffic at a time. Ideally a
pedestrian should not have to cross more than two lanes at a time unless the
crossing is signal-controlled.
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Design considerations include:

Median and island crossings should be at least 1.5 m wide so that more than
one pedestrian can wait and 0.6 m detectable warning space can be provided
at both sides of the refuge. Where practical, a width of 2.4 m may be provided
to accommodate bicycles, wheelchairs, scooters, and groups of pedestrians.
(refer to DPWH Department Order No. 34, Series 2012)

Landscaping on the approach to a refuge must not block the sight lines of
pedestrians and motorists at the crossing area.

Curb ramps or full cut-throughs should be installed in all median crossings
islands (refer to DPWH Department Order No. 37, Series 2009). Cut-throughs
are more common because the median width is often insufficient to
accommodate ramps. Cut-throughs should be designed with a slope, up to a
maximum of 2%, to allow water, silt and debris to drain from the area.
Detectable warnings should be placed at both curb ramps and cut-throughs
to identify the street edge for pedestrians with vision impairments.

Median and island cut-throughs should provide a 2 m wide travel path.
Pedestrian Zebra Crossing
Zebra crossings are a type of crossing with the distinguishing feature of
alternating dark and light stripes on the road surface, from which it derives its
name. A zebra crossing typically gives extra rights of way to pedestrians, and in
the Philippines they are not controlled by traffic lights.
Their use is only justified when crossing volumes are high – refer to DPWH
Department Order No. 62, Series 2011.
Also refer to the DPWH May 2012 Highway Safety Design Standards, Part 1, Road
Safety Design Manual.
Pedestrian Actuated Traffic Signal Crossing
Where pedestrians cross roadways at signalized intersections, adequate time
should be provided to cross the entire roadway during the pedestrian phase. A
walking speed of 1.2 m/sec can be assumed in the development of phasing for
signalized intersections. However, where pedestrians who travel more slowly
may not be able to cross the roadway in one cycle, a median or crossing island (or
refuge island) should be considered. Pedestrians who often travel slowly include
very young pedestrians; older pedestrians; wheelchair, cane, and prosthesis
users; and pedestrians with vision impairments.
The placement of midblock signals may be appropriate at some locations. If used
at a location where a median is present, pedestrian actuator buttons should be
provided in the median for times when some pedestrians start too late, or when
slower pedestrians lack time to cross.
Where there are other nearby signals, midblock signals should be made part of a
coordinated signal system to increase the efficiency of traffic operations. At
locations where no other nearby signals are present and there is no coordinated
signal system, it is desirable for a midblock pedestrian signal to provide a nearby
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immediate response to induce pedestrians to walk out of their way to use the
crossing.
Pedestrian Subway or Bridge Crossing
Grade-separated pedestrian facilities provide pedestrians with a safe refuge for
crossing roadways without vehicle interference or conflict. They should be
provided where pedestrian volume, traffic volume, intersection capacity, and
other conditions favor their use, although their specific location and design need
individual study. They are warranted to accommodate heavy peak pedestrian
movements at locations such as central business districts, factories, schools, or
athletic fields, in combination with moderate to heavy vehicular traffic or where
unusual risk or inconvenience to pedestrians may result.
Subways and over-bridges are very unpopular with pedestrians and they will not
use them unless it is extremely difficult or dangerous to cross at-grade. On a
divided highway a tall wall can be built on the median to prevent pedestrians
crossing at-grade.
Generally, pedestrians prefer to use overcrossings rather than subways. Good
sight lines and lighting are needed for subways to enhance a sense of security.
Ventilation may be needed for very long subways.
Where there are frontage roads adjacent to the arterial highway, the pedestrian
crossing may be designed to span the entire facility or only the through roadway,
depending on frontage road volumes and speeds.
Fencing may be needed to prevent pedestrians from crossing arterial roads at
locations where a grade-separated facility is not provided.
Pedestrian ramps should be provided at all pedestrian separation structures.
Elevators should be considered where the length of ramp would result in a
difficult path of travel for a person with or without a disability. Facilities should
be well lit and barriers/railings provided where necessary.
Walkways for pedestrian separations should have a minimum width of 2.4 m, but
wider provisions may be needed through tunnels, where overpass screening
creates a tunnel effect, and where there are high volumes of pedestrian traffic
such as around sports stadiums and arenas.
A serious problem associated with pedestrian overcrossings and highway
overpasses with sidewalks is vandals dropping objects into the path of traffic
moving under the structure. The need for enclosing overcrossing / overpass
screens to be included should be considered when the location is:
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
Near a school, a playground, or where it could be expected that the overpass
would be frequently used by children unaccompanied by adults,

In large urban areas on overpasses used exclusively by pedestrians and not
easily kept under surveillance by police, or

Where the history of incidents on nearby structures indicates a need for
screens.
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
3.7.15
Curb-Cut Ramps
When designing a project that includes curbs and adjacent sidewalks proper
attention should be given to the needs of persons with disabilities, such as those
with mobility issues or visual impairment. Curb ramps are necessary to provide
access between the sidewalk and the street at pedestrian crossings. Detectable
warnings are needed where the curb has been removed to alert visually impaired
pedestrians that they have arrived at the street/sidewalk interface.
Design details of curb ramps will vary in relation to:

Sidewalk width

Sidewalk location with respect to the curb

Height and width of curb cross section

Design turning radius and length of curb along the curb face

Angle of street intersection

Planned or existing location of sign and signal control devices

Stormwater inlets and public utilities

Potential sight obstructions

Street width

Border width
The minimum curb ramp width should be 0.9 m, the maximum curb ramp grade
should be 6%, and the maximum cross slope on a sidewalk should be no more
than 2%. A level landing area, with a maximum cross slope of 2%, at the top of
each curb ramp should be at least 1.2 m by 1.2 m. In addition, 0.6 m detectable
warning strips are required at the bottom of curb ramps to improve detectability
by people with visual impairment.
The bottom of the curb ramp should be situated within the parallel boundaries of
the crosswalk markings and should be perpendicular to the face of the curb, or
bottom grade break, without warping in the sidewalk or curb ramp. Curb ramps
may be located either within the corner radius or on the tangent section beyond
the corner radius.
Curb ramps for persons with disabilities are not limited to intersections and
marked crosswalks, but may also be provided at midblock pedestrian crossings
and loading islands where warning signs have been installed and parking is
prohibited.
Drainage inlets should be located on the upstream side of all crosswalks and curb
ramps.
Refer to the standard design in DPWH Department Order No.37, Series 2009.
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3.7.16
Bicycle Facilities
Most of the facilities needed for bicycle travel are provided by the street and
highway system. Improvements that provide for bicycle traffic include:

Paved shoulders

Wider 4.2 m minimum outside traffic lanes if no shoulders exist

Bicycle compatible drainage grates

Manhole covers that have been adjusted to grade

Maintaining a smooth, clean riding surface
However, at certain locations, or in certain corridors, it is appropriate to
supplement the existing highway system with specifically designated bikeways.
To provide adequately for bicycle traffic, the designer should be familiar with
bicycle dimensions, operating characteristics and needs. These factors determine
acceptable turning radii, grades and sight distance. In many instances, design
features of separate bike facilities are controlled by the adjoining roadway and by
the design of the highway itself.
Bicycles are not normally allowed on fully access controlled facilities where
vehicle speeds are high.
Refer to AASHTO Guide for the Development of Bicycle Facilities, 4th Edition 2012
for further information.
Bicycle facility plan should be developed within a framework of possible
recreational and utilitarian network. This recognizes that there are two general
classes of bicycle trips: recreational trips and utilitarian trips.
Potential suitable bicycle facility corridor should be identified preferably
following closely the desired lines between origins and destinations. The length
of designated routes vary widely, but routes serving mainly recreational trips
should be over 8.0 km in length. Generally, commuting trips will be shorter than
8.0 km bike routes should take advantage of good views, historic sights, and
should be compatible with existing land use.
Bicycle facilities may be grouped into three classes (refer Figure 3-19 and Figure
3-20):
Class I
Bike path or trail. A completely separate roadway designated for the
exclusive use of bicycles; typically separated from motor-vehicle
roadway by open space or barrier.
Class II
Bike lane. A portion of roadway, which has been designated for
exclusive use by bicycle normally distinguished by a paint stripe, curb
or barrier.
Class III
Shared roadway or bike route. A roadway that has been officially
designated and marked as bicycle route but which is used by both
motor vehicle and bicycle traffic.
- Width of Bicycle Facilities
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The minimum width of one lane bikeway is 1.22 m. This allows
0.60 m for the bike width and 0.31 m for weaving space on each
side or desirable surface width of 1.22 m. or 2.44 m for a 2-lane
bikeway.
- Bikeway Capacity
Empirical studies indicates that a one way bicycle lane with an
effective width of 1.22 m. has a capacity of approximately 1275
bikes per hour. A two-way lane with an effective width of 2.44 m
has a capacity of about 1900 bikes per hour.
- Bikeway Speed
Studies show that nearly all bikes travel within a range of 11 to 24
kph with an average of 16-18 kph. Higher speeds are noted on
downgrades. AASHTO recommends the use of design speed of 24
kph and 32 kph for long downgrades.
- Grades
The maximum grade that a cyclist will be able to negotiate depends
on the capability of the individual biker, the length of grade, as well
as the condition of the bicycle and road surface, weather condition;
where the grade exceeds 5%, the length should not be not more
than 90 m and preferably not less than 30 m. AASHTO
recommends a maximum grade of 10%.
- Sight Distance
Sight distance criteria are similar to those for the design of motorvehicle facilities.
- Cross–slope
The cross-slope of the bicycle lane is the same as the slope of the
adjoining carriage way or a minimum slope of 2%.
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Figure 3-19
Bike Path - Class I & II
Source: Highway Engineering 5th Edition, Paul H. Wright and Ragner J. Paquette
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Figure 3-20
Bike Route Class I & II
Source: Highway Engineering 5th Edition, Paul H. Wright and Ragner J. Paquette
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3.7.17
Bus Turnouts
Bus turnouts serve to remove the bus from the traveled way. The location and
design of turnouts should provide ready access in the safest and most efficient
manner.
DPWH Department Order No. 58 dated 29 October 2010 on the subject ‘Revised
Guidelines in the Design and Location of Turnouts (Loading and Unloading Bays)
Along National Roads’ specifies:

Length of turnouts shall be a minimum of 60 m and a maximum of 185 m
based on a bus length of 15 m.

Minimum width of turnouts shall be 3.6 m.

Turnouts shall not be placed on or adjacent to horizontal and vertical curves
that limit sight distance in either direction.

Specific location of turnouts shall take into consideration (a) proximity to
where pedestrians are concentrated, (b) being ‘downstream’ of any road
intersections, (c) a minimum spacing of 500 m in urban areas and 1000 m in
rural areas, and (d) an offset stagger of at least 30 m for turnouts on opposite
sides of the road.

Pavement type for a turnout shall be the same as for the through carriageway.

Pavement thickness shall be at least 100 mm for asphalt and 230 mm for
concrete plus supporting base layers.

Pedestrian sidewalk or platform, in no case lower than the existing sidewalk,
of minimum width 2.0 m shall be provided alongside the turnout.

For adequate turnout surface drainage (a) the cross slope of the turnout shall
be 0.50% steeper than the cross slope of the adjacent through lane, (b) the
gutter alongside the turnout shall have the same slope as the existing
carriageway, (c) on carriageway with existing storm drains the turnout shall
be provided with inlets at 20 m spacing and (d) on level carriageways with no
existing storm drains, the gutter alongside the turnout shall be sloped at
0.30%.
The following design details are also relevant in the specific case discussed
below:
Expressways–Buses should leave the expressway at an intersection, pick up and
drop passengers at a bus bay, and then regain the expressway.
Arterials – The deceleration lane should be tapered at a minimum angle of 5
longitudinal to 1 transverse. The loading area should provide at least 15 m length
for each bus, with a width of at least 3.0 m and preferably 3.6 m. The merging or
re-entry taper may be more abrupt than the deceleration taper, but preferably
not sharper than 3:1.
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3.7.18
Park-and-Ride Facilities
Park-and-ride facilities should be located adjacent to the street or highway and
be visible enough to attract use by commuters. They should be located as close to
residential areas as practical to minimize travel by vehicles with only one
occupant. Bicycle and pedestrian access to park-and-ride facilities should also be
considered.
The size of the park-and-ride lot is dependent upon its design volume, the
available land area, and the size and number of other parking lots in the area. The
lot should include a drop-off facility close to the station entrance, plus a holding
or short-term parking area for passenger pickup. This area should be clearly
separated from park-and-ride areas.
Consideration should be given to the location for bus loading and unloading, taxi
service, bicycle parking, and special parking for persons with disabilities.
Conflicts between pedestrians and vehicles should be minimized by locating
parking aisles perpendicular to the bus roadway. All bus roadways should have a
minimum width of 6.0 m to permit the passing of standing buses. Parking spaces
should be 2.7 m by 6.0 m for full-sized cars. Pedestrian paths from parking spaces
to loading areas should be as direct as practical.
Sidewalks should be a minimum of 1.5 m wide and loading areas should be 3.6 m
wide. Principal loading areas should be provided with sidewalk curb ramps.
Facilities for locking bicycles should be provided where needed.
Grades of parking areas should be set for effective drainage. Recommended
grades along vehicle paths within the parking area are 1% minimum and 2%
desirable with a maximum of 5%. Grades of over 8% parallel to the length of the
parked vehicles should be avoided. Curvature, radius of planned vehicular paths
within the parking area, and access roads should be sufficiently large to
accommodate the vehicles they are intended to serve.
Access to lots should be at points where they will avoid disruption to through
traffic, at least 90 m from other intersections with sufficient sight distance to exit
and enter the lot. There should be at least one exit and entrance for every 500
spaces in a lot. If practical, exits and entrances should be provided at separate
locations and should access different streets. It is also desirable to provide
separate access for public transport vehicles.
Curb corner radius should be at least 9.0 m, although 4.5 m radius is suitable for
access points used exclusively by passenger vehicles.
Passenger loading areas should be provided with shelters sufficient to at least
accommodate off-peak passenger volumes with provision for extension at a later
date. The size of the shelter can be determined by multiplying the number of
passengers by a factor of 0.3 to 0.5 m2. Accessories to be provided with the
shelter include lighting, benches, route information, trash receptacles and public
telephones. The area delineating the passenger refuge area should be curbed to
reduce the height between the ground and the first bus step, and to reduce
encroachment by buses on the passenger area.
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3.8
Highway Capacity
The term ‘highway capacity’ pertains to the ability of a roadway to accommodate
traffic. Highway capacity is considered in two broad categories – uninterrupted
flow or open highway conditions and interrupted flow as at intersections.
3.8.1
General Characteristics
The term ‘capacity’ is defined as the maximum number of vehicles that can pass
over a given section of a lane or roadway in one direction (or both directions in
the case of a 2-lane highway) during a given period under prevailing roadway
and traffic conditions. The range of traffic flow on a highway can vary from very
light to volumes that equal the capacity of the facility. When the traffic volume on
a highway is equal to the capacity of that highway, operation is congested with all
vehicles travelling at nearly the same speed, which is established by the speed of
the slowest vehicle. There is little or no opportunity for passing and operating
conditions are not only generally unsatisfactory but are unstable. Any slight
disruption can cause stop and go operation or complete stoppage. Between the
extremes of capacity volume and a very low volume, the average speed at which
traffic can travel and have freedom to maneuver is directly related to traffic
volume. Speed is the most sensitive measure of operating conditions as related to
traffic volume that has yet been established. Three appropriate ranges of average
running speeds are:

Average running speed 70-80 kph: Applicable for most main rural 2-lane,
two-way highways and all rural multi-lane highways in level and in rolling
terrain.

Average running speed 65-70 kph: Applicable for highways approaching
urban areas, for multilane highways in mountainous terrain and wherever
feasible for two-lane highways in mountainous terrain.

Average running speed 55-65 kph: Applicable for 2-lane rural highways in
mountainous terrain where design for higher running speed is not feasible.
Also applicable to controlled access highways in urban areas where during
the design hour it is expected that freedom to travel at high speed will be
curtailed by DHV (design hourly volume) traffic.
Traffic volumes resulting in running speeds lower than those indicated above
would have to operate in a manner considered too restrictive and is generally not
economically feasible to provide a facility that will permit running speeds during
the design hour higher than those above.
Capacities in the following table are for highways constructed to high standard;
namely 3.65 m lanes, adequate shoulders, lateral clearances of about 1.83 m or
more, adequate stopping sight distance throughout, no trucks and no restrictive
passing sight distance when the highway is 2-lane, two-way.
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Table 3-47
Possible and Design Capacities of Highways Constructed to High Design
Standard in Terms of Passenger Cars per Hour
Possible
Capacity
Design Capacity for Average Running Speed* of
55-65
65-70
70-80
Vehicles / hour
Vehicles / hour
Vehicles / hour
Vehicles / hour
Type of Highway
2-lane, two way (Total)
2,000
1,500
1,500
900
Multilane (per lane)
2,000
1,500
1,200
1,000
* Running speed for the faster vehicles will exceed the average running speed shown by an amount varying
from about 8 kph (in the case of 55-65 kph average) to about 11 kph (in the case of the 70-80 kph average).
3.8.2
Capacity as a Design Control
Design Service Flow Rate versus Design Volume
The design volume is the volume of traffic projected to use a particular facility
during the design life, which is usually 10 to 20 years in the future. Design
volumes are estimated in the planning process and are often expressed as the
expected traffic volume during a specified design hour.
Design service flow rate is the maximum hourly flow rate that a highway with
particular design features would be able to serve without the degree of
congestion falling below a pre-selected level.
A major objective in designing a highway is to create a facility with dimensions
and alignment that can serve the design service flow rate, which should be at
least as great as the flow rate during the peak 15-minute period of the design
hour, but not so great as to represent an extravagance in the design. Where this
objective is accomplished, a well-balanced, economical highway facility will
result.
Measures of Congestion
Three key considerations in geometric design are the roadway design, the traffic
using the roadway, and the congestion on the roadway. The first two items can be
measured in exact units, but the third is more difficult.
For uninterrupted traffic flow (i.e. flow not influenced by signalized
intersections), traffic operational conditions are defined by using three primary
measures: speed, volume (or rate of flow), and density. Density describes the
proximity of vehicles to one another and reflects the freedom to maneuver within
the traffic stream. As density increases from zero, the rate of flow also increases
because of more vehicles on the roadway. However, as density continues to
increase, a point is reached at which speed declines due to vehicle interactions.
The maximum flow rate is reached at which the high density of traffic results in
markedly decreased speeds and a reduced flow rate. This maximum rate of flow
for any given facility is defined as its capacity. As capacity is approached, flow
becomes more unstable because available gaps in the traffic stream become
fewer and fewer. At capacity, there are no usable gaps in the traffic stream, and
any conflict from vehicles entering or leaving the facility, or from internal lane
changing maneuvers, creates a disturbance that cannot be effectively damped or
dissipated. Thus, operation at or near capacity is difficult to maintain for long
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periods of time without the formation of upstream queues, and forced or
breakdown flow becomes almost unavoidable. For this reason, most facilities are
designed to operate at volumes less than their capacity.
For interrupted flow, such as that occurring on streets where traffic is controlled
by signals, the highway user is not as concerned with attaining a high travel
speed as with avoiding lengthy stops at intersections or a succession of stops at
several intersections. Average stopped-time delay is the principal measure of
effectiveness used in this case, as it is reasonably easy to measure, and is closely
related to motorist perceptions of the quality of traffic flow.
Relationship between Congestion and Traffic Flow Rate
Congestion does not necessarily involve a complete stoppage of traffic flow,
although it does restrict normal free flow. For any given class of highway,
congestion increases with an increase in flow rate until the flow rate is almost
equal to the facility’s capacity. As the traffic flow rate approaches capacity, any
minor disruption in the free flow of traffic may cause traffic on a roadway to
operate on a stop-and-go basis, with a resulting decrease in traffic flow.
Highway sections where the paths of traffic merge and diverge within relatively
short distances are called ‘weaving sections’. Average running speed within
weaving sections, and hence the degree of congestion, is a function not only of the
volume of traffic involved in the weaving movements but also of the distance
within which the weaving maneuvers are completed.
On arterial streets within the urban environment, average running speed varies
only slightly with changes in traffic flow rate. However, delay at signalized
intersections may increase dramatically as flow rates approach capacity. In such
cases greater degrees of congestion occur, and these result in reduced overall
travel speeds, higher average travel times, and traffic spill-back into upstream
intersections.
Acceptable Degrees of Congestion
Motorists will generally accept a higher degree of congestion in those areas
where improvements can be made only at a substantial cost. Normally they are
also more willing to accept a higher degree of restraint on short trips than they
are on long trips, but they are not satisfied with the type of operation that occurs
when the volume of traffic approaches the facility’s capacity.
For highway administrators, the degree of congestion that highway users
experience is related to the availability of resources. Historically, funds have
never been sufficient to meet all needs.
Principles for Acceptable Degrees of Congestion
The appropriate degree of congestion that should be used in planning and
designing highway improvements is determined by weighing the desires of
motorists against the resources available for satisfying those desires.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Reconciliation of Principles for Acceptable Degrees of Congestion
The degree of congestion that should not be exceeded during the design year on a
proposed highway can be realistically assessed by:

Determining the operating conditions that the majority of motorists will
accept as satisfactory.

Determining the most extensive highway improvement that the DPWH
considers practical.

Reconciling the demands of the motorist and the general public with the
finances available to meet those demands.
This requires an early decision to be made as to the degree of congestion that
should not be exceeded during the design period.
Expressways
Expressway capacity needs are determined from directional design hourly
volumes (DDHV) for the appropriate design period. In large metropolitan areas,
the selection of appropriate design traffic volumes and design periods may be
influenced by system planning. Segments of expressways may be constructed or
reconstructed to be commensurate with either intermediate traffic demands or
with traffic based on the completed system. Actual in-service capacity may be
affected by the design of weaving sections and ramp terminals.
Other Multilane Highways
For multilane highways other than expressways, capacity is affected by
intersections, both un-signalized and signalized; driveways and interference
from traffic entering and leaving through-traffic lanes; and sharp curves, steep
grades and cross-sectional dimension limitations. All of these conditions combine
to cause congestion at lower traffic volumes than would otherwise be the case for
highways designed with ideal features and protected by full access control.
3.8.3
Factors Other Than Traffic Volume That Affect Operating Conditions
Highway Factors
Most modern expressways have adequate cross-sectional dimensions, but many
are not ideal with respect to design speed, weaving section design, and ramp
terminal design.
On other classes of multilane highways, intersections often interfere with the
free-flow operation of traffic. Development adjacent to the highway with
attendant driveways and interference from traffic entering and leaving the
through-traffic lanes cause an increase in congestion and may increase crash
frequency even at relatively low volumes. Sharp curves and steep grades cannot
always be avoided, and it is sometimes appropriate to compromise on crosssectional dimensions. All of these conditions combine to cause congestion to be
perceived at lower traffic volumes than would be the case for highways designed
with ideal features and protected by full access control.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
For urban streets with signalized intersections at relatively close intervals, the
traffic volumes that could otherwise be served are reduced because a portion of
each signal cycle is assigned exclusively to the crossing highway.
For a highway that is deficient in some of its characteristics and where the traffic
stream is composed of a mixture of vehicle classes, compensatory adjustment
factors need to be applied to the traffic flow rates used as design values for ideal
highway conditions. These adjustments are needed to determine the volume of
mixed traffic that can be served under minimum acceptable operating conditions
on the highway under consideration.
Alignment
For traveling at any given speed, the better the roadway alignment, the more
traffic it can carry. It follows that congestion will generally be perceived at lower
volumes if the design speed is low. The highway should be subdivided into
sections of consistent geometric design characteristics for analysis. A single
limiting curve or steep grade in an otherwise gentle alignment will thus be
identified as the critical feature limiting roadway capacity.
Weaving Sections
Weaving sections are highway segments where the pattern of traffic entering and
leaving at contiguous points of access results in vehicle paths crossing each other.
Where the distance in which the crossing is accomplished is relatively short in
relation to the volume of weaving traffic, operations within the highway section
will be congested. A reduction in operating speed of about 10 kph below that for
which the highway as a whole operates can be considered a tolerable degree of
congestion for weaving sections.
Ramp Terminals
Ramps and ramp terminals are features that can adversely influence operating
conditions on expressways if the demand for their use is excessive or if their
design is deficient. When congestion develops at expressway ramp junctions,
some through vehicles avoid the outside lane of the expressway, thereby adding
to the congestion in the remaining lanes. Thus, if there are only two lanes in one
direction, the efficiency per lane is not as high on the average as that for three or
more lanes in one direction.
Apart from the effect on through traffic, traffic that uses the ramp is exposed to a
different form of congestion related to the total volume of traffic in the outside
lane of the expressway in the vicinity of the ramp junction.
Traffic Factors
Traffic streams are usually composed of a mixture of vehicles: passenger cars,
trucks, buses and occasionally recreational vehicles and bicycles. Furthermore,
traffic does not flow at a uniform rate throughout the hour, day, season, or year.
Consideration should be given to these two variables in deciding upon volumes of
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
traffic that will result in acceptable degrees of congestion, and also upon the
period of time over which the flow should extend.
Acceleration, Deceleration and Climbing Lanes
Drivers entering a divided highway from a turning roadway accelerate until the
desired highway speed is reached. Drivers leaving a divided highway at an
interchange are required to reduce speed as they exist onto a ramp. Because the
change in speed is usually substantial, provision should be made for acceleration
and deceleration to be accomplished on auxiliary lanes to minimize interference
with through traffic and to reduce crash potential. Such an auxiliary lane,
including tapered areas, is referred to as a speed change lane. Refer to
recommended minimum deceleration and acceleration taper lengths in Table 348 to Table 3-51.
In the case of drivers accelerating to pass a slow vehicle that has moved into a
climbing lane, the climbing lane provides an auxiliary lane even though the
change in speed actually occurs in the through lane.
Deceleration distances indicated by Table 3-48 should be increased for a
downgrade and may be reduced for an upgrade in accordance with Table 3-49.
The ratio from this Table 3-51 multiplied by length in Table 3-50 gives length of
speed change lane on grade.
Table 3-48
Deceleration Distances Required for Cars on a Level Grade
Length of Deceleration including Diverge Taper (m)
Design Speed of
approach road (kph)
Table 3-49
Design Speed of Exit Curve (kph)
20
30
40
50
60
70
80
90
50
30
25
15
60
50
40
30
15
70
70
60
50
40
20
80
95
85
75
60
45
25
90
120
110
100
85
70
50
25
100
150
140
130
115
100
80
55
30
110
180
175
160
150
130
110
90
60
Correction to Deceleration Distance as a Result of Grade
Ratio of ‘Length on Grade’ to ‘Length on Level’
Grade
Upgrade
Downgrade
0 – 2%
1.0
1.0
3 – 4%
0.9
1.2
5 – 6%
0.8
1.35
Source: Austroads Guide to Road Design, Part 4A Unsignalized and Signalized Intersections, Table 5.3
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Table 3-50
Length of Acceleration Lanes for Cars on Level Grade
Design Speed
of
Road Entered
(kph)
Length of Acceleration Lane including Merge Taper (m)
0
20
30
40
50
105
105
105
105
60
125
125
125
125
125
70
165
150
150
150
150
150
80
235
220
210
195
170
170
170
Design Speed of Entry Curve (kph)
50
60
70
80
90
330
315
305
290
260
220
190
190
100
450
435
425
410
380
340
290
220
110
610
595
585
570
540
500
450
320
Source: Austroads Guide to Road Design, Part 4A Unsignalised and Signalised Intersections, Table 5.4
Table 3-51
Correction of Acceleration Distances as a Result of Grade
Ratio of Length on Grade to Length on Level* for Design Speed of Turning
Roadway Curve (kph)
Design Speed
of
Road Entered
(kph)
Stop
30
50
1.3
1.3
60
1.3
1.3
1.3
80
1.3
1.3
1.4
1.4
100
1.3
1.4
1.5
1.5
110
1.4
1.5
1.6
1.6
3 to 4% Upgrade
50
60
5 to 6% Upgrade
80
Stop
30
1.4
1.5
50
60
80
1.5
1.5
1.5
1.5
1.5
1.7
1.9
1.6
1.6
1.7
1.9
2.2
2.5
1.8
1.8
2.0
2.2
2.6
3.0
3 to 4% downgrade for all speeds
5 to 6% downgrade for all speeds
50
0.70
0.60
60
0.70
0.60
80
0.65
0.55
100
0.60
0.50
110
0.60
0.50
Source: Austroads Guide to Road Design, Part 4A Unsignalized and Signalized Intersections, Table 5.5ble
U-Turn Slots
The provision of U-turn slots requires careful consideration as maneuvers in
urban or heavily developed residential or commercial sectors may create
inefficient traffic operations. Vehicles that slow down or stop in a lane primarily
used by through traffic may cause a decrease in the capacity for through traffic
and an increase in the potential for rear-end collisions. Also U-turns where
medians are present may have limited sight distance, which may also increase
the accident potential.
3.8.4
Levels of Service
The ‘level of service’ characterizes the operating conditions on a facility in terms
of traffic performance measures related to speed and travel time, freedom to
maneuver, traffic interruptions, and comfort and convenience. The levels of
service range from A (least congested) to F (most congested), as shown in Table
3-52.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Table 3-52
Level of Service
General Definitions of Levels of Service
General Operating
Conditions
Min. Speed
(kph)
Service Flow
Rate
Max. VIC (4
LANE)
A
Free flow
97
700
0.318
B
Reasonably free flow
97
1120
0.509
C
Stable flow
97
1844
0.247
D
Approaching unstable flow
92
2015
0.918
E
Unstable flow
85-80
2200-2300
1.00
F
Forced or breakdown flow
Variable
VAR
VAR
Source: Table 2-4 (Modified) AASHTO, 2011, A Policy on Geometric Design of Highway and Streets 6th
Edition. Used by Permission.
The relationship between highway type and location and the level of service
appropriate for design is summarized in Table 3-53.
Table 3-53
Guidelines for Selection of Design Levels of Service
Appropriate Level of Service for Specified Combinations of Area & Terrain Type
Functional
Class
Rural – Level
Rural – Rolling
Rural –
Mountainous
Urban and
Suburban
Expressway
B
B
C
C or D
Arterial
B
B
C
C or D
Collector
C
C
D
D
Local
D
D
D
D
Source: Table 2-5, AASHTO, 2011, A Policy on Geometric Design of Highway and Streets 6th Edition. Used
by Permission.
A highway design should aim to provide the highest level of service practical to
balance the operating conditions that the majority of motorists will accept as
satisfactory, the most extensive highway improvement that the DPWH considers
practical, and the demands of the motorist and the general public with the
finances available to meet these demands.
3.8.5
Design Service Flow Rates
The traffic flow rates that can be served at each level of service are termed
‘service flow rates’. Once a particular level of service has been identified as
applicable for design, the corresponding service flow rate logically becomes the
design service flow rate. The design service flow rate recognizes that the longterm average hourly flow rate of a section of a highway network will depend on
entry and exit links and the degree of congestion.
Once a level of service has been selected, it is desirable that all elements of the
roadway are designed consistent to this level. This consistency of design service
flow rate results in near-constant freedom of traffic movement and operating
speed, and flow interruptions due to bottlenecks can be avoided.
Whether designing an intersection, interchange, arterial, or expressway, the
selection of the desired level of service should be carefully considered because
the traffic operational adequacy of the roadway is dependent on this choice.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Weaving Sections
Weaving sections occur where one-way traffic streams cross by merging and
diverging maneuvers. The design level of service of a weaving section is
dependent on its length, number of lanes, acceptable degree of congestion, and
relative volumes of individual movements. Weaving sections are designed,
checked, and adjusted so that the level of service is consistent with the remaining
highway. Large-volume weaving movements usually result in considerable
friction and reduction in speed of all traffic. Further, there is a definite limit to the
amount of traffic that can be handled on a given weaving section without undue
congestion. This limiting volume is a function of the distribution of traffic
between the weaving movements, the length of weaving section, and the number
of lanes.
Weaving sections may be simple or multiple. A simple weaving section comprises
a single entrance followed by a single exit. A multiple weaving section consists of
two or more overlapping weaving sections. Multiple weaving sections occur
frequently in urban areas where there is need for collection and distribution of
high concentrations of traffic.
The weaving section should have a length and number of lanes based on the
appropriate level of service. Refer Table 3-54.
Table 3-54
Service Flow Rates Under Ideal Conditions of a Major Weaving Section (pc/h)
Length of Weaving Section (ft)
LOS
500
1,000
1,500
2,000
2,500
500
1,000
N=3; NWL=2
1,500
2,000
2,500
N=3; Nwl=3
A
17450
1750
1760
1765
1770
1800
1805
1805
1805
1805
B
3200
3250
3260
3270
3285
3360
3380
3400
3400
3400
C
4210
4280
4310
4335
4350
4460
4520
4550
4560
4570
D
5010
5110
5150
5150
5190
5360
5450
5480
5500
5510
E
5957
6071
6186
6301
6416
6316
6431
6545
6600
6775
N=4; NWL=2
N=4; NWL=3
A
2280
2300
2320
2320
2320
2370
2380
2380
2385
2385
B
4140
4210
4230
4250
4260
4390
4440
4450
4460
4470
C
5400
5510
5550
5580
5600
5820
5900
5940
5970
5980
D
6300
6530
6580
6620
6640
6960
7080
7140
7160
7180
E
7942
8095
8248
8401
8554
8421
8574
8717
8880
9033
N=5; NWL=2
N=5; NWL=3
A
2800
2840
2850
2860
2860
2920
2930
2950
2955
2955
B
5040
5120
5150
5180
5190
5400
5450
5470
5500
5510
C
6530
6650
6710
6750
6770
7100
7230
7270
7300
7330
D
7680
7840
7910
7950
7970
8480
8630
8700
8740
8740
E
8889
8889
8889
8889
8889
10527
10718
10909
11100
11292
Note:
NWL =
N
=
number of lanes from which weaving movement can be made with one or no lane changes
number of lanes in the weaving section
Source: Analysis of Freeway Weaving sections
National Cooperative Highway Research Program, Transportation Research Board
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Multilane Highways without Control of Access
Multilane highways may be treated similarly to expressways if major crossroads
are infrequent, many of the crossroads are grade separated, adjacent
development is sparse so as to generate little interference with traffic flow, or
some combination thereof. Even on those highways where such interference is
currently only marginal, the designer should anticipate that by the design year
the interference may be extensive unless access to the highway is well managed.
Where there are major crossroads or where adjacent development results in
more than slight interference, the facility should be treated as a multilane
highway without access control.
Arterial Streets and Urban Highways
It is often difficult to establish design service flow rates for arterial streets and
urban highways, because the level of service provided by such facilities does not
remain stable with the passage of time, and tends to deteriorate in an
unpredictable manner. The capacity of an arterial is generally dominated by the
capacity of its individual signalized intersections. The level of service for a
section of an arterial is defined by the average overall travel speed for the
section.
Intersections
Design capacities of intersections are affected by a very large number of
variables. To the extent that these variables can be predicted for the design year,
design capacities can be estimated by procedures for signalized and unsignalized intersections given in Section 4.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
4
Intersection Design
An intersection is the general area where two or more highways join or cross,
within which are included the roadway and roadside facilities for traffic
movements in that area. It is an important part of a highway since much of the
efficiency, safety, speed, cost of operation, and capacity are dependent upon its
design.
There are three types of intersections, namely, at-grade intersections and grade
separated intersections without ramps, and interchanges.
4.1
Intersection at Grade
4.1.1
Introduction
Except for expressways, all highways have intersections at-grade, so that the
intersection area is a part of every connecting road or street. In this area must
occur all crossing and turning movements.
Un-channelized intersections are the cheapest and least elaborate.
An at-grade intersection in which traffic is directed into definite paths by islands
for the efficient operation of all traffic, through cross and turning, is termed a
channelized intersection. Extensive intersection areas where drivers may have
considerable latitude in movement should be avoided. Likewise, intersections
which are ‘over channelized’ may be undesirable in that drivers may be confused
as to proper paths to follow between several islands. The design and location of
the islands must be given careful study and several alternate plans should be
considered. In designing, the position and shape of the islands are best
determined graphically on a scale drawing of the intersection after the desired
paths of all movements have been delineated thereon, with due recognition of the
volumes and types of traffic making each turn. (Refer Figure 4-1 for General
Types of AT-Grade Intersections). The guidelines in designing at-grade
intersections are:
4-1

Provide sight distance at least equal to the stopping distance for the design
speed of the road, and preferably more. In line with this suggestion, avoid if
possible intersections in cuts or near the crest of vertical curves. Where
necessary to protect the intersection from future obstruction by billboards or
houses, purchase additional right-of-way at the time the road is built.

If possible, avoid placing the intersection where the major road is on a sharp
horizontal curve.

Intersections where either road is on a steep grade are difficult to design, so
avoid them if possible. Where they cannot be avoided try to preserve the
grade of the major road with as little change as possible and warp the minor
road into it.

Where an intersection occurs in fill with the major road considerably higher
that the minor road, make certain that the ramps of the minor road begin
some distance from the edge of the major road. This will provide an easy and
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
safe place for the minor road traffic to pause. If this is not done there is a
temptation for the minor road traffic to ‘make a run for it’.

4.1.2
Make the intersection as nearly right angled as possible. Right angle
intersections are safer and cheaper to construct. For every acute intersection
it is usually better to stagger the minor road, or resort to some simple form of
channelization.
Factors affecting Design
In varying degrees, four principal factors determine the character of any
intersection. These factors are Traffic, Physical factors, Economic factors and
Human factors. These factors should be known and evaluated prior to selecting
the type of design to be used.
Human Factors
Human factors include:

Driving habits

Ability of drivers to make decisions

Driver expectancy

Decision and reaction times

Conformance to natural paths of movement

Pedestrian use and habits

Bicycle traffic use and habits
Traffic Engineering Considerations
Traffic engineering considerations include:

Classification of each intersecting roadway

Design and actual capacities

Design-hour turning movements

Size and operating characteristics of vehicle

Variety of movements, such as diverging, merging, weaving and crossing

Vehicle speeds

Bus involvement

Crash experience

Bicycle movements

Pedestrian movements
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Figure 4-1
4-3
General Types of At-Grade Intersections
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Physical Elements
Physical elements include:

Character and use of abutting property

Vertical alignments at the intersection

Sight distance

Angle of the intersection

Conflict area

Speed-change lanes

Geometric design features

Traffic control devices such a sign, signal, marking or other service

Lighting equipment

Roadside design features

Environmental factors

Cross walks

Driveways

Access management treatments
Economic Factors
Economic factors include:
4.1.3

Cost of improvements

Effect of controlling or limiting rights-of-way on abutting residential or
commercial properties where channelization restricts vehicular movements

Energy consumption
Types of Intersections
The basic types of intersections are three-leg (T), four-leg, multi-leg, and
roundabouts. Further classification of these basic types includes such variables as
un-channelized, flared, and channelized. Additional variations include offset
intersections, which are two adjacent T intersections that function similar to a
four-leg intersection, and indirect intersections that provide one or more of the
intersection movements at a location away from the primary intersection.
Three-Leg Intersections
The most common type of three-leg intersection maintains the normal pavement
width of both highways except for the paved corner radii or where widening is
needed to accommodate the selected design vehicle.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Figure 4-2
Three-Leg Intersections
This type of un-channelized intersection is generally suitable for junctions of
minor or local roads, and junctions of minor roads with more important
highways where the angle of intersection is not generally more than 30 degrees
from perpendicular.
In rural areas, this intersection type is usually used in conjunction with two-lane
highways carrying light traffic. In suburban or urban areas, it may be suitable for
higher volumes and for multilane roads. Where speeds, or turning movements, or
both, are high, an additional area of surfacing or flaring may be provided for
maneuverability. Additional control can be gained by marking a separate lane
exclusively for left-turning vehicles. Where right-turning movement from the
through highway is substantial, a right-turn lane can be added. Where leftturning movement from the through highway and the through movement are
substantial, a left-turn lane or a less safe right-hand passing lane can be added.
Where channelization is provided, islands and turning roadways should be
designed with path analysis using turning circle templates or CAD software to
accommodate the wheel tracks of each vehicle movement while providing
optimum crossing paths and storage for pedestrians within the intersection.
Where traffic demand at an intersection approaches or exceeds the capacity of a
two-lane highway and where signal control may be needed in rural areas, it may
be desirable to convert the two-lane highway to a divided section through the
intersection.
Four-Leg Intersections
The overall design principles, island arrangements, use of auxiliary lanes, and
many other aspects of the previous discussion of three-leg intersection design
also apply to four-leg intersections.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Figure 4-3
Four-Leg Intersections
The simplest form of un-channelized four-leg intersection is suitable for minor or
local roads, and often for intersections of minor roads with major highways. A
skewed intersection leg should not be more than 30 degrees from perpendicular.
Approach pavements are continued through the intersection, and the corners are
rounded to accommodate turning vehicles.
A flared intersection has additional capacity for through and turning movements.
Auxiliary lanes on each side of the normal pavement at the intersection enable
through vehicles to pass slow-moving vehicles preparing to turn right. Depending
on the relative volumes of traffic and the type of traffic control used, flaring can
be accomplished by parallel auxiliary lanes or by pavement tapers. Parallel
auxiliary lanes are essential where traffic volume on the major highway is near
the uninterrupted-flow capacity of the highway or where through and cross
traffic volumes are sufficiently high to warrant signal control. The length of
added pavement should be determined as it is for speed-change lanes, and the
length of uniform lane width, exclusive of taper, should normally be greater than
45 m on the approach side of the intersection.
A flared intersection with a median lane for left-turn movements may be suitable
for two-lane highways where speeds are high, intersections are infrequent, and
the left-turning movements from the highway could create a conflict. Such an
arrangement is also better suited for intersections with signal control.
Channelized four-leg intersections are often provided at major intersections
where turning movements by large vehicles are to be accommodated and at
minor intersections where the angle of turn greatly exceeds 90 degrees.
A simple configuration with right-turn roadways in all four quadrants of the
intersection is suitable where sufficient space is available and right-turn volumes
are high. Where one or more of the right-turning movements need separate
turning roadways, additional lanes are generally needed for the complementary
left-turning movements.
Intersections with divisional islands on the crossroad fit a wide range of volumes,
with its capacity governed by the roadway widths provided through the
intersection.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
For an intersection on a two-lane highway operating at near capacity or carrying
moderate volumes at high speeds, a configuration with channelized left-turn
lanes may be considered. Auxiliary lanes are used for speed changes,
maneuvering, and storage of turning vehicles. The form of channelization on the
crossroad should be determined based on the cross and turning volumes and the
sizes of vehicles to be accommodated.
The simplest form of intersection on a divided highway has paved areas for right
turns and a median opening. Often the speeds and volumes of through and
turning traffic justify a higher type of channelization suitable for the predominant
traffic movements. Channelization is often used at intersections on divided
highways. Right-turning roadways with speed-change lanes and median lanes for
left turns afford both a high degree of efficiency in operation and high capacity
and permit through traffic on the highway to operate at reasonable speed.
Intersection configurations with dual left-turn lanes where required for high
volumes need traffic signal control with a separate single phase for the dual leftturn movement. Auxiliary lanes in the median may be separated from the
through lanes by pavement markings or by an elongated island. Pavement
markings, contrasting pavements, and signs should be used to discourage
through drivers from entering the median lane inadvertently.
Where roadways cross one another at an angle other than 90 degrees, the effects
of the skew can be mitigated by providing right-turn roadways or realigning the
cross street to reduce the impact of the skew.
Un-signalized four-leg intersections are very high risk and are not to be used.
Conversion to two three-leg or a roundabout would provide safer options in such
cases.
Multi-Leg Intersections
Multi-leg intersections, with five or more intersection legs, should be avoided
wherever practical. At locations where multi-leg intersections are used, it may be
satisfactory to have all intersection legs intersect at a common paved area if
volumes are light and stop control is used. At other than minor intersections,
traffic operational efficiency can often be improved by reconfigurations that
remove some conflicting movements from the major intersection. Such
reconfigurations are accomplished by realigning one or more of the intersecting
legs and combining some of the traffic movements at adjacent subsidiary
intersections. Other options include redesigning the intersection to a roundabout
or converting one or more legs to one-way operation away from the intersection.
Roundabouts
The properly designed roundabout is the safest form of at-grade intersection,
with the least number of conflict points and ability to control speeds to a safe
level.
A roundabout is an intersection with a central island around which traffic must
travel counter-clockwise and in which entering traffic must yield to circulating
traffic. Other common traffic control features of roundabouts include:
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design

Channelized approaches.

Appropriate curvature designed into the intersection geometry so that travel
speeds on the circulatory roadway are typically less than 50 kph.

Splitter islands on each leg of the roundabout that have multiple roles to (a)
separate entering and exiting traffic, (b) deflect and slow entering traffic, and
(c) provide a pedestrian refuge. Refer Figure 4-4 and Figure 4-5.

On high speed roads, the splitter island should generally extend across the
full width of the approach lanes as seen by the approaching driver. The length
should provide for adequate deflection and deceleration. Refer Figure 4-6.
Figure 4-4
Urban Splitter Island Details: Low Speed Approach
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Figure 4-5
Urban Splitter Island
Figure 4-6
Splitter Island for High Speed Approach
Source: DPWH Road Safety Design Manual May 2012
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Figure 4-7
Roundabouts
Source: DPWH Road Safety Design Manual May 2012
Roundabouts can be classified into three basic categories according to size and
number of lanes:

Mini-roundabouts

Single-lane roundabouts

Multilane roundabouts
Any of these categories may be appropriate for application in suburban, or urban
areas. Single-lane and multilane roundabouts may be used in rural areas, but
mini-roundabouts are not suitable for high-speed rural roads. Roundabouts in
urban areas may need smaller circle diameters due to smaller design vehicles,
constraints of existing right-of-way, and more extensive pedestrian and bicycle
features. Roundabouts in rural areas typically have higher approach speeds, and
this may need special attention to visibility, approach alignment, and crosssectional details. Suburban roundabouts may combine features of both urban and
rural roundabouts. Further details and discussion of roundabout could be found
in DPWH Road Safety Design Manual May 2012.
4.1.4
Plan of Traffic Volume
The first step in the development of intersection geometric design should be a
complete analysis of current and future traffic demand, including pedestrian,
bicycle and transit users. The need for right- and left-turn lanes to minimize the
interference of turning traffic with the movement of through traffic should be
evaluated concurrently with the potential need for obtaining any additional
right-of-way needed. Along a highway with a number of signalized intersections,
the locations where turns will, or will not, be accommodated should also be
examined to permit optimal traffic signal coordination.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
4.1.5
Basic Principles of Intersection Design
A junction has to fulfill a number of general design requirements:

Recognizable: if a limited number of junction forms are used, with uniform
(main) characteristics, then the road user will recognize the situation as such
more quickly and the situation will comply with expectations.

Visible: a junction must be visible in time, conspicuous and clearly
recognizable and locatable as such. To see something from a distance, it must
have at least a certain size to which the road user’s attention and perception
can be directed. Contrast, color, shape and movement are important factors
here. Finally, the information ‘signs’ need to be installed in logical, clearly
visible places in the field of vision.

Overseeable: when approaching a junction the road user must be able to
oversee the junction and part of the approaching roads and any traffic on
them, in time.

Comprehensible: a junction is comprehensible to the road user when
perceptions of shape, scope, signposting, marking and traffic regulations can
be interpreted quickly, correctly and unambiguously on approach.

Negotiable: negotiability of a junction means that the various design elements
fit together sufficiently smoothly. The elements themselves must also be
easily negotiated.

Balance: a balanced junction structure means that the various design
elements (including the approach roads) and the traffic measures must form
an integrated whole.

Completeness: a junction is complete when the traffic at the site of the
junction can continue on its way in all possible and intended directions.
Safer junctions tend to have a more compact layout than others, in the interests
of simplicity and speed control. Having said this, it is also important to avoid a
cramped layout.
Reduction of conflicts is very important, but where conflicts cannot be avoided
they should happen at slow speed, so the consequences of a collision are not so
serious. This is why roundabouts are by far the safest form of at-grade
intersection.
Continuity of intersection type along a route is helpful for safety.
Closely spaced offset intersections should be avoided, wherever practical.
Table 4-1 lists the key traffic management considerations to be taken into
account, in association with cost, in selecting the type of intersection to be used
in any given situation.
4.1.6
Geometric Design at Intersections
Alignment and grades are subject to greater constraints at or near intersections
than on the open road. Combinations of grade lines that make vehicle control
difficult should be avoided at intersections. Substantial grade changes should also
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
be avoided as much as possible. Normally, the grade line of the major road should
be carried through the intersection and that of the minor road should be adjusted
to it. This involves transition in the crown of the minor road to an inclined cross
section at its junction with the major road. Reference point of radius is at the
edge of carriageway. For simple un-channelized intersections involving low
design speeds and stop or signal control, it may be desirable to warp the crowns
of both roads into a plane at the intersection. Changes from one cross slope to
another should be gradual. Intersections at which a minor road crosses a
multilane divided highway with a narrow median on a superelevated curve
should be avoided whenever practical.
Local Urban Streets (within a municipality or city, providing access to property, usually lowspeed and low-volume)
The intersection and approach areas where vehicles are stored while waiting to
enter the intersection should be designed with a relatively flat grade; the
maximum grade on the approach leg should not exceed 2% where practical and
at least 30 m in length.
At street intersections, there are two distinct radii that need to be considered –
the effective turning radius of the turning vehicle and the radius of the curb
corner radius (or return). The effective turning radius is the minimum radius
appropriate for turning from the right-hand travel lane on the approach street to
the appropriate lane of the receiving street. This radius is determined by the
selection of a design vehicle appropriate for the streets being designed and the
lane on the receiving street into which that design vehicle will turn. For local
urban streets this radius should be at least 7.5 m.
The radius of the curb corner radius (or return) should be no greater than that
needed to accommodate the design turning radius. However, the curb return
radius should be at least 1.5 m to enable effective use of street-sweeping
equipment.
In industrial areas with no on-street parking, the radius of the curb return should
not be less than 10 m.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Table 4-1
Intersection
Type
Unsignalized
Key Traffic Management Considerations in Selection of At-Grade Intersection
Type
Key Traffic Management Selection Considerations
Basic
Used at urban locations where low-volumes and low-speeds occur and at rural sites with low cross and turning volumes
Design to be compact and low cost, and can be used with any road surface
Offers no protection to turning traffic and causes through traffic to slow when such movements occur
Skewed T-intersection layouts may have safety problems
Capacity
Unsignalized intersections rely on gap selection for the entry of minor road traffic into or across the major road and
for left-turn movements from the major road
Higher conflicting volumes result in increased delays and higher risk of crashed
Auxiliary lanes
Auxiliary lanes may be added to the basic intersection to improve safety
Typically used in rural areas where high-speed, low-volume traffic occurs and the volume and slow maneuvering of
turning traffic is sufficient to create conflict with following traffic
Generally intended to provide separation for the maneuvering of a single vehicle
Right passing lane allows traffic to bypass a vehicle waiting to turn left and is not intended for locations with regular
queing
Left-turning lane allows traffic to decelerate and turn without affecting through vehicles
Lanes should be installed on a needs basis and may not be required on all approaches
Channelized
Used where there is a need to define vehicle paths where there would otherwise be a large area of pavement; also used
where conflicting vehicle travel paths need to be separated and where approaches are at odd angles or multi-leg
Applicable where turning traffic movements are heavy with frequent queuing
Necessary where refuges for pedestrians are required or where street furniture requires protection
Used to cater for unusual maneuvers or where unwanted movements are to be eliminated
Staggered T-intersection
Generally used to treat left-angle crashes at existing low-volume rural cross intersections
Left-right configuration on two-lane, two-way roads may develop safety problems at high traffic volumes
Roundabout
Generally much safer than traffic signals in terms of crash severity
Usually less delay than traffic signals during the off-peak periods, leading to less overall delay to traffic throughout the day
Readily caters for heavy left-turns
Can be used in local streets
Controls vehicle speeds as a traffic calming measure (e.g. at the extremities of high pedestrian activity area)
May not be suitable where strong coordination of movement required along a route
May not be able to provide sufficient capacity for high-volume sites
Dominant flows on one approach may lead to excessive delay on the subsequent approach
Does not allow positive regulation of particular movements (e.g. access to local street from a busy road)
Are less safe than signals for on-road cyclists, particularly at multi-lane roundabouts
Need to consider pedestrians of all types (young, aged and impaired) and cyclist movement and numbers
Need to consider bus and long-vehicle requirements (e.g. movement and numbers)
Signalized
Provides the most suitable treatment for very high-volume sites
Enables efficient coordination along traffic routes
Can readily accommodate priority measures for public transport
May provide controlled crossings for pedestrians and cyclists
Safer for cyclists than multi-lane roundabouts
Preferred for sites with high pedestrian activity
Generally preferred to roundabouts for intersections along freight routes
Are not desirable from a safety perspective in high-speed environments
The following guidelines indicate those circumstances where signals are of significant benefit. The terms ‘major’
and ‘minor’ are used respectively to indicate the roads carrying the larger and smaller traffic volume:
1. Traffic volume: Where the volume of traffic is the principal reason for providing a control device, traffic signals may be
considered when the major road carries at least 600 vehicles/hour (two-way) and the minor road concurrently carries at
least 200 vehicles/hour (highest approach volume) on one approach over any four hours of an average day.
2. Continuous traffic: Where traffic on the major road is sufficient to cause undue delay or hazard for traffic on a minor road, traffic
signals may be considered when the major road carries at least 900 vehicles/hour (two-way) and the minor road concurrently
carries at least 100 vehicles/hour (highest approach volume) on one approach over any four hours of an average day.
3. Pedestrian safety: To help pedestrians cross a road in safety, signals may be considered when over any four
hours of an average day, the major road carries 600 vehicles/hour (two-way); or where there is a central
pedestrian refuge at least 1.2m wide, the major road flow exceeds 1000 vehicles/hour; and 150
pedestrians/hour or more cross the major road.
4. Crashes: Where the intersection has an average of three or more reported casualty crashes per year over a
three-year period where the accidents could have been prevented by traffic signals, and traffic flows are at least
80% of the volume warrants in (1) and (2) above.
5. Combined factors: In exceptional cases, where no single guideline is satisfied but where two or more of the
warrants given in (1), (2) and (3) above are satisfied to the extent of 80% or more of the stated criteria.
Source: AustRoads Guide to Traffic Management, Part6 Intersections, Interchanges and Crossings, Table 2.4
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Rural Collector Roads (outside municipalities or cities, linking important centers)
Where turning volumes are substantial, speed-change lanes and channelization
should be considered.
A stopping area that is as level as practical should be provided for approaches on
which vehicles may be required to stop.
Urban Collector Roads (within municipalities or cities, linking local streets to important centers)
The pattern of traffic movements at intersections and the volume of traffic on
each approach during one or more peak periods of the day, including pedestrian
and bicycle traffic, are relevant for the design of traffic control devices, lane
width, and where applicable, the type and extent of channelization needed. The
arrangement of islands and the shape and length of auxiliary lanes may differ
depending on whether or not signal control is used. The composition and
character of traffic is a design control, with movement of large trucks needing
larger intersection areas and flatter approach grades.
The number and location of approach roadways and their angles of intersection
are major controls for intersection geometric design, the location of islands, and
the types of control devices. Intersections at grade should preferably be limited
to no more than four approach legs. When two crossroads intersect the collector
highway in close proximity, they should be combined into a single intersection.
Sight Distance
Intersections have the potential for several different types of vehicular conflicts.
The likelihood of these conflicts actually occurring can be greatly reduced
through the provision of adequate intersection sight distances and appropriate
traffic controls.
Sight distance is provided at intersections to allow drivers to perceive the
presence of potentially conflicting vehicles in sufficient time to stop or adjust
their speed as appropriate. The driver of a vehicle approaching an intersection
should have an unobstructed view of the entire intersection, including any trafficcontrol devices, and sufficient lengths along the intersecting highway. The sight
distance needed under various assumptions of physical conditions and driver
behavior is directly related to vehicle speeds and to the resultant distances
traversed during perception-reaction time and braking.
Sight distance is also provided at intersections to allow the drivers of stopped
vehicles a sufficient view of the intersecting highway to decide when to enter the
intersecting highway or to cross it. In some cases, a major-road vehicle may need
to stop or slow to accommodate a maneuver by a minor-road vehicle. In such a
case, to enhance traffic operations, intersection sight distances that exceed
stopping sight distances are desirable along the major road.
Specified areas along intersection approach legs and across their included
corners should be clear of obstructions that might block a driver’s view of
potentially conflicting vehicles. These specified areas are known as clear sight
triangles. The dimensions of the legs of the sight triangles depend on the design
speeds of the intersecting roadways and the type of traffic control used at the
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
intersection. Two types of clear sight triangles are considered in intersection
design – approach sight triangles and departure sight triangles.
Approach Sight Triangles
Each quadrant of an intersection should contain a triangular area free of
obstructions that might block an approaching driver’s view of potentially
conflicting vehicles. The length of the legs of this triangular area, along both
intersecting roadways, should be such that drivers can see any potentially
conflicting vehicles in sufficient time to slow or stop before colliding within the
intersection. Note that greater than necessary sight distance can lead to excessive
approach speeds.
Although desirable at higher volume intersections, approach sight triangles are
not needed for intersection approaches controlled by stop signs or traffic signals.
In such cases, the need for approaching vehicles to stop at the intersection is
determined by the traffic control devices and not by the presence or absence of
vehicles on the intersecting approaches.
Departure Sight Triangles
A second type of clear sight triangle provides sight distance sufficient for a
stopped driver on a minor-road approach to depart from the intersection and
enter or cross the major road. Departure sight triangles should be provided in
each quadrant of each intersection approach controlled by stop or yield signs.
Departure sight triangles should also be provided for some signalized
intersection approaches.
Identification of Sight Obstructions within Sight Triangles
The profiles of the intersecting roadways should be designed to provide the
recommended sight distances for drivers on the intersection approaches. Within
a sight triangle, any object at a height above the elevation of the adjacent
roadways that would obstruct the driver’s view should be removed or lowered, if
practical. Such objects may include buildings, parked vehicles, highway
structures, roadside hardware, hedges, trees, bushes, un-mowed grass, tall crops,
walls, fences, and the terrain itself. Particular attention should be given to the
evaluation of clear sight triangles at interchange ramp/crossroad intersections
where features such as bridge railings, piers, and abutments are potential sight
obstructions.
The determination of whether an object constitutes a sight obstruction should
consider both the horizontal and vertical alignment of both intersecting
roadways, as well as the height and position of the object. It should be assumed
that the driver’s eye is 1.08 m above the roadway surface and that the object to
be seen is 1.08 m above the surface of the intersecting road.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Intersection Control
The recommended dimensions of the sight triangles vary with the type of traffic
control used at an intersection because different types of control impose
different legal constraints on drivers. Procedures to determine sight distances at
intersections are presented below according to different types of traffic control,
as listed below.
Case A – Intersections with No Control
For intersections not controlled by yield signs, stop signs, or traffic signals, the
driver of a vehicle approaching an intersection should be able to see potentially
conflicting vehicles in sufficient time to stop before reaching the intersection. The
location of the decision point of the sight triangles on each approach is
determined from a model that is analogous to the stopping sight distance model,
with slightly different assumptions based on determined driver detection,
recognition, reaction time gaps and intersection behavior characteristics.
Table 4-2 shows the distance traveled by an approaching vehicle during
perception-reaction and braking time as a function of the design speed of the
roadway on which the intersection approach is located. These distances should
be used as the legs of the sight triangles. The distances shown in this table are
generally less than the corresponding values of stopping sight distance for the
same design speed, since field observations show that motorists slow down to
some extent on approaches to uncontrolled intersections.
Table 4-2
Case A ‘No Traffic Control’ – Length of Sight Triangle Leg
Design Speed
Length of Leg (m)
20
20
30
25
40
35
50
45
60
55
70
65
80
75
90
90
100
105
110
120
120
135
130
150
Source: Table 9-3 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
Where the grade along an intersection approach exceeds 3%, the leg of the clear
sight triangle along that approach should be adjusted by multiplying the
appropriate sight distance from Table 4-2 by the appropriate adjustment factor
from Table 4-3.
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Table 4-3
Approach
Grade (%)
Adjustment Factors for Sight Distance Based on Approach Grade
Design Speed (kph)
20
30
40
50
60
70
80
90
100
110
120
130
-6
1.1
1.1
1.1
1.1
1.1
1.1
1.2
1.2
1.2
1.2
1.2
1.2
-5
1.0
1.0
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.2
1.2
1.2
-4
1.0
1.0
1.0
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
-3 to +3
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
+4
1.0
1.0
1.0
1.0
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
+5
1.0
1.0
+6
1.0
1.0
1.0
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
Note: Based on ratio of stopping sight distance on specified approach grade to stopping sight distance on
level terrain.
Source: Table 9-4 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
If the sight distance given in Table 4-2, as adjusted for grades, cannot be
provided, consideration should be given to installing regulatory speed signing to
reduce speeds or installing stop signs on one or more approaches.
No departure sight triangle is needed at an uncontrolled intersection because
such intersections typically have very low traffic volumes.
Case B – Intersections with Stop Control on Minor Road
Intersection sight distance criteria for stop-controlled intersections are longer
than stopping sight distance to allow the intersection to operate smoothly.
Minor-road drivers can wait until they can proceed safely without forcing a
major-road vehicle to stop. For intersections with stop control on the minor road,
departure sight triangles should be considered for the three cases below.
Case B1 – Left Turn from Minor Road
Departure sight triangles for traffic approaching from either the right or the left
should be provided for left turns from the minor road onto the major road for all
stop-controlled approaches. The length of the departure sight triangle along the
major road in both directions is the recommended intersection sight distance in
this case.
The design values for intersection sight distance for passenger cars are shown in
Table 4-4. No adjustment of the recommended sight distance values for the
major-road grade is generally needed because both the major and minor-road
vehicle will be on the same grade when departing from the intersection.
However, if the minor-road design vehicle is a heavy truck and the intersection is
located near a sag vertical curve with grades over 3%, then an adjustment to
extend the recommended sight distance based on the major-road grade should be
considered.
Sight distance for left turns at divided-highway intersections should consider
multiple design vehicles and median width.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
If the sight distance along the major road, including any appropriate adjustments,
cannot be provided, consideration should be given to installing regulatory speed
signing on the major-road approaches.
Table 4-4
Case B1 ‘Left Turn from Stop’ – Design Intersection Sight Distance
Intersection Sight Distance for Passenger Cars
Design Speed (kph)
Stopping
Sight Distance (m)
Calculated (m)
Design (m)
20
20
41.7
45
30
35
62.6
65
40
50
83.4
85
50
65
104.3
105
60
85
125.1
130
70
105
146.0
150
80
130
166.8
170
90
160
187.7
190
100
185
208.5
210
110
220
229.4
230
120
250
250.2
255
130
285
271.1
275
Note: Intersection sight distance shown is for a stopped passenger car to turn left onto a two-lane highway
with no median and grades 3% or less. For other conditions, the time gap should be adjusted and the sight
distance recalculated.
Source: Table 9-6 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
Case B2 – Right Turn from Minor Road
A departure sight triangle for traffic approaching from the left should be
provided for right turns from the minor road onto the major road. The
intersection sight distance for right turns is determined in the same manner as
for Case B1, except that the time gaps should be adjusted. Table 4-5 provides the
resulting design values for intersection sight distance for passenger cars.
When the minimum recommended sight distance for a right-turn maneuver
cannot be provided consideration should be given to installing regulatory speed
signs or other traffic control devices on the major-road approaches.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Table 4-5
Case B2 ‘Right Turn from Stop’ and Case B3 ‘Crossing Maneuver’ – Design
Intersection Sight Distance
Design Speed (kph)
Stopping Sight
Distance (m)
Intersection Sight Distance for
Passenger Cars
Calculated (m)
Design (m)
20
20
36.1
40
30
35
54.2
55
40
50
72.3
75
50
65
90.4
95
60
85
108.4
110
70
105
126.5
130
80
130
144.6
145
90
160
162.6
165
100
185
180.7
185
110
220
198.8
200
120
250
216.8
220
130
285
234.9
235
Note: Intersection sight distance shown is for a stopped passenger car to turn left onto a two-lane highway
with no median and grades 3% or less. For other conditions, the time gap should be adjusted and the sight
distance recalculated.
Source: Table 9-8 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
Case B3 – Crossing Maneuver from Minor Road
In most cases, the departure sight triangles for left and right turns onto the major
road, as for Cases B1 and B2, will also provide adequate sight distance for minorroad vehicles to cross the major road. However it is advisable to check the
availability of sight distance for the following crossing maneuvers:

Where left or right turns or both are not permitted from a particular
approach and the crossing maneuver is the only legal maneuver;

Where the crossing vehicle would cross the equivalent width of more than six
lanes; or

Where substantial volumes of heavy traffic cross the highway and steep
grades may slow the progress of a vehicle through the intersection.
Case C – Intersections with Yield Control on Minor Road
Drivers approaching yield signs are permitted to enter or cross the major road
without stopping if there are no potentially conflicting vehicles on the major
road. The sight distances needed by drivers on yield-controlled approaches
exceed those for stop-controlled approaches. The following two cases of sight
triangles require consideration.
Case C1 – Crossing Maneuver from Minor Road
The length of the leg of the approach sight triangle along the minor road to
accommodate the crossing maneuver from a yield-controlled approach is given in
Table 4-6. The distances in this table are based on the same assumptions as those
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
for Case A except that minor-road vehicles that do not stop are assumed to
decelerate to 60% of the minor-road design speed rather than 50%. Distances
and times should be adjusted as necessary for other design vehicles, the grade of
the minor-road approach, and the width of median on the major-road.
The length of the sight triangle leg of the approach along the major road is given in
Table 4-7.
Table 4-6
Design
Speed (kph)
Case C1 – Crossing Maneuvers from Yield-Controlled Approaches, Length of
Minor Road Leg and Travel Times
Minor-Road Approach
Length of Leg (m)
Travel Time tga,b (s)
Travel Time (tg) (s)
Calculated Value
Design Value c,d
20
20
3.2
7.1
7.1
30
30
3.6
6.2
6.5
40
40
4.0
6.0
6.5
50
55
4.4
6.0
6.5
60
65
4.8
6.1
6.5
70
80
5.1
6.2
6.5
80
100
5.5
6.5
6.5
90
115
5.9
6.8
6.8
100
135
6.3
7.1
7.1
110
155
6.7
7.4
7.4
120
180
7.0
7.7
7.7
130
205
7.4
8.0
8.0
a
For minor-road approach grades that exceed 3%, multiply the distance or the time in this table by the
appropriate adjustment factor from Table 4-3.
b Travel time applies to a vehicle that slows before crossing the intersection but does not stop.
c The value of tg should equal or exceed the appropriate time gap for crossing the major road from a stopcontrolled approach.
d Values shown are for a passenger car crossing a two-lane highway with no median and with grades of
3% or less.
Source: Table 9-9 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
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Table 4-7
Major Road
Design
Speed (kph)
Case C1 ‘Crossing Maneuver at Yield-Controlled Intersections’ – Length of Sight
Triangle Leg along Major Road
Stopping
Sight
Distance (m)
Design Values (m) for Minor-Road Design Speed (kph)
20
30-80
90
100
110
120
130
20
20
40
40
40
40
45
45
45
30
35
60
55
60
60
65
65
70
40
50
80
75
80
80
85
90
90
50
65
100
95
95
100
105
110
115
60
85
120
110
115
120
125
130
135
70
105
140
130
135
140
145
150
160
80
130
160
145
155
160
165
175
180
90
160
180
165
175
180
190
195
205
100
185
200
185
190
200
210
215
225
110
220
220
200
210
220
230
240
245
120
250
240
220
230
240
250
260
270
130
285
260
235
250
260
270
280
290
Note: Values in the table are for passenger cars and are based on the unadjusted distances and times in
Table 4-6, which may be adjusted using the factors in Table 4-3.
Source: Table 9-10 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
Case C2 – Left or Right Turn from Minor Road
The length of the leg of the approach sight triangle along the minor road to
accommodate right turns without stopping should be 25 m. This distance is
based on the assumption that drivers making left and right turns without
stopping will slow to a turning speed of 16 kph.
The leg of the approach sight triangle along the major road is similar to the
major-road leg of the departure sight triangle for a stop-controlled intersection in
Cases B1 and B2, however the time gaps should be increased. The appropriate
lengths of the sight triangle leg are shown in Table 4-8.
Yield-controlled approaches generally need greater sight distance than stopcontrolled approaches, especially at four-leg yield-controlled intersections where
the sight distance needs of the crossing maneuver should be considered. If sight
distance sufficient for yield-control is not available, use of a stop sign instead of a
yield sign should be considered. In addition, at locations where the
recommended sight distance cannot be provided, consideration should be given
to installing regulatory speed signing or other traffic control devices at the
intersection on the major road to reduce the speeds of approaching vehicles.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Table 4-8
Case C2 ‘Left or Right Turn at Yield-Controlled Intersections’ – Design
Intersection Sight Distance
Design Speed (kph)
Stopping Sight
Distance (m)
Length of Leg for Passenger Cars
Calculated (m)
Design (m)
20
20
44.5
45
30
35
66.7
70
40
50
89.0
90
50
65
111.2
115
60
85
133.4
135
70
105
155.7
160
80
130
177.9
180
90
160
200.2
205
100
185
222.4
225
110
220
244.6
245
120
250
266.9
270
130
285
289.1
290
Note: Intersection sight distance is shown for a passenger car making a right or left turn without stopping,
onto a two-lane road.
Source: Table 9-12 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
Case D – Intersections with Traffic Signal Control
At signalized intersections, the first vehicle stopped on one approach should be
visible to the driver of the first vehicle stopped on each of the other approaches.
Left-turning vehicles should have sufficient sight distance to select gaps in
oncoming traffic and complete left-turns. Apart from these sight conditions, there
are generally no other approach or departure sight triangles needed for
signalized intersections. Signalization may be an appropriate crash
countermeasure for higher volume intersections with restricted sight distance
that have experienced a pattern of sight-distance related crashes.
Case E – Intersections with All-Way Stop Control
At intersections with all-way stop control, the first stopped vehicle on one
approach should be visible to the drivers of the first stopped vehicle stopped on
each of the other approaches. There are no other sight distance criteria
applicable to intersections with all-way stop control.
Case F – Left Turns from Major Road
All locations along a major highway from which vehicles are permitted to turn
left across opposing traffic, including intersections and driveways, should have
sufficient sight distance to accommodate the left-turn maneuver. Left-turning
drivers need sufficient sight distance to decide when to turn left across the
lane(s) used by opposing traffic. Sight distance design should be based on a left
turn by a stopped vehicle, since a vehicle that turns left without stopping would
need less sight distance. The sight distance along the major road to accommodate
left turns is provided in Table 4-9.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Table 4-9
Case F ‘Left Turn from the Major Road’ – Design Intersection Sight Distance
Design Speed (kph)
Stopping Sight
Distance (m)
Length of Leg for Passenger Cars
Calculated (m)
Design (m)
20
20
30.6
35
30
35
45.9
50
40
50
61.2
65
50
65
76.5
80
60
85
91.7
95
70
105
107.0
110
80
130
122.3
125
90
160
137.6
140
100
185
152.9
155
110
220
168.2
170
120
250
183.5
185
130
285
198.8
200
Note: Intersection sight distance is shown for a passenger car making a left turn from an undivided highway.
For other conditions and design vehicles, the time gap should be adjusted and the sight distance
recalculated.
Source: Table 9-14 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
Effect of Skew
Intersecting streets and roads should intersect at right angles, wherever
practical, and should not intersect at an angle less than 60 degrees.
When two highways intersect at an angle less than 60 degrees, some of the
factors for determination of intersection sight distance may need adjustment. For
example, the length of the travel paths for some turning and crossing maneuvers
will be increased. In addition, in the acute-angle quadrant, drivers often need to
turn their heads considerably to see across the entire clear sight triangle. For
these reasons, it is recommended that the sight distance criteria for Case A
should not be applied to oblique-angle intersections and that sight distances at
least equal to those for Case B should be provided.
Railroad Grade Crossing
For Local Rural and Urban Roads and Streets, plus Collector Roads, appropriate
grade-crossing warning devices should be installed at railroad-highway grade
crossings on local roads and streets.
Sight distance is an important consideration at railroad-highway grade crossings.
There should be sufficient sight distance along the road and along the railroad
tracks for an approaching driver to recognize the crossing, perceive the warning
device, determine whether a train is approaching, and stop if necessary.
Sufficient sight distance along the track is also needed for drivers of stopped
vehicles to decide when it is safe for proceed across the tracks.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
The roadway width at all railroad crossings should be the same as the width of
the approach roadway.
Crossings that are located on bicycle routes that are not perpendicular to the
railroad may need additional paved shoulder for bicycles to maneuver over the
crossing.’
Sidewalks should be provided at railroad grade crossings to connect existing or
future walkways that approach these crossings. Provisions for future sidewalks
should be incorporated into design, if they are be anticipated.
Curbs
General Considerations
Concrete curbs are generally classified as barrier curbs or mountable curbs.
Either type can be constructed in many different shapes, depending on regional
preferences, purposes and construction costs. Typical cross sections of the most
commonly used curbs and curb and gutter sections are shown in Figure 4-8.
Barrier curbs, also known as straight curbs, resemble the stone slabs used
originally for curbs and form abrupt obstacles to vehicles leaving pavements.
Mountable curbs, sometimes referred to as roll curbs, have sloping faces that
allow vehicles to encroach on them without damaging tires and wheels; and if the
slopes are gentle enough, cars can cross them to access driveways. Curbs that
cannot be crossed without damage or discomfort must have sections where the
heights of the curbs are reduced for vehicular entrances. The low portions are
usually referred to as depressed curbs. When curbs are constructed in areas
where buildings have already been erected and driveways established, the
depressed portions can be easily designated, but in developing areas where the
driveways have not been located, mountable curbs are usually preferred.
Either type of curb can have an apron or gutter section attached and become a
combined curb and gutter. Combined curb and gutter sections are commonly
used along streets and parking lots in urban areas, especially with asphalt
pavements, to provide the advantages of stable concrete gutters with sustainable
flow lines along the curbs.
Because concrete can be readily shaped to transition between cross-sections,
curbs can be tapered to meet ramps for pedestrian crossings where these are
preferred or to meet requirements for the disabled.
Curbs built monolithically with concrete pavements project above the pavement
at the edges. These are referred to as monolithic curbs or integral curbs, as
opposed to separate curbs. As the edges of concrete pavements with the added
thickness of curbs are stronger and stiffer, deflections caused by heavy wheels
close to the outside edges are reduced. Where curbs are cast on hardened
concrete slabs, resulting in cold joints between the curbs and slabs, there are
opportunities for planes of weakness and water penetration, which can result in
shortened service life.
A separate curb and gutter must be tied to the pavement slab with deformed
steel bars if there is to be effective load transfer. If a curb is separate from the
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
pavement the joint between the pavement and the curb may require
maintenance.
Basic Requirements
Curbs must meet several basic requirements if they are to serve their intended
purposes and have long service life. Curbs must have the required mass, stability
and strength to withstand the impacts of traffic and the effects of their
environments and to maintain their positions even when crossed by traffic or
struck by snowplows. They must have the strength to bridge small areas where
subgrade support is inadequate.
The standard sections shown here have been proven to have the necessary mass
for strength and stability. Separate curb and gutter sections should be at least
two feet wide with greater widths having more stability for a relatively small
amount of construction costs.
Another important requirement is visibility. Because of their light and reflective
surfaces, concrete curbs can be easily seen, even at night when pavements are
wet. The washings of rain and the removal of debris by street sweeping are
sufficient to meet this requirement.
Design Requirements
The design of curbs is more dependent on successful experience and regional
preferences, and less on rigorous analyses compared to the design of other
concrete structures. The review of a few available publications on concrete curbs
reveals what types have been used but there is no specification regarding the
forces acting on curbs or calculations on reducing stresses to acceptable limits.
This is because experience has shown that curb sections proportioned to have
adequate mass to provide the required stability are unlikely to fail from any
imposed loads or impacts. Like other concrete members, curbs should be jointed
or reinforced to accommodate the effects of volume changes due to shrinkage,
temperature, or moisture changes.
Besides meeting the basic requirements discussed above, good curb design
should allow economical and efficient construction. Economical construction
results from designs that reduce labor, permit the use of any of the efficient curb
forming machines available today, and take advantage of standardized cross
sections that provide the necessary properties. Minor variations in shapes or
dimensions that add nothing to the strength or utility should be avoided.
Templates or "mules" can be manufactured for any desired cross- sections form
curb shapes, but they are costly. If the entire cost of a special mule must be
amortized on a single project, the cost of the curb must necessarily be increased
to cover that expense, even though the utility of the curb is not increased over
that of a similar standard section.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Figure 4-8
Typical Highway Curbs
Source: AASHTO, 2011, A Policy on Geometric Design of Highway and Streets 6th Edition. Used by
Permission.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
4.1.7
Turning Roadways and Channelization
Turning roadways and channelization are a key aspect of intersection design.
Types of Roadways to Intersections
General
The widths of roadways to intersections are governed by the volumes of turning
traffic and the types of vehicles to be accommodated. In most cases, turning
roadways are designed for use by right-turning traffic. The widths for rightturning roadways may also be applied to other roadways within an intersection.
There are three typical types of right-turning roadways at intersections: (1) a
minimum edge-of-traveled-way design, (2) a design with a corner triangular
island, and (3) a free-flowing design using a simple radius or compound radii.
The turning radii and pavement cross slopes for free-flowing right turns are
functions of design speed and type of vehicle, as discussed in Chapter 3 A Policy
on Geometric Design of Highways and Streets, AASHTO 2011 6th Edition.
Minimum Edge-of-Traveled-Way Designs
Where it is appropriate to provide for turning vehicles within minimum space, as
at un-channelized intersections, the corner radii should be based on minimum
turning path of the selected design vehicles – refer to Sections 2.1.1 and Table
2.1.2, Figures 9-23 to 9-30 Chapter 2, A Policy on Geometric Design of Highway
and Streets, AASHTO 2011 6th Edition.
At an intersection with a low right-turn volume, the designer may determine that
a right-turn lane is not warranted. In this case, the shoulder may be improved for
greater load capacities to permit right-turning vehicles to utilize the shoulder.
Where right-turning volumes are high, consideration should be given to
providing a right-turn lane along with appropriate provisions for vehicle
deceleration. In rural areas, the appropriate shoulder width should be considered
in conjunction with the design of right-turn lanes.
Design for Specific Conditions (Right-Angle Turns)
Combinations of curves with radii other than the minimums discussed above may
also provide satisfactory operations. The choice of design for a specific
intersection or turning movement where pedestrians are present is a particular
concern, and it is desirable to keep the intersection area to a minimum. The
selection of any specific design depends on:

Type and size of vehicles that will be turning and the extent to which they
should be accommodated.

Number and frequency of larger vehicles involved in turning movements, and
their effect on other traffic.

Type, character, and location of the intersecting roads.

Vehicular and pedestrian traffic volumes.
From the analysis of these maneuvers and corresponding paths, the appropriate
type of minimum design can be selected. Minimum designs are appropriate for
locations with low turning speeds, low turning volumes, or high property values.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Three minimum edge-of-traveled-way designs for turns may be considered at an
intersection based on the turning paths of the design vehicles identified below:

P design vehicle – Used at intersections in conjunction with parkways where
minimum turns are appropriate, at local road intersections with major roads
where turns are made only occasionally, and at intersections of two minor
roads carrying low volumes.

Single-unit truck design vehicles–Generally the SU-9 and SU-12 design
vehicles provide the recommended minimum edge-of-traveled-way design
for rural highways. Important turning movements on major highways,
particularly those involving a large percentage of trucks, should be designed
with larger radii, speed-change lanes, or both.
See Figure 4-9 Minimum Turning Path for Single Truck (SU) Design Vehicle.

Semitrailer combination design vehicles – These design vehicles should be
used where truck combinations will turn repeatedly. Symmetrical
arrangements of three-centered compound curves are generally preferred if
smaller truck combinations make up a sizable percentage of the turning
volume. Because designs for semitrailer combination vehicles produce large
paved areas, it may be desirable to provide somewhat larger radii and use a
corner triangular island.
See Figure 4-10 and Figure 4-11 for Minimum Turning Path for Intermediate
semi-trailer and Interstate semi-trailer design vehicle
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Figure 4-9
Minimum Turning Path for Single-Unit (SU) Truck Design Vehicle
Source: AASHTO, 2011, A Policy on Geometric Design of Highway and Streets 6th Edition. Used by
Permission.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Figure 4-10
Minimum Turning Path for Intermediate Semitrailer (WB-12 [WB-40]) Design
Vehicle
Source: AASHTO, 2011, A Policy on Geometric Design of Highway and Streets 6th Edition. Used by
Permission.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Figure 4-11
Minimum Turning Path for Interstate Semitrailer (WB-20, WB-65 and WB-67)
Design Vehicle
Source: AASHTO, 2011, A Policy on Geometric Design of Highway and Streets 6th Edition. Used by
Permission.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Effect of Curb Radii on Pedestrians
For arterial street design, adequate radii for vehicle operation should be
balanced against the needs of pedestrians and the difficulty of acquiring
additional right-of-way or corner setbacks. Because the corner radius is often a
compromise, its effect on both pedestrians and vehicular movements should be
examined.
The following is offered as a guide:

Radii of 4.5 to 7.5 m are adequate for passenger vehicles. These radii may be
provided at minor cross streets where there is little occasion for trucks to
turn or at major intersections where there are parking lanes. Where the
street has sufficient capacity to retain the curb lane as a parking lane, parking
should be restricted for appropriate distances from the crossing.

Radii of 7.5 m or more should be provided at minor cross streets, on new
construction, and on reconstruction projects where space permits.

Radii of 9 m or more should be provided at minor cross streets where
practical so that an occasional truck can turn without too much
encroachment.

Radii of 12 m or more, or preferably three-centered curves or simple curves
with tapers to fit the paths of large truck combinations, should be provided
where such combinations or buses turn frequently. Where speed reductions
would cause problems, longer radii should be considered.
See Table 4-10 for the Edge of Travelled Way for Turns at Intersection and
Figure 4-12 for Design Formula for 3- Centered Compound Curve.
Curb radii should be coordinated with crosswalk distances or special designs
should be used to make crosswalks efficient for all pedestrians.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Table 4-10
Edge of Traveled Way Designs for Turns at Intersection –Three Centered Curves
Metric
Angle of
Turn
(Degrees)
Design
Vehicle
30
45
60
75
4-33
Three-Centered Compound
Three-Centered
Compound
Curve Radii
(m)
Symmetric
Offset (m)
Curve Radii
(m)
Asymmetric
Offset (m)
P
-
-
-
-
SU-9
-
-
-
-
SU-12
-
-
-
-
WB-12
-
-
-
-
WB-19
-
-
-
-
WB-20
140-53-140
1.2
91-53-168
0.6-1.4
WB-28D
168-16-168
1.2
61-46-168
0.6-1.8
WB-30T
67-24-67
1.4
61-24-91
0.8-1.5
WB-33D
168-76-168
1.5
76-61-198
0.5-2.1
P
-
-
-
-
SU-9
-
-
-
-
SU-12
-
-
-
-
WB-12
-
-
-
-
WB-19
140-72-140
0.6
36-43-150
1.0-2.6
WB-20
140-53-140
1.2
76-38-183
0.3-1.8
WB-28D
160-47-160
1.5
61-43-152
0.5-1.8
WB-30T
76-24-76
1.4
61-24-91
0.8-1.7
WB-33D
168-61-168
1.5
61-52-198
0.5-2.1
P
-
-
-
-
SU-9
-
-
-
-
SU-12
-
-
-
-
WB-12
-
-
-
-
WB-19
120-30-120
4.5
34-30-67
3.0-3.7
WB-20
122-30-122
2.4
76-38-183
0.3-1.8
WB-28D
146-34-146
1.8
46-34-152
0.9-2.7
WB-30T
76-24-76
1.4
61-24-91
0.6-1.7
WB-33D
198-46-198
1.7
61-43-183
0.5-2.4
P
30-8-30
0.6
-
-
SU-9
36-14-36
0.6
-
-
SU-12
61-9-61
1.5
18-14-61
0.3-1.4
WB-12
36-14-36
1.5
36-14-60
0.6-2.0
WB-19
134-23-134
4.5
43-30-165
1.5-3.6
WB-20
128-23-128
3.0
61-24-183
0.3-3.0
WB-28D
152-29-152
2.1
46-30-152
0.3-2.4
WB-30T
76-24-76
1.4
30-24-91
0.5-1.5
WB-33D
213-38-213
2.0
46-34-168
0.5-3.5
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Metric
Angle of
Turn
(Degrees)
Design
Vehicle
90
105
120
135
150
Three-Centered Compound
Three-Centered
Compound
Curve Radii
(m)
Symmetric
Offset (m)
Curve Radii
(m)
Asymmetric
Offset (m)
P
30-6-30
0.8
-
-
SU-9
36-12-36
0.6
-
-
SU-12
61-9-61
2.1
18-14-61
0.3-1.4
WB-12
36-12-36
1.5
36-12-60
0.6-2.0
WB-19
120-21-120
3.0
48-21-110
2.0-3.0
WB-20
134-20-134
3.0
61-21-183
0.3-3.4
WB-28D
143-23-143
3.0
46-27-152
0.5-2.6
WB-30T
76-21-76
1.4
61-21-91
0.3-1.5
WB-33D
213-34-213
2.0
30-29-168
0.6-3.5
P
30-6-30
0.8
-
-
SU-9
30-11-30
1.0
-
-
SU-12
61-11-61
1.8
18-12-58
0.5-1.8
WB-12
30-11-30
1.5
30-17-60
0.6-2.5
WB-19
160-15-160
4.5
110-23-180
1.2-3.2
WB-20
152-15-152
4.0
61-20-183
0.3-3.4
WB-28D
152-24-152
2.4
46-24-152
0.6-3.0
WB-30T
76-18-76
1.5
30-18-91
0.5-1.8
WB-33D
213-29-213
2.4
46-24-152
0.9-4.6
P
30-6-30
0.6
-
-
SU-9
30-9-30
1.0
-
-
SU-12
61-11-61
1.8
18-12-58
0.5-1.5
WB-12
36-9-36
2.0
30-9-55
0.6-2.7
WB-19
160-21-160
3.0
24-17-160
5.2-7.3
WB-20
168-14-168
4.6
61-18-183
0.6-3.8
WB-28D
152-21-152
3.0
46-21-137
0.9-3.2
WB-30T
76-18-76
1.5
30-18-91
0.5-1.8
WB-33D
213-26-213
2.7
46-21-152
2.0-5.3
P
30-6-30
0.5
-
-
SU-9
30-9-30
1.2
-
-
SU-12
61-12-61
1.2
18-12-55
0.5-1.5
WB-12
36-9-36
2.0
30-8-55
1.0-4.0
WB-19
180-18-180
3.6
30-18-195
2.1-4.3
WB-20
168-14-168
5.0
61-18-183
0.6-3.8
WB-28D
137-21-137
2.7
46-20-137
2.1-4.1
WB-30T
76-18-76
1.7
30-18-91
0.8-2.0
WB-33D
213-21-213
3.8
46-20-152
2.1-5.6
P
23-6-23
0.6
-
-
SU-9
30-9-30
1.2
-
-
4-34
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Metric
Angle of
Turn
(Degrees)
180
Design
Vehicle
Three-Centered Compound
Three-Centered
Compound
Curve Radii
(m)
Symmetric
Offset (m)
Curve Radii
(m)
Asymmetric
Offset (m)
SU-12
61-11-61
2.0
18-12-61
0.3-1.4
WB-12
30-9-30
2.0
28-8-48
0.3-3.6
WB-19
145-17-145
4.5
43-18-170
2.4-3.0
WB-20
168-14-168
5.8
61-17-183
2.0-5.0
WB-28D
107-18-107
4.6
37-20-137
1.8-4.0
WB-30T
76-18-76
2.1
30-18-91
1.5-2.4
WB-33D
213-20-213
4.6
61-20-152
2.7-5.6
P
15-5-15
0.2
-
-
SU-9
30-9-30
0.5
-
-
SU-12
46-11-46
1.9
15-11-40
1.7-2.1
WB-12
30-6-30
3.0
26-6-45
2.0-4.0
WB-19
245-14-245
6.0
30-17-275
4.5-4.5
WB-20
183-14-183
6.2
30-17-122
1.8-4.6
WB-28D
122-17-122
5.1
37-18-122
2.7-4.4
WB-30T
76-17-76
2.9
30-17-91
2.6-3.2
WB-33D
213-17-213
6.1
61-18-152
3.0-6.4
Source: Table 9-16 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6th Edition.
Used by Permission.
4-35
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Figure 4-12
Symmetrical Three-Centered Compound Curve
Equations for Any Two-Centered
Compound Curves:
I = Total Deflection Angle = Δ1 +
Δ2
X = R2 sin I + (R1 - R2) sin Δ1
Y = R1 - R2 cos I – (R1 - R2) cos Δ1
Tb = Y / sin I
Ta = X - Tb cos I
Equations for any Three-Centered
Compound Curves:
I = Total Deflection Angle = Δ1 +
Δ2 + Δ3
X = (R1 – R2) sin Δ1 + (R2 – R3) sin
(Δ1 + Δ2) + R3 sin I
Y = R1 - R3 cos I – (R1 - R2) cos Δ1
- (R2 - R3) cos (Δ1 + Δ2)
Tb = Y / sin I
Ta = X – Tb cos I
Equations for Symmetrical ThreeCentered Compound Curve (R1 =
R3; Δ1 = Δ3, as shown in Figure):
Note: This is only one example of how a compound curve can be designed.
I = Total Deflection Angle = 2 Δ1
+ Δ2
X = (R1 – R2) sin Δ1 + (R2 – R1) sin
(Δ1 + Δ2) + R1 sin I
Y = R1 - R2 cos I – (R1 – R2) cos Δ1
- (R2 – R1) cos (Δ1 + Δ2)
Tb = Y / sin I
Ta = X - Tb cos I
Note: R1 ≤ 1.5 R2
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Corner Radii into Local Urban Streets
Because of space limitations, presence of pedestrians, and generally lower
operating speeds in urban areas, curve radii for turning movements may be
smaller than those normally used in rural areas. Corner radii to accommodate
right-turning movements depend largely on the number and type of turning
vehicles and the volume of pedestrians.
Right-turning radii into minor side streets in urban areas usually range from 1.5
to 9 m, with most between 3 and 4.5 m. Where a substantial number of
pedestrians are present, the lower end of the range is appropriate. On arterial
streets carrying heavy traffic volumes, it is desirable to provide corner radii of
4.5 to 7.5 m for passenger vehicles, and 9 to 15 m for most trucks and buses,
provided there are no significant pedestrian conflicts. Where large truck
combinations turn frequently, somewhat larger radii should be provided for
turns.
WB-19 and larger trucks generally are used between trucking terminals or
industrial or commercial areas. Ideally, such destinations are located near major
highway facilities that are designed to accommodate the larger combination
units. Such trucks may be present on urban arterials, but seldom turn into or out
of local urban streets.
Channelization
Channelization is the separation or regulation of conflicting traffic movements
into definite paths of travel by traffic islands or pavement marking to facilitate
the orderly movements of both vehicles and pedestrians. Proper channelization
increases capacity, provides positive guidance to motorists, increases operational
efficiency and reduces crash frequencies; improper channelization has the
opposite effect and may be worse than none at all. Separation of left-turn
movements from through movements is a common use of channelization.
Channelization of intersections is generally considered for one or more of the
following factors:
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
Paths of vehicles are confined by channelization so that not more than two
paths cross at any one point.

Angle and location at which vehicles merge, diverge, or cross are controlled.

Amount of paved area is reduced and thereby decreases the potential for
vehicles to wander and narrows the area of conflict between vehicles.

Clearer indications are provided for the proper path in which movements are
to be made.

Predominant movements are given priority.

Areas are provided for pedestrian refuge.

Separate storage lanes permit turning vehicles to wait clear of through-traffic
lanes.
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design

Space is provided for traffic control devices so that they can be more readily
perceived.

Prohibited turns are controlled.

Speeds of vehicles are restricted to some extent.
Design controls for a channelized intersection include: the type of design vehicle,
the cross sections on the crossroads, the projected traffic volumes in relation to
capacity, the number of pedestrians, the speed of vehicles, the location of any
needed bus stops, and the type and location of traffic control devices. Physical
controls such as right-of-way and terrain have an effect on the extent of
channelization that is economically practical.
Principles to be followed in the design of a channelized intersection are:

Motorists should not be confronted with more than one decision at a time.

Unnatural paths that involve turns greater than 90 degrees or sudden and
sharp reverse curves should be avoided.

Channelization should keep vehicles within well-defined paths that minimize
conflict. However, merging and weaving areas should be as long as conditions
permit.

Where the distance to the downstream driveway or intersection is less than
the desirable distance for merging or weaving and where pedestrians are
present, turning roadways should be controlled with a yield, stop, or signal
control and the angle of intersection should be greater than 60 degrees.

Traffic streams that intersect without merging and weaving should intersect
at angles as close to 90 degrees as practical, and certainly within a range of 60
to 120 degrees.

Angle of intersection between merging streams of traffic should be
appropriate to provide adequate sight distance.

Points of crossing or conflict should be studied carefully to determine if such
conditions would be better separated or consolidated to simplify design with
appropriate control devices added to provide efficient operation.

Refuge areas for turning vehicles should be provided separate from through
traffic.

Islands used for channelization should not interfere with or obstruct bicycle
lanes at intersections.

Prohibited turns should be blocked by channelizing islands, wherever
practical.

Location of essential control devices should be established as a part of the
design of a channelized intersection.
Channelization may be desirable to separate the various traffic movements
where multiple phase signals are used. Intersection design including
channelization can be used to discourage wrong-way entry of expressway-ramps,
one-way streets, and turning roadways.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
The storage length should be sufficient long to store the number of vehicles likely
to accumulate during the average daily peak period:

At unsignalized intersection, length to be based on the number of vehicles
likely to arrive in an average 2 minute period within the peak hour.

At signalized intersection, the required length depends on the signal cycle
length, the signal phasing arrangement and the rate of arrivals and departure
of left turning vehicles.
Islands
General Characteristics
An island is a defined area between traffic lanes used for control of traffic
movements. Islands also provide an area for pedestrian refuge and traffic control
devices. Within an intersection, a median or an outer separation is also
considered an island. It may range from an area delineated by a raised curb to a
pavement area marked out by paint or thermoplastic markings. Where traffic
entering an intersection is directed into definite paths by islands, the design
feature is termed a channelized intersection.
Channelizing islands are usually included in intersection design for one or more
of the following purposes:

Separation of conflicts

Control of angle of conflict

Reduction in excessive pavement areas

Regulation of traffic and indication of proper use of intersection

Arrangements to favor a predominant turning movement

Protection of pedestrians

Protection and storage of turning and crossing vehicles

Location of traffic control devices
Islands generally are either elongated or triangular in shape and are normally
situated in areas unused for vehicle paths. Islands should be located and
designed to offer little obstruction to vehicles, be relatively inexpensive to build
and maintain, occupy a minimum of roadway space, and be commanding enough
that motorists will not drive over them. It is desirable to provide a common
geometric design for all intersections along a route.
When designing an island, attention should be given to the fact that the driver’s
eye view is different from the plan view. Also the use of a few large islands is
usually less confusing than a number of smaller islands. Temporary layouts of
movable stanchions or sandbags can be used to observe traffic flow with several
variations of size and shape of islands before finalizing a design and constructing
permanent islands.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Curbed islands can sometimes be difficult to see at night because of glare, and
therefore should ideally have fixed-source lighting or appropriate delineation
such as curb-top reflectors.
Painted, flush medians and islands or traversable type medians are not well
respected by drivers but may be preferable to raised curb type islands in the
following circumstances;

Lightly developed areas that will not be considered for access management.

Intersections where approach speeds are relatively high.

Areas where there is little pedestrian traffic.

Areas where fixed-source lighting is not provided.

Median or corner islands where signals, signs, or luminaire supports are not
needed.

Areas where extensive development exists along a street and may demand
left-turn lanes into many entrances.
Painted islands may be used at the traveled way edge. At some intersections,
both curbed and painted islands may be desirable. All pavement markings should
be reflectorized. The use of thermoplastic striping, raised dots, spaced and raised
retro-reflective markers, and other forms of long-life markings is also desirable.
Channelizing Islands
Channelization for minor intersections on two-lane highways in rural areas is
often not necessary. Curbed islands generally should not be used in rural areas
and at isolated locations unless the intersection is lighted and curbs are
delineated.
The use of curbed islands generally should be reserved for multilane highways or
streets and for the more important intersections on two-lane highways.
Channelization can work well in or near urban areas where speeds are low and
drivers are accustomed to confined facilities.
Divisional Islands
Divisional islands are often used on undivided highways at intersections. They
alert drivers to the crossroads ahead and regulate traffic through the
intersection. The islands are particularly advantageous in controlling left turns at
skewed intersections and at locations where separate roadways are provided for
right-turning traffic.
Widening a roadway to include a divisional island should be done in such a
manner that the proper paths to follow are unmistakably evident to drivers.
Often the highway is on a tangent, and introducing a dividing island involves the
use of a reverse curve. In rural areas where speeds are generally high, reversals
in curvature should preferably be with radius of at least 1,165 m tapers can also
be used, provided they are consistent with lane shifts at the design speed.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Refuge Islands
In both rural and urban areas, many of the islands designed for the function of
channelization also serve as refuge for pedestrians. The general principles for
island design also apply to providing refuge islands.
A refuge island for pedestrians is one at or near a crosswalk or bicycle path that
aids and protects pedestrians and bicyclists who cross the roadway. Raised-curb
corner islands and center channelizing or divisional islands can be used as refuge
areas. Refuge islands for pedestrians and bicyclists crossing a wide street are
used primarily in urban areas.
The location and width of crosswalks, the location and size of bus loading zones,
and the provision of wheelchair ramps influence the size and location of refuge
islands. Pedestrians and bicyclists should have a clear path through the island
and should not be obstructed by poles, sign posts, utility boxes, etc. Refuge
islands should be a minimum of 1.8 m wide when they will be used by bicyclists.
Island Size
The smallest curbed corner island normally should have an area of
approximately 5 m2 for urban and 7 m2 for rural intersections, however at least 9
m2 is preferable for both. Elongated or divisional islands should not be less than
1.2 m wide and 6 to 8 m long, although in special cases a minimum width of 0.5 m
may be used for elongated islands. Curbed divisional islands at isolated
intersections on high-speed highways should be highly visible and a minimum of
30 m in length. The approach end should be extended when situated in the
vicinity of a high point in the roadway profile, or at the beginning of a horizontal
curve.
In many cases, the central area of large channelizing islands has a turf or
vegetative cover. Low plant material may also be included, but it should not
obstruct sight distance. Where pavement cross slopes are outward, large islands
should be depressed to avoid draining water across the pavement. For small
curbed islands, and in areas where growing conditions are not favorable, some
type of paved surface is used on the island.
Island Delineation and Approach Treatment
Delineation of small islands is effected primarily by curbs and curb-top reflectors.
Large curbed islands may be sufficiently delineated by color and texture contrast
via vegetative cover, mounded earth, shrubs, reflector posts, signs or any
combination of these.
The most commonly used height of curb is 150 mm. Vertical or sloping curbs are
appropriate in urban areas, depending on the conditions. High-visibility sloping
curbs may be appropriate at critical locations. The approach corner of a curbed
island should be designed with an approach nose treatment. The approach nose
of a curbed island should be conspicuous to approaching drivers and should be
clear of vehicle paths, both physically and visually.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Where a curbed corner is proposed on an approach roadway with shoulders, the
face of the curb on the corner island should be offset by an amount equal to the
shoulder width.
Curbed corner islands and median noses should be ramped down and provided
with devices to give advance warning to approaching drivers, especially for
night-time driving. Pavement markings in front of the approach nose, and
reflectorized curb-top markers mounted on the curb or median surface are
advantageous.
The approach should consist of a gradual widening of the divisional island,
accompanied by a gradual change to a raised surface with texture or jiggle bars
that may be crossed readily. This transition section should be as long as practical.
Free-Flow Turning Roadways at Intersections
A free-flow alignment for right turns at intersections can be provided by
designing compound curves preceded by a right-turn deceleration lane. The
shape and length of these curves should (1) allow drivers to avid abrupt
deceleration, (2) permit development of some superelevation in advance of the
maximum curvature, and (3) enable vehicles to follow natural turning paths. The
design speed of the turning roadway may be equal to or within 20 to 30 kph less
than the through roadway design speed.
Turning Roadways with Corner Islands
Where the inner edges of the traveled way for right turns are designed to
accommodate semitrailer combinations, or where the design permits passenger
vehicles to turn at speeds of 15 kph or more, the pavement area within the
intersection may become excessively large. To avoid this situation, a corner
island can be provided to form a separate turning roadway between the two
intersection legs.
Right-Angle Turns with Corner Islands
A turning roadway should be designed to provide at least the minimum size
island and the minimum width of roadway. The turning roadway should be wide
enough to permit the right and left wheel tracks of a selected vehicle to be within
the edges of the traveled way by about 0.6 m on each side. Generally, the turning
roadway width should not be less than 4.2 m, unless a wider roadway is needed
for a semitrailer combination. To discourage passenger vehicles from using this
wider roadway as two lanes, the roadway may be reduced in size by marking out
part of the roadway with paint or thermoplastic markings.
In urban areas, islands should be located about 0.6 m outside the traveled way
edges. For high-speed highways, the offset from the through lanes to the face of
curb normally should be equal to the shoulder width. In rural areas, the use of
painted corner islands may be considered.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Oblique-Angle Turns with Corner Islands
The minimum design dimensions for oblique-angle turns are determined on a
basis similar to that for right-angle turns, and values are given in Table 4-11.
Curve design for the inner edge of the traveled way, the turning roadway width,
and the approximate island size are indicated for the three chosen design
classifications described at the bottom of the table. For a particular intersection,
the designer may choose from the three minimum designs shown in accordance
with vehicle size, the volume of traffic anticipated, and the physical controls at
the site.
Table 4-11 does not provide design values for angles of turn less than 75 degrees.
In general, angles of intersection less than 75 degrees should not be used, but if
unavoidable such turning angles should have individual designs to fit site control
and traffic conditions.
Table 4-11
Typical Designs for Turning Roadways
Angle of Turn
(°)
Design
Classification
75
A
45-23-45
1.0
4.2
5.5
B
45-23-45
1.5
5.4
5.0
C
67-41-67
1.5
6.7
33.5
A
45-15-45
1.0
4.2
5.0
90
105
120
135
150
Three-Centered Compound Curve
Radii (m)
Offset (m)
Width of
Lane (m)
Approx. Island
Size (m2)
B
45-15-45
3.4
6.4
14.0
C
61-21-61
3.4
7.6
25.0
A
36-12-36
0.6
4.5
6.5
B
46-11-46
3.5
8.8
6.0
C
55-18-55
2.9
9.8
24.0
A
30-9-30
0.8
4.8
11.0
B
46-9-46
3.2
10.0
12.0
C
43-17-43
2.1
13.7
20.0
A
30-9-30
0.8
4.8
43.0
B
46-9-46
3.0
11.6
37.0
C
43-14-43
2.1
15.8
45.0
A
30-9-30
0.8
4.8
130.0
B
46-9-46
2.7
12.8
125.0
C
49-12-49
1.8
16.1
150.0
Notes: Asymmetric three-centered compound curve and straight tapers with a simple curve can also be used
without significantly altering the width of roadway or corner island size. Painted island delineation is
recommended for islands less than 7 m2 in size.
Design classification:
A – Primarily passenger vehicles; permits occasional design single-unit trucks to turn with restricted
clearances.
B – Provides adequately for the SU-9 and SU-12 design vehicles; permits occasional WB-19 design vehicles
to turn with slight encroachment on adjacent traffic lanes.
C – provides fully for the WB-19 design vehicle.
Source: Table 9-18 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6th Edition.
Used by Permission.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Superelevation for Turning Roadways at Intersections
General Design Guidelines
The general factors that control the maximum rates of superelevation for open
highway conditions also apply to turning roadways at intersections. Maximum
superelevation rates up to 10% may be used where climatic conditions are
favorable.
In intersection design, the free flow of turning roadways is often of limited radii
and length. When speed is not affected by other vehicles, drivers on turning
roadways anticipate sharp curves and accept operation with higher side friction
than they accept on open highway curves of the same radii. When other traffic is
present, drivers will travel more slowly on turning roadways than on open
highway curves of the same radii because they must diverge from and merge
with through traffic. Therefore, in designing for safe operation, periods of light
traffic volumes and corresponding speeds will generally control.
Designs with gradually changing curvature, affected by the use of compound
curves, spirals, or both, permit desirable development of superelevation. The
design superelevation rates and corresponding radii listed in Table 3-13 to 3-16
are applicable.
Superelevation Runoff
The principles of superelevation runoff discussed in Section 3.6.2.3 generally
apply to free-flow turning roadways at intersections.
Usually, the profile of one edge of the traveled way is established first, and the
profile on the other edge is developed by stepping up or down from the first edge
by the amount of desired superelevation at that location. This step is done by
plotting a few control points on the second edge by using the maximum relative
gradients in Table 4-12 and then plotting a smooth profile for the second edge of
traveled way. Drainage may be an additional control, particularly for curbed
roadways.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Table 4-12
Effective Maximum Relative Gradients (%)
Rotated Width (m)
Design Speed (kph)
3.6 m
5.4 m
7.2 m
20
0.80
0.96
1.00
30
0.75
0.90
1.00
40
0.70
0.84
0.93
50
0.65
0.78
0.87
60
0.60
0.72
0.80
70
0.55
0.66
0.73
80
0.50
0.60
0.67
90
0.47
0.57
0.63
100
0.44
0.53
0.59
110
0.41
0.49
0.55
120
0.38
0.46
0.51
130
0.35
0.42
0.47
Note: Based on maximum relative gradients listed in Table 3-15 and the adjustment factors in Table 3-16.
One lane is assumed to equal 3.6 m. Gradients for speeds of 80 kph or faster are applicable to turning
roadways at interchanges (i.e., ramps).
Source: Table 9-19 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
Development of Superelevation at Turning Roadway Terminals
For design of a highway, the through traffic lanes may be considered fixed in
profile and cross slope. As the exit curve diverges from the through traveled way,
the curved edge of the widening section can only gradually vary in elevation from
the edge of the through lane. Shortly beyond the point where the full width of the
turning roadway is attained, an approach nose separates the two pavements.
Where the exit curve is relatively sharp and without taper or transition, little
superelevation in advance of the nose can be developed in the short distance
available. Beyond the nose, substantial superelevation usually can be attained,
the amount depending on the length of the turning roadway curve.
Where this curve deviates gradually from a traveled way, a desirable treatment
of superelevation may be effected.
Stopping Sight Distance at Intersections for Turning Roadways
The values for stopping sight distance for open highway conditions are applicable
to turning roadway intersections of the same design speed, are shown in Table
4-13.
Table 4-13
Stopping Sight Distance at Intersections for Turning Roadway
Design Speed (kph)
Stopping sight distance (m)
15
20
30
40
50
60
15
20
35
50
65
85
70
105
th
Source: Table 9-21 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 Edition.
Used by Permission.
These sight distances should be available at all points along a turning roadway.
Wherever practical, longer sight distances should be provided. They apply as
controls in design of both vertical and horizontal alignment.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
4.1.8
Auxiliary Lanes
General Design Considerations
In general, auxiliary lanes are used preceding median openings and are also used
at intersections preceding right and left-turning movements. Auxiliary lanes may
also be added to increase capacity and reduce crashes at an intersection. In many
cases, an auxiliary lane may be desirable after completing a right-turn movement
to provide for acceleration, maneuvering, and weaving.
Auxiliary lanes should be at least 3 m wide and desirably should equal that of the
through lanes. Shoulders adjacent to auxiliary lanes should be the same width as
the shoulders adjacent to through lanes. A minimum 1.8 m wide shoulder is
preferred adjacent to auxiliary lanes on rural high speed roadways. Shoulders
may be omitted adjacent to auxiliary lanes in urban areas and on right and left
turn lanes, as they serve as a useable shoulder for emergency use and to
accommodate stopped or disabled vehicles. Auxiliary lanes subject to heavy truck
usage or off-tracking vehicles or both, a paved shoulder 0.6 to 1.2 m wide may be
needed. Where curbing is to be used adjacent to the auxiliary lane, an
appropriate curb offset should be provided.
Auxiliary lanes are commonly provided on highways having expressway
characteristics, and are frequently used at other intersections on main highways
and streets. An auxiliary lane, including the tapered area, serves as a speedchange lane primarily for the acceleration or deceleration of vehicles entering or
leaving the through-traffic lanes.
In general:

Auxiliary lanes are warranted on high-speed and on high-volume highways
where a change in speed is needed for vehicles entering or leaving the
through-traffic lanes.

All drivers do not use auxiliary lanes in the same manner, but overall these
lanes are used sufficiently to improve highway operation.

Use of auxiliary lanes varies with volume, the majority of drivers using them
at high volumes.

The directional type of auxiliary lane consisting of a long taper fits the
behavior of most drivers and does not involve maneuvering on a reverse
curve path.

Deceleration lanes on the approaches to intersections that also function as
storage lanes for turning traffic are particularly advantageous.
Deceleration lanes are advantageous on higher-speed roads, otherwise the driver
of a vehicle leaving the highway has no choice but to slow down on the throughtraffic lane. Failure to brake by following drivers may result in rear-end
collisions.
Acceleration lanes are not always desirable at stop-controlled intersections
where entering drivers can wait for an opportunity to merge without disrupting
through traffic. They are advantageous on roads without stop control and on all
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
high-volume roads even with stop control where openings between vehicles in
the peak-hour traffic streams are infrequent and short.
Deceleration Lanes and Storage Length
Ideally the total length of an auxiliary lane for deceleration should allow for the
perception-reaction distance, the full deceleration length, and storage length.
However, common practice is to accept some deceleration within the through
lanes and to consider the taper length as part of the deceleration within the
through lanes.
Estimated distances needed by drivers to maneuver from the through lane into a
turn bay and brake to a stop are provided in Table 4-14.
Table 4-14
Desirable Full Deceleration Lane Lengths
Speed (kph)
Distance(m) – rounded to 5 m
30
20
50
45
65
85
80
130
95
185
110
245
Source: Table 9-22 AASHTO, 2011, A Policy on Geometric Design of Highway and Streets 6th Edition. Used
by Permission.
Storage Length
The auxiliary lane should be sufficiently long to store the number of vehicles, or
queue, likely to accumulate during a critical period.
At un-signalized intersections, the storage length should be determined by an
intersection traffic analysis based on the number of turning vehicles likely to
arrive in an average two-minute period within the peak hour. Space for at least
two passenger cards should be provided; with over 10% turning truck traffic,
provision should be made for at least one car and one truck. The two-minute
interval may need adjustment, depending on opportunities for completing the
left-turn maneuver through opposing traffic. Where the volume of turning traffic
is high, a traffic signal will be needed.
At signalized intersections, the storage length needed should be determined by
an intersection traffic analysis which considers (1) the signal cycle length, (2) the
signal phasing arrangement, and (3) the rate of arrivals and departures of leftturning vehicles.
On high-speed highways, it is common practice to use a taper rate that is between
8:1 and 15:1 (longitudinal: transverse). Long tapers approximate the path drivers
follow when entering an auxiliary lane from a high-speed through lane, but they
can allow through drivers to drift into the deceleration lane.
For urban areas, short tapers give more positive identification to an added
auxiliary lane, and are preferred for the slow speeds during peak periods. The
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
total length of taper and deceleration length should be the same as if a long taper
was used.
Design Treatments for Left-Turn Maneuvers
Guidelines for Design of Left-Turn Lanes
In designing an intersection, left-turning traffic should be removed from the
through lanes whenever practical. Ideally, left-turn lanes should be provided at
driveways and street intersections along major arterial and collector roads
wherever left turns are permitted. In some cases or at certain locations,
providing for indirect left turns (jughandles, U-turn lanes, and diagonal
roadways) may be appropriate to reduce crash frequencies and preserve
capacity. Left-turn facilities should be established on roadways where traffic
volumes are high enough or crash histories are sufficient to warrant them. They
are often needed to provide adequate service levels for intersections.
Exclusive left-turn lanes are required at signalized intersections where:

Left-turn signal phasing is provided.

Left-turn volumes exceed 100 vehicles per hour.

Double left turn lanes should be considered where left-turn volumes exceed
300 vehicles per hour.
A median left-turn lane is an auxiliary lane for storage or speed change of leftturning vehicles located at the left of a one-directional roadway within a median
or divisional island. Median lanes should be provided at intersections and at
other median openings where there is a high volume of left-turns or where the
vehicular speeds are high.
The form of treatment given to the end of the median adjacent to lanes of
opposing traffic depends largely on the available width. The narrowed median
may be curbed to delineate the lane edge, to separate opposing movements, to
provide a space for signs, markers, and luminaire supports, and to protect
pedestrians.
Parallel offset left-turn lanes may be used at both signalized and un-signalized
intersections. The advantages of offsetting the left-turn lanes are (1) better
visibility of opposing through traffic, (2) decreased possibility of conflict between
opposing left-turn movements within the intersection, and (3) more left-turn
vehicles served in a given period of time, particularly at a signalized intersection.
4.1.9
Median Openings
General Design Considerations
Medians are discussed in Section 3.7.9 as an element of the cross section. Median
openings should reflect street or block spacing and the access classification of the
roadway, while being consistent with traffic signal spacing criteria.
Spacing of openings should be consistent with access management classifications
or criteria. Where the traffic pattern at an intersection shows that nearly all
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
traffic travels through on the divided highway and the volume is well below
capacity, a simple low cost median opening may be sufficient to permit cross and
turning movements. Where a traffic pattern shows appreciable cross and turning
movements or through traffic of high speed and high volume, the shape and
width of the median opening should provide for turning movements to be made
without encroachment on adjacent lanes and with little or no interference
between traffic movements.
The design of a median opening and median ends should be based on traffic
volumes, urban/rural area characteristics, and the type of turning vehicles.
Design should be based on the volume and composition of all movements
occurring simultaneously during the design hours. The design of a median
opening becomes a consideration of what traffic is to be accommodated, choosing
the design vehicle to use for layout controls for each cross section and turning
movement, investigating whether larger vehicles can turn without undue
encroachment on adjacent lanes, and checking the intersection capacity. If the
capacity is exceeded by the traffic demand, the design should be expanded.
Intersections with narrow medians in urban/suburban areas generally operate
with lower crash frequencies; un-signalized intersections with wider medians in
rural areas also operate with lower crash frequencies; and traffic signals at
intersections with wide medians can be inefficient. Traffic control devices such as
yield signs, stop signs, or traffic signals may be needed to regulate the various
movements effectively and improve the effectiveness of operations.
Control Radius for Minimum Turning Paths
An important factor in designing median openings is the path of each design
vehicle making a minimum left turn at 15 to 25 kph.
By considering the range of radii for minimum right-turns and the need for
accommodation of more than one type of vehicle at the usual intersections, the
following control radii can be used for minimum practical design of median ends:

A control radius of 12 m accommodates P design vehicles suitably and
occasional SU-9 design vehicles with some swinging wide. (Refer Table 4-15).
Table 4-15
Minimum Median Opening for P Design Vehicle
Minimum Length of Median Opening (m)
Width of Median (m)
Semicircular
Bullet Nose
1.2
22.8
22.8
1.8
22.2
18.0
2.4
21.6
16.8
3.0
21.0
16.8
3.6
20.4
16.8
4.2
19.8
16.8
4.8
19.2
16.8
6.0
18.0
16.8
7.2
16.8
16.8
Source: Table 9-25 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design

A radius of 15 m accommodates SU-9 design vehicles and occasional SU-12
and WB-12 design vehicles with some swinging wide. (Refer Table 4-16).
Table 4-16
Minimum Median Opening for SU-9 Design Vehicle
Width of Median (m)
Minimum Length of Median Opening (m)
Semicircular
Bullet Nose
1.2
28.2
28.8
1.8
28.2
22.8
2.4
27.6
20.4
3.0
27.0
18.6
3.6
26.4
17.4
4.2
25.8
16.8
4.8
25.2
16.8
6.0
24.0
16.8
7.2
22.8
16.8
8.4
21.6
16.8
9.6
20.4
16.8
10.8
19.2
16.8
12.0
18.0
16.8
Source: Table 9-26 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.

A radius of 23 m accommodates SU-12, WB-12, and WB-19 design vehicles
with minor swinging wide at the end of the turn. (Refer Table 4-17).
Table 4-17
Minimum Median Opening for SU-12, WB-12 and WB-19 Design Vehicles
Width of Median (m)
Minimum Length of Median Opening (m)
Semicircular
Bullet Nose
1.2
43.8
36.6
1.8
43.2
36.3
2.4
42.6
33.6
3.0
42.0
31.2
3.6
41.4
29.4
4.2
40.8
27.6
4.8
40.2
26.4
6.0
39.0
23.4
7.2
37.8
21.6
8.4
36.6
19.5
9.6
35.4
18.0
10.8
34.2
16.2
12.0
30.0
14.7
18.0
27.0
13.3
24.0
21.0
13.2
30.0
15.0
13.2
Source: Table 9-27 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design

A control radius of 40 m accommodates WB-19 design vehicles and
occasional WB-20 vehicles with minor swinging wide at the end of the turn.
A semicircular median end can be used for narrow medians up to 3.0 m wide. For
a median width of 3.0 m or more, a bullet nose design is used which closely fits
the path of the inner rear wheel and results in less intersection pavement and a
shorter length of opening than the semicircular end.
Minimum Length of Median Opening
For any three or four-leg intersection on a divided highway, the length of median
opening should be as great as the width of the crossroad traveled way plus
shoulders. Where the crossroad is a divided highway, the length of opening
should be at least equal to the width of the crossroad traveled ways plus that of
the median.
The median opening should not be longer than needed at rural un-signalized
intersections.
Design controls for minimum openings for left-turns are summarized in Table 418.
Table 4-18
Minimum Length of Median Opening for Left-Turn
Design Vehicles
Accommodated
Predominant
Occasional
Control Radius (m)
12
15
23
40
P
SU-9
WB-12
WB-19
SU-9
SU-12
-
WB-20
Source: Table 9-29 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.
Above-Minimum Designs for Direct Left Turns
Median openings that enable vehicles to turn on minimum paths and at 15 to 25
kph are adequate for intersections where traffic for the most part proceeds
straight through the intersection. Where through-traffic volumes and speeds are
high and left-turning movements are important, undue interference with through
traffic should be avoided by providing median openings that permit turns
without encroachment on adjacent lanes. This arrangement would enable turns
to be made at speeds greater than the minimum vehicle paths and provide space
for vehicle protection while turning or stopping. The general pattern for
minimum design can be used with larger dimensions.
4.1.10
Indirect Left-Turns and U-Turns
General Design Considerations
Divided highways need median openings to provide access for crossing traffic in
addition to left-turning and U-turning movements. However provision for direct
left-turns is not practical at some locations where right-of-way or cultural
features constrain the space available for improved traffic movement. In such
situations, the only safe way for traffic to gain access to the opposite traveled way
is by indirect movements.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
One option for access to adjacent properties is to use the interconnecting street
patterns. This involves making a series of right-turns around the block to a
median opening that services the secondary crossroads, and then turning left.
This approach needs careful examination of existing turning radii to
accommodate single-unit truck design vehicles and estimation of the number of
WB vehicles that might use this method of indirect left-turns or indirect U-turns.
Another alternative is to construct jug-handle-type ramps or at-grade
intersection loops.
Intersections with Jughandle or Loop Roadways
Jughandles are one-way roadways in two quadrants of the intersection that allow
for removal of left-turning traffic from the through stream without providing leftturn lanes. All turns – right, left and U-turns – are made from the right side of the
roadway. Drivers wishing to turn left exit the major roadway on the right and
turn left onto the minor road at a terminus separated from the main intersection.
Less right-of-way is needed along the roadway because the left-turn lanes are not
needed. However, more right-of-way is needed at the intersection to
accommodate the jughandles.
Jughandle roadways may be appropriate at intersections with high major-street
through movements, low-to-medium left turns from the major street, low-tomedium left turns from the minor street, and any amount of minor-street
through volumes. Jughandles can reduce left-turn collision and improve
operations by providing more available green time for major-street through
movements. In the Philippines it is essential to consider the traffic impact on a
minor street with large pedestrian traffic.
The jughandle should operate with stop control at the minor street approach.
Right turns onto the cross street may operate with yield control. Signing is
needed in advance of the jughandle ramp to indicate that motorists destined to
the left need to exist the roadway from the right-hand lane. Signalized
intersection phases can be set to minimize vehicle queuing.
Another alternative is to provide a loop roadway beyond the intersection. The
left-turn movement becomes a right-turn movement at the intersection of a farside loop roadway with the crossroad, resulting in fewer conflicts and higher
capacity for the left-turn movement. The loop design may be considered when
the right-of-way for the far-side quadrant is less expensive than that for the
nearside quadrant.
Displaced Left-Turn Intersections
A displaced left-turn intersection, also known as a continuous-flow intersection
or a crossover-displaced left-turn intersection, removes the conflict between leftturning vehicles and oncoming traffic at the main intersection by introducing a
left-turn bay placed to the left of oncoming traffic. Vehicles access the left-turn
bay at a midblock signalized intersection on the approach where continuous flow
is desired.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
The left turns potentially stop two times: once at the midblock signal on
approach and once at the main intersection on departure. Careful signal
coordination can minimize the number of stops, particularly at the main
intersection.
Intersections with high through and left-turn volumes may be appropriate for
displaced left-turn intersections. U-turns are prohibited with this design, and
right-of-way adjacent to the intersection is needed for the left-turn roadways.
Wide Medians with U-Turn Crossover Roadways
Median U-turn crossover roadways eliminate left turns at intersections and move
them to median crossovers beyond the intersection. For median U-turn
crossovers located on the major road, drivers turn left off the major road by
passing through the intersection, making a U-turn at the crossover, and turning
right at the cross road. Drivers wishing to turn left onto the major road from the
cross street turn right onto the major road and make a U-turn at the crossover.
Median U-turn crossovers need a wide median to enable the U-turn movement.
Median U-turn roadways may be appropriate at intersections with high majorstreet through movements, low-to-medium left turns from the major street, lowto-medium left turns from the minor street and any amount of minor street
volumes.
Locations with high left-turning volumes are not suitable, the distance for
pedestrians to cross is increased, turning paths of vehicles making median Uturns may encroach into bike lanes, and additional right-of-way may be needed.
Signing, visual clues, education, and enforcement are needed to guide drivers to
the intended turning path without illegal turns.
Location and Design of U-Turn Median Openings
Normally, U-turns should not be permitted from through lanes. However, where
medians have adequate width to shield a vehicle stored in the median opening,
through volumes are low and left-turns/U-turns are infrequent, this type of
design may be permissible.
Median openings designed to accommodate vehicles making U-turns only are
needed on some divided highways in addition to openings provided for cross and
left-turning movements. Separate U-turn median openings may be used at the
following locations:
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
Locations beyond intersections to accommodate minor turning movements
not otherwise provided in the intersection or interchange area.

Locations just ahead of an intersection to accommodate U-turn movements
that would interfere with through and other turning movements at the
intersection.

Locations occurring in conjunction with minor crossroads where traffic is not
permitted to cross the major highway but instead is required to turn right,
enter the through traffic stream, weave to the left, U-turn, and then return.
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design

Locations occurring where regularly spaced openings facilitate maintenance
operations, policing, repair service of stalled vehicles, or other highwayrelated activities.

Locations occurring on highways without control of access where median
openings at optimum spacing are provided to serve existing frontage
developments and at the same time minimize pressure for future median
openings. A preferred spacing of 0.40 to 0.80 km is suitable in most instances.
For a satisfactory design for U-turn maneuvers, the width of the highway,
including the median, should be sufficient to permit the design vehicle to turn
from an auxiliary left-turn lane in the median into the lane next to the outside
shoulder or outside curb and gutter on the roadway of the opposite traffic lanes.
Medians of 5.0 m and 15 m or wider are needed to permit U-turn maneuvers by
passenger and single-unit truck traffic, respectively. Wide medians are
uncommon in highly developed areas.
Alternatively, a jughandle layout can be provided whereby the vehicle turns
across the opposing traffic stream, leaves the road and is then turned back to
enter the main road via a slip road.
4.1.11
Roundabout Design
A roundabout is an intersection with a central island around which traffic must
travel counter-clockwise and in which entering traffic must yield to circulating
traffic.
The geometric design of a roundabout involves the balancing of competing
design objectives. Roundabouts operate with the lowest crash frequencies when
their geometry forces traffic to enter and circulate at slow speeds. Many of the
geometric parameters are governed by the maneuvering capabilities of the
design vehicle. Thus designing a roundabout is a process of determining the
appropriate balance between operational performance, reduced conflict
frequency, and accommodation of the design vehicle. Refer Figure 4-13.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Figure 4-13
Geometric Elements of Roundabout
Source: AASHTO, 2011, A Policy on Geometric Design of Highway and Street 6th Edition. Used by
Permission.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Geometric Elements of Roundabouts
Basic geometric elements are provided in Table 4-19.
Table 4-19
Basic Geometric Elements of Roundabouts
Element
Geometry
Central Island
The central island is the raised area in the center of a roundabout
around which traffic circulates. The central island does not necessarily
need to be circular in shape, but circular islands make driver steering
easier. It must be clearly visible to approaching drivers – a landscaped
dome shape is recommended.
Entry and Exit Curves
The exit curve radius is normally a little larger than the entry curve
radius, on the principle of sharp turn entries and easy exits. Entry curve
radius should not be greater than the center island radius.
Splitter Island
A splitter island is a raised or painted area on an approach used to
separate entering from exiting traffic, deflect and slow entering traffic,
and allow pedestrians to cross the roadway in two stages.
Circulatory Roadway
The circulatory roadway is the curved path used by vehicles to travel in
a counter-clockwise fashion around the central island. It is generally a
little wider than the entry width.
Truck/Bus Apron
If needed on smaller roundabouts to accommodate the wheel tracking
of large vehicles, an apron is the mountable portion of the central island
adjacent to the circulatory roadway. It is commonly 30mm high and 1 to
2 m wide.
Yield line at Entrance to
Circulating Roadway
The yield line marks the point of entry into the circulatory roadway.
Entering vehicles must yield the right of way to any circulating vehicles
coming from the left before crossing this line into the circulatory
roadway.
Accessible Pedestrian
Crossings
Accessible pedestrian crossings should be provided at all roundabouts.
The crossing location is set back from the entrance line, and the splitter
island is cut to allow pedestrians, wheelchairs, strollers, and bicycles to
pass through.
Landscape Strip
Landscape strips are provided at most roundabouts to separate
vehicular and pedestrian traffic and to lead pedestrians to the
designated crossing locations. Landscape strips can also significantly
improve the aesthetics of the intersection.
The key indicator of the required space for a roundabout intersection is the
inscribed circle diameter. Fundamental design and operational elements for each
of the three basic roundabout categories are summarized in Table 4-20.
Table 4-20
Design and Operational Elements for Basic Roundabout Categories
Design Element
Mini-Roundabout
Single-Lane
Roundabout
Multi-Lane
Roundabout
25 to 30 kph
30 to 40 kph
40 to 50 kph
1
1
2+
Typical Inscribed Circle
Diameter
13 to 27 m
27 to 46 m
40 to 76 m
Central Island Treatment
Mountable*
Raised
Raised
Typical Daily Volumes on 4Leg Roundabout (vehicles per
day)
0 to 15,000
0 to 20,000
20,000+
Recommended Maximum
Entry Design Speed
Maximum Number of Entering
Lanes per Approach
Source :Table 9-2 in AASHTO, 2011, A Policy on Geometric Design of Highway and Street 6th Edition. Used
by Permission.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
*In cases where the volume of heavy vehicles is low, or at existing intersections
where space is restricted, it may be appropriate to provide encroachment areas
(paved areas behind mountable curbs) which allow a smaller width of circulating
carriageway to be used via encroachment onto the central island area and/or the
approach splitter islands. Encroachment areas should:

Be constructed of appropriate load bearing pavement.

Have semi-mountable or fully mountable curbs.

Not have drainage pits within them, or have suitably reinforced pits to carry
heavy vehicle loads.

Not accommodate road furniture.
The number of entering and circulating lanes affects the capacity of the
roundabout and the size of the roundabout footprint. The capacity of a
roundabout is dependent upon directional distribution of traffic and ratio of
minor-street to total entering traffic. The designer may select a volume-tocapacity ratio between 0.85 and 1.00. A single circulating lane will normally
accommodate 1,400 vehicles per hour and may accommodate up to 2,400
vehicles per hour. A two-lane circulating roadway will normally accommodate at
least 2,200 vehicles per hour and may accommodate up to 4,000 vehicles per
hour.
A single-lane entry is likely to be sufficient when the sum of the entering and
conflicting volumes is less than 1,300 vehicles per hour. A two-lane entry (and
circulation roadway) is likely to be sufficient when the sum of the entering and
conflicting volumes is less than 1,800 vehicles per hour. A detailed capacity
evaluation should be conducted to verify lane numbers and arrangements.
Multilane roundabout may contain a minimum of one entry with two or more
lanes and may requires under circulatory roadways to accommodate more than
one vehicle travelling side by side. The roundabout may have a different number
of lanes or transition on one or more legs. The number of lanes should be the
minimum needed for the anticipated traffic demand to effect smooth flow of the
traffic on the roundabout. The design speed at the entry, on the circulatory
roadway and at the exit may be slightly higher than those for single-lane
roundabout. Multi-lane roundabout include raised splitter island, truck aprons a
non-traversable central island and appropriate entry path deflection.
The size of multilane roundabout is typically determined by balancing two
critical design objectives, the need to achieve deflection, and providing sufficient
natural vehicle path alignment. To achieve both of these objectives requires a
larger diameter than those for single-lane roundabout. Generally, the inscribed
circle diameter of a multilane roundabout ranges from 46 to 68 m (two lanes)
and 61 to 90 m (three lane) to achieve adequate speed control and alignment.
Truck apron are recommended to accommodate larger design vehicles and keep
the inscribed circle diameter reasonable.
For a multilane roundabout circulatory roadway, the width is dependent on the
types of vehicles that the roundabout, where traffic is mainly passenger cars (P)
and single-unit truck (SU), the appropriate width maybe either two passenger
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
cars or a car/truck combination. For semi-trailer traffic (WB-50) greater than
10%, it may be acceptable to design for a semi-trailer/passenger vehicle
combination.
Typical lane width for multilane circulatory roadways 8.5 to 9.8 m for two lane
and 12.8 m to 15.0 m for three-lane circulatory roadways.
Fundamental Principles
The goal of any roundabout design should be to achieve:

Slow entry speed and consistent circulation speed through the roundabout by
using deflection.

Appropriate number of lanes and lane assignment to achieve adequate
capacity, lane volume, and lane continuity.

Smooth channelization that is intuitive to drivers and results in vehicles
naturally using the intended lanes.

Adequate accommodation of the design vehicles.

A design to meet the needs of pedestrians and cyclists.

Appropriate sight distance and visibility.
Advance direction signs of the map-type are always necessary.
Figure 4-14
Example of an Urban Roundabout
Source: AASHTO,2011, A Policy on Geometric Design of Highway and Streets 6th Edition. Used by
Permission.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Figure 4-15
A Rural Roundabout
Source: AASHTO, 2011, A Policy on Geometric Design of Highway and Streets 6th Edition. Used by
Permission.
In some cases, a roundabout designed to accommodate design year traffic
volumes typically projected 20 years from the present, can result in substantially
more entering, exiting, and circulating lanes than needed in the earlier years of
operation. In such cases, the designer may wish to consider a phased design.
Two types of sight distance should be considered in roundabout design: (1)
stopping sight distance, and (2) intersection sight distance. As with other
intersection forms, the design should check that stopping sight distance is
provided at every point within the roundabout and on each entering and exiting
approach. Intersection sight distance should be verified for circulating vehicles.
However it is recommended that no more than the minimum required
intersection sight distance should be provided at each approach to discourage
high vehicle entry speed. Landscaping within the central island can be effectively
used to both provide forewarning of an intersection ahead and restrict approach
intersection sight distance.
Non-Motorized Users
These users are vulnerable and span a wide range of ages and abilities that can
have a significant effect on the design of a facility. Roundabouts to accommodate
non-motorized users must be designed to control speeds to less than 50 kph, and
to encourage motorists to give way on entry. Basic design dimensions that need
to be considered are listed below in Table 4-21.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Table 4-21
User
Bicyclist
Basic Design Details for Non-Motorized Roundabout Users
Characteristic
Dimension (m)
Affected Roundabout Feature
Length
1.8
Splitter island width at crosswalk
Minimum operating
width
1.2
Bike lane width on approach
roadways; shared use path width
Pedestrian
Width
0.5
Sidewalk width, crosswalk width
Wheelchair User
Minimum width
0.75
Sidewalk width, crosswalk width
Operating width
0.9
Sidewalk width, crosswalk width
Person pushing
stroller
Length
1.7
Splitter island width at crosswalk
Skaters
Typical operating
width
1.8
Sidewalk width
Source: Table 9.31 AASHTO, 2011, A Policy on Geometric Design of Highway and Streets 6th Edition. Used
by Permission.
Pedestrian crossings are typically provided approximately one car length behind
the yield line. Provision of a landscape buffer strip between the pedestrian path
and the circulating roadway can direct pedestrians to the pedestrian crossings on
each leg of the roundabout and discourage pedestrians from crossing to the
central island.
Bicycle lanes should not be provided through the roundabout as serious conflicts
occur where exiting traffic crosses the lane. Cyclists should negotiate
roundabouts as if they were in cars.
4.1.12
Other Intersection Design Considerations
Intersection Design Elements with Frontage Roads
Frontage roads are generally needed adjacent to arterials or expressways where
adjacent property owners are not permitted direct access to the major facility.
The improvement in capacity provided by the use of frontage roads can be offset
by the added conflicts introduced where the frontage road and arterial intersect
the crossroad.
In lightly developed areas, an intersection designed to fit minimum turning paths
of passenger vehicles may operate satisfactorily. However, in heavily developed
areas it is necessary to design intersections with expanded dimensions, and
particularly the width of outer separation.
For satisfactory operation with moderate-to-heavy traffic volumes on the
frontage roads, the outer separation should be 50 m or more on the basis of the
following considerations:

This dimension is the shortest acceptable length needed for placing signs and
other traffic control devices to provide proper direction to traffic on the
crossroad.

It usually affords acceptable storage space on the crossroad in advance of the
main intersection to avoid blocking the frontage road.

It enables turning movements to be made from the main lanes onto the
frontage roads without seriously disrupting the orderly movement of traffic.
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
It facilitates U-turns between the main lanes and two-way frontage roads.

It alleviates the potential of wrong-way entry onto through lanes of the
predominant highway.
The design year traffic volumes, turning movements, signal phasing, and storage
needs should determine the ultimate outer separation distance.
Except for the width of outer separation, the design elements for intersections
involving frontage roads are much the same as those for conventional
intersections.
Traffic Control Devices
Traffic control devices are used to regulate, warn, and guide traffic and are a
primary determinant in the efficient operations of intersections. It is essential
that intersection design be accomplished simultaneously with the development
of signal, signing, and pavement marking plans so that sufficient space is
provided for proper installation of traffic control devices.
At intersections that do not need signal control, the normal roadway widths of
the approach highways are carried through the intersection with the possible
addition of median lanes, auxiliary lanes, or pavement tapers.
Where volumes are sufficient to require signal control, the number of lanes for
through movements may also need to be increased where the volume approaches
the uninterrupted flow capacity of an intersection leg. Other geometric features
that may be affected by signalization are length and width of storage areas,
location and position of turning roadways, spacing of other subsidiary
intersections, access connections, and the possible location and size of islands to
accommodate signal posts or supports. At high-volume intersections at grade, the
design of the signals should respond to varying traffic demands, the objective
being to keep vehicles moving through the intersection. Signalized intersections
are designed by jointly considering the geometric design, capacity analysis,
design hour volumes, and physical controls. The number and arrangement of
lanes are crucial to successful operation of signalized intersections. The crossing
distances for both vehicles and pedestrians should normally be kept as short as
practical to reduce exposure to conflicting movements.
The first step in the development of intersection geometrics should be a complete
analysis of current and future traffic demand, including pedestrian, bicycle, and
transit users. The need to provide right and left-turn lanes to minimize the
interference of turning traffic on the movement of through traffic should be
evaluated concurrently with the potential for obtaining any additional right-ofway needed. Along a highway or street with a number of signalized intersections,
the locations where turns will or will not be accommodated should also be
examined to facilitate optimal traffic signal coordination.
Bicycles
When on-street bicycle lanes or off-street bicycle paths or both enter an
intersection, the design of the intersection should be modified accordingly.
Modifications may include special sight distance considerations, wider roadways
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
to accommodate on-street lanes, special lane markings to channelize and
separate bicycles from right-turning vehicles, provisions for left-turn bicycle
movements, or special traffic signal designs.
Pedestrians
Pedestrian facilities include sidewalks, crosswalks, traffic control features, and
curb ramps for persons with wheeled accessories and persons with mobility
impairments. When designing a project that involves curbs and adjacent
sidewalks to accommodate pedestrian traffic, proper attention should be given to
location and design of ramps and traffic control devices to accommodate the
needs of persons whose mobility depends on wheelchairs and other devices, and
persons with sight impairment who depend on texture and sound for mobility.
Lighting
Lighting can reduce crashes at highway and street intersections, as well as
increase the efficiency of traffic operations, particularly in urban and suburban
areas. Whether or not rural intersections should be lighted depends on the
planned geometrics and the turning volumes involved. Intersections that are not
channelized are seldom lighted.
Intersections with channelization, including roundabouts, should include
lighting. Large channelized intersections especially need illumination. Each gore
area should be illuminated to help drivers make decisions at diverge locations
and to be able to see the location for diverge movements in advance of headlight
range.
Driveways
Access to driveways introduces conflicts and friction into the traffic stream as
vehicles enter and leave the roadway. The function of driveways is similar to that
of public intersections, consistent with their intended use. Ideally, driveways
should not be located within the functional area of an intersection.
The regulation and design of driveways are intimately linked with the type of
road and zoning of the roadside. On new highways, right-of-way can be obtained
to provide the desired degree of driveway regulation and control. In some cases,
additional right-of-way can be acquired with the reconstruction of an existing
highway or agreements can be made to improve existing undesirable access
conditions. The main objectives of driveway regulation are to provide desirable
spacing of driveways and to provide a proper internal layout. Achieving these
objectives depends on the type and extent of legislative authority granted the
highway agency.
Midblock Left-Turns on Streets with Flush Medians
Paved flush or traversable-type medians are often used in commercial and
industrial areas where property values are high, and rights-of-way for wide
medians are often difficult to acquire. In general, two-way, left-turn lanes should
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
only be used in an urban setting where operating speeds are relatively low and
where there are no more than two through lanes in each direction.
4.1.13
Railroad-Highway Grade Crossings
A railroad-highway crossing, like any highway-highway intersection, involves
either a separation of grades or a crossing at-grade. The geometrics of a highway
and structure that involves the overcrossing or undercrossing of a railroad are
substantially the same as those for a highway grade separation without ramps.
Refer Figure 4-16.
Figure 4-16
Railroad-Highway Grade Crossings
Source: AASHTO, 2011, A Policy on Geometric Design of Highway and Streets 6th Edition. Used by Permission.
Horizontal Alignment
To the extent practical, crossings should not be located on either highway or
railroad curves, and the highway should intersect the tracks at a right angle with
no nearby intersections or driveways.
Vertical Alignment
The intersection of highway and railroad should be as level as practical. Where
used, vertical curves should be of sufficient length to provide an adequate view of
the crossing.
The crossing surface should be at the same plane as the top of the rails for a
distance of 0.6 m outside the rails. Rails that are superelevated, or a roadway
approach section that is not level, need a site-specific analysis for rail clearances.
Crossing Design
The geometric design of railroad-highway grade crossings involves the elements
of alignment, profile, sight distance, and cross section, and should be made jointly
when determining the warning devices to be used. For low-volume crossings
where adequate sight distance is not available, additional signing may be needed.
Traffic control devices for railroad-highway grade crossings consist primarily of
signs, pavement markings, flashing light signals, and automatic gates.
When only passive warning devices such as signs and pavement markings are
used, highway drivers are warned of the crossing location but need to determine
for themselves whether or not there are train movements for which they should
stop.
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When active warning devices such as flashing light signals or automatic gates are
used, the driver is given a positive indication of the presence or the approach of a
train at the crossing. Considerations for evaluating the need for active warning
devices at a grade crossing include the type of highway, volume of vehicular
traffic, volume of railroad traffic, maximum speed of the railroad trains,
permissible speed of vehicular traffic, volume of pedestrian traffic, crash history,
sight distance, and geometrics of the crossing.
The highway traveled way at a railroad crossing should be constructed for a
suitable length will all-weather surfacing. A roadway section equivalent to the
current or proposed cross section of the approach roadway should be carried
across the crossing. The crossing surface itself should have a riding quality
equivalent to that of the approach roadway.
Sight Distance
The two considerations for vehicle drivers intending to cross a railroad-highway
grade intersection without train-activated warning devices are:
4.1.14

The vehicle operator can observe the approaching train in a sight line that
will allow the vehicle to pass through the grade crossing prior to the train’s
arrival at the crossing.

The vehicle operator can observe the approaching train in a sight line that
will permit the vehicle to be brought to a stop prior to encroachment in the
crossing area.
Lay-By
A paved area at the side of an expressway designated for driver to stop in, for
emergency parking or where vehicles can wait.
In addition to acting as short-term stopping places, lay-by may be provided for
more specialized function such as emergency lay-by for broken down vehicles,
bus lay-by and hardstanding where maintenance vehicles may pull off the road.
Several factors need to be taken into account when considering where to site a
lay-by as well as the land-take requirements and consequently it should be
considered at an early stage in the design process in order to reach a balanced
solution.
Lay-by should not be sited on the inside of a left hand curve of radius of less than
the appropriate value for the design speed of the road given in Table 4-22 as this
increases the risk that a fatigued driver may unintentionally cater the lay-by at
high speed.
Table 4-22
Appropriate Value for the Design Speed of the Road
Design Speed (kph)
120
100
85
70
60
Minimum Curve Radius (m)
2040
1440
1020
720
510
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Advance signing enables driver to decide whether or not to stop a lay-by in good
time, thus avoiding sudden changes in direction and speed. The recommended
spacing is 2.5 km. The cross slope of the lay-by is the same as the cross slope of
the expressway.
4.2
Grade Separations and Interchanges
4.2.1
Introduction and General Types of Interchanges
The ability to accommodate high volumes of traffic safely and efficiently through
intersections depends largely on the arrangements provided for handling
intersecting traffic. The greatest efficiency, safety, and capacity are attained when
intersecting traveled ways are grade separated. An interchange is a system of
interconnecting roadways in conjunction with one or more grade separations
that provides for the movement of traffic between two or more roadways or
highways on different levels.
The selection of the appropriate type of grade separation and interchange, along
with its design, depends upon:

Highway classification

Character and composition of traffic

Design speed

Degree of access control

Signing needs

Economics

Terrain

Right-of-way
Essential interchange elements include the expressway, cross street, median,
ramps, and auxiliary lanes.
Interchanges vary from single ramps connecting local streets to complex and
comprehensive layouts involving two or more highways.
4.2.2
Warrants for Interchanges and Grade Separation
An interchange can be a useful solution to improve many intersection conditions
either by reducing traffic bottlenecks or by reducing crash frequencies, but the
high cost of construction limits their use to those cases where the additional
expenditure can be justified.
The following conditions should be considered to determine if an interchange is
justified at a particular site:
1. Design designation – A decision to develop a highway with full access control
between selected terminals becomes a warrant for providing highway grade
separation or interchanges for all intersecting roadways crossing the
highway.
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2. Reduction in bottlenecks or spot congestion – Inability to provide essential
capacity with an at-grade facility resulting in congestion provides a warrant
for an interchange where development and available right-of-way permit.
3. Reduction of crash frequency and severity –A disproportionate frequency of
serious crashes may warrant introduction of a highway grade separation or
interchange where low cost crash reduction strategies are ineffective or
impractical.
4. Site topography – The topography at some sites may be such that grade-
separation designs are the economically preferable.
5. Road-user benefits – The relationship between road-user benefits (such as
reduced fuel and oil usage, reduced tire wear, repairs, delay to motorists, and
crashes) and the cost of improvement provides an economic warrant for the
improvement.
6. Traffic volume warrant–Interchanges that greatly improve the movement of
traffic compared to the capacity of an at-grade intersection.
Additional warrants applicable specifically to grade separations include:
1. Servicing local roads and streets that cannot be terminated outside the right-
of-way limits of expressways.
2. Providing access to areas not served by frontage roads or other means of
access.
3. Elimination of a railroad-highway grade crossing.
4. Serving unusual concentrations of pedestrian traffic.
5. Serving bikeways and routine pedestrian crossings.
6. Providing access to mass transit stations within the confines of a major
arterial.
7. Providing free-flow operation of certain ramp configurations and serve as
part of an interchange.
8. Ideally, rural arterial highway intersections and railroad crossings should be
grade separated where they cannot be provided with adequate sight distance.
4.2.3
Adaptability of Highway Grade Separations and Interchanges
The general types of intersections are at-grade intersections, highway grade
separations without ramps, and interchanges. The selection of which type to use
for a particular site is often a compromise after consideration of design traffic
volume and pattern, cost, topography, and availability of right-of-way.
Traffic and Operation
Each intersection type accommodates through traffic to varying degrees of
efficiency:

At-grade intersections cause minimal delay to through traffic where the
traffic on the minor crossroad is minor and the topography is flat. Where the
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minor crossroad traffic volume is sufficient to justify a traffic signal, delay is
experienced by all through traffic. Where through and crossroad volumes are
nearly equal, approximately 50% of the traffic on each approach needs to
stop.

Highway grade separations do not delay through traffic, except where
approach gradients are long and steep and there are many heavy trucks in
the traffic stream.

Interchange ramps have no severe effect on through traffic, except where
their capacity is not adequate, the merging or speed-change lanes are not of
adequate length, or a full complement of turning roadways is not provided.
Turning movements can affect traffic operations at an intersection and are
accommodated to varying degrees by each type of intersection.

Interchanges provide ramps for turning movements. Where these movements
are light, a one quadrant ramp design may suffice. However, left-turning
movements on both highways may be no better accommodated than at an
intersection at grade. Ramps provided in two quadrants may keep the major
highway free of interference, while crossings of through movements occur at
the crossroad. An interchange with a ramp for every turning movement is
suitable for heavy volumes of through traffic, and for any volume of turning
traffic provided the ramps and terminals are designed with sufficient
capacity.

Interchange right-turning movements follow simple direct or nearly direct
paths on which there is little potential for driver confusion. Cloverleaf
interchanges involve loop paths for left-turn movements, which may cause
confusion, involve additional distance, and induce weaving movements. The
diamond pattern of ramps is simpler and more adaptable, with direct left
turns or roundabouts on the minor road.
Except on expressways, interchanges are usually provided only where crossing
and turning traffic cannot readily be accommodated by an at-grade intersection.
Interchanges are adaptable to various traffic mixes, and help maintain the
capacity of the intersecting highways by minimizing vehicle delays caused by
relatively slow moving heavy trucks.
Site Conditions
Interchanges can often be satisfactorily fitted to rolling or hilly topography.
Meanwhile interchange design may be straightforward in flat terrain, but may
require ramp grades that do not favor some types of vehicle.
Right-of-way needed for an interchange is largely dependent on the number of
turning movements that need separate ramps, but also depends on highway type,
topography, overall interchange criteria, and the impact on adjacent property.
Type of Highway and Intersecting Facility
High-speed highways have greater need for interchanges than low-speed roads
with similar volumes. With full control of access, and grade separation at all
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crossroads, they are essential components of expressways. Configuration will
vary with terrain, development along the highway, frontage roads, right-of-way,
and ramp layouts to expedite entrance to or exit from the expressway.
The extent to which local services should be maintained or provided is also a
consideration in selection of intersection type.
4.2.4
Access Separations and Control on the Crossroad at Interchanges
Interchanges are expensive to build and equally expensive to upgrade. It is
essential that they be designed and operated as efficiently as practical to
preserve their intended function. Future access control needs careful
consideration to avoid congestion on the crossroad from queue spillback, stopand-go travel, and heavy weaving volumes as development occurs in close
proximity to the ramp terminus and the crossroad.
4.2.5
Safety
Grade separation structures themselves may be a roadside obstruction, but their
related safety concerns can be minimized by the use of adequate clear roadside
widths and use of protective devices. More importantly, by separating the grades
of the intersecting roadways, grade separation substantially reduces the
incidence and severity of crashes caused by crossing and turning movements.
Common safety problems at grade separated intersections include:
4.2.6

Over-complicated, confusing layouts

Lack of clear advance signing

Exit ramps that require too much reduction in speed

Gores that are not crashworthy
Staged Development
Staged development of grade-separation structures should be considered, but is
not always practical where the ultimate development consists of a single
structure.
Ramps are well adapted to stage development.
The provision of grade separated intersections on undivided roads violates
driver expectancy.
4.2.7
Economic Factors
Initial Costs
The combined cost of an interchange structure, ramps, throughways, grading and
landscaping of large areas, and possible adjustments in existing roadways and
utilities, generally exceeds the cost of an at-grade intersection.
Directional ramp interchanges involve more than one structure, and usually cost
much more than a simple interchange.
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Maintenance Costs
Interchanges generally have large paved and variable slope areas that have to be
maintained, as well as the structure, signs, landscaping and lighting. These costs
are usually more than for maintenance of an at-grade intersection.
Vehicular Operating Costs
While evaluation varies considerably for different vehicles and intersection
arrangements, for intermediate-to-heavy traffic, the total vehicle operating costs
at an intersection usually will be lower with an interchange than with an at-grade
design, especially where through movements predominate.
4.2.8
Grade Separation Structures
Introduction
Various types of structures are used to separate the grades of two intersecting
roadways, or a highway and a railroad. The discussion below is confined to the
geometric features of grade separation structures, and not structural design.
Grade-separated intersections should never be used on single carriageway roads
as they violate driver expectancy.
Types of Separation Structures
There are three general types of grade-separation structures: deck type, through,
and partial through. The deck type is the most common, but through and partial
through types are often used for railroad structures. In special cases where long
spans are involved, truss bridges may also be used.
Separation structures should aim at maintaining a constant clear roadway width
with a uniform protective railing or parapet, and give the impression to drivers
that their surroundings are similar to other points on the highway. The structure
design needs to fit the environment in a functional manner without drawing
excessive or distracting attention. Overpass structures should have liberal lateral
offset on roadways at each level; all piers and abutment walls should be suitably
offset from the traveled way; the underpass roadway median and off-shoulder
slopes should be rounded, and there should be a transition to backslopes to
redirect errant vehicles away from structural elements.
Grade separation structures should conform to the lines of the highway
approaches in alignment, profile, and cross section. This may result in varying
structure details, resulting in the need for individual designs for each separate
structure. Consideration of with the bridge should start with alignment, and close
coordination should be maintained throughout the design process to achieve the
most functional and economic result.
A deck-type structure is the most suitable for highway overpasses. The upper
roadway has unlimited vertical clearance, while lateral offset is controlled by the
location of protective barriers. Parapets and railings are designed with the
strength and ability to serve as roadside barriers.
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For vehicular operation the most desirable structure for the underpass highway
is one that will span the entire highway cross section and provide lateral offset
from structural supports. Center supports may be used on divided highways
where the median is wide enough to provide sufficient lateral offset, or protective
barriers will be required. Provision for future highway widening should be
considered during design, so that such provisions can be utilized as interim clear
space.
A single simple-span girder bridge may be used with spans of up to 45 m and can
accommodate severe skew and horizontal curvature. Spans of greater length
require greater structure depth and higher approach embankments. The
structure depth for single-span girder bridges is usually 1/15 to 1/30 of the span.
The conventional type of overpass structure over divided highways is usually a
deck-type bridge of two spans or more. Two or more structures are not
uncommon at interchanges.
Highway planning in urban areas needs to consider which arterial and cross
streets are important enough to warrant interchange ramps to preserve the
continuity of traffic flow on the local street system, and where frontage roads
may be used each side of the main facility, with due allowance for the land
development that commonly follows the construction of major roadways. Factors
that may influence planning of interchanges include the location of factories,
schools, churches, recreational areas, other public facilities, school bus routes,
and emergency response routes. Cross-street planning should also consider the
needs of pedestrians and bicyclists, and the need for separate dedicated facilities
for their access. Cross-street planning may also consider the phased development
of interchanges as development progressively takes place along the corridor.
Interchange design should also consider:

The need for adequate sight distances and clear roadside recovery areas.

Any necessary improvements needed to approaching streets, such as land
and shoulder widening, control of parking and pedestrian movements, line
marking, signage, and channelizing.

The aesthetics of the underside of an overpass structure.

The provision for expansion of approach width and vertical clearance at
structural openings to elevated viaducts.
Overpass Versus Underpass Roadways
A detailed study should be made at each proposed highway grade separation to
determine whether the major roadway should be carried over or under the
crossroad. Often this decision is based on whether:

The influence of topography predominates.

The topography does not favor either arrangement.

The alignment and gradeline controls of one highway predominate.
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A design that best fits the existing topography is usually the most visually
pleasing and economical to construct and maintain. Where topography does not
govern, the following guidelines should be examined:

Designs are often governed by the need for economy, not only for the
interesting highways but also for all area ramps and slopes. Alternative cost
analyses needs to consider the bridge types, span lengths, roadway cross
sections, angles of skew, soil conditions and cost of approaches.

Desirably, the roadway carrying the highest traffic volume should have the
fewest number of bridges for better readability and fewer issues when repair
and reconstruction are needed.

An underpass may be advantageous where the major road can be built close
to the existing ground, with no pronounced grade changes.

An undercrossing highway has a general advantage in that an approaching
interchange is easily seen by drivers.

The wide overlook from an overpass gives drivers a minimal feeling of
restriction.

Where turning traffic is significant, the ramp profiles are best fitted when the
major road is at the lower level. The ramp grades assist turning vehicles to
decelerate as they leave the major highway, and accelerate as they approach
it. Also ramp terminals are visible to drivers as they leave the major highway.

In rolling or mountainous terrain, the design that provides the better sight
distance on the major road should be preferred.

An overpass offers the best possibility for staged construction.

Troublesome drainage challenges may dictate that the major highway should
be carried over, rather than under, the crossroad.

The choice of underpass or overpass may be determined by design of the
highway as a whole, such as where the proposed grade separation is part of a
depressed or elevated expressway.

In some instances, it may be appropriate to locate the higher volume facility
in a depressed roadway to reduce noise impact.

Where a new highway will cross an existing route carrying a large volume of
traffic, overcrossing by the new highway will cause least disturbance to the
existing route.

Overcrossing structures have no limitation of vertical clearance, which can be
important for oversized loads on a major highway.
When determining the appropriate width of the roadway over or under a grade
separation, the design should aim to provide a facility on which driver reaction
and vehicle placement will be essentially the same as elsewhere on the
intersection roads. However, the width should not be so great as to result in a
high cost of structure without proportionate value in usefulness or crash
reduction.
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Underpass Roadways
The type of structure used should be determined by the dimensional, load,
foundation, and general site needs for the particular location. An underpass
should be consistent with the design standards for the rest of the facility, to the
extent practical. It is desirable that the entire roadway cross section be carried
through the structure without change.
The minimum lateral offset from the edge of the traveled way to the face of the
protective barrier should be the normal shoulder width. On divided highways,
the offset on the left side of each roadway is usually governed by the median
width. A minimum median width of 3.0 m may be used on a four-lane roadway
with 1.2 m shoulders and a rigid median barrier. For a roadway with six or more
lanes, the minimum median width should be 6.6 m with 3.0 m shoulders and a
rigid median barrier.
Where conditions preclude a full clear zone, all abutments, piers, and columns
should be shielded with suitable protective devices unless they are continuously
walled sections of adequate impact strength. Guardrail installed along the face of
an exposed pier or abutment should have an offset appropriate to the dynamic
lateral deflection of the particular rail type.
Vertical clearance is typically determined for an entire route. The vertical
clearance of all structures above the traveled way and shoulders should be at
least 0.3 m greater than the legal vehicle height to allow for future resurfacing
and an occasional slightly over-height load.
Overpass Roadways
The roadway dimensional design of an overpass should be the same as that of the
basic roadway. Overpasses are usually deck structures with their major features
being the parapet rail system, lateral offset, and median treatment where
applicable.
Bridge railings typically have some form of concrete base or parapet on which
metal or concrete rail(s) are mounted on structurally adequate posts. Solid rails
may also be used. The railing is designed to withstand impact from the design
vehicle without penetrating, vaulting, snagging, or causing the vehicle to roll
over.
Where an expressway overpass includes a pedestrian walkway or bicycle path,
barrier-type bridge rails are installed between the walkway and the roadway. A
pedestrian rail or screen is also provided on the outer edge of the walkway.
When the full approach roadway width is continued across the structure, the
parapet rail should align with the guardrail on the approach roadway. At some
interchanges, additional width for speed-change lanes or weaving sections is
needed across overpass structures. Where the auxiliary lane is a continuation of a
ramp, a weaving lane connecting entry and exit ramps, or a parallel speed-change
lane, the lateral offset to the bridge rail should be uniform and at least equal to
the width of the shoulder on the approach ramp.
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On a divided highway with a wide median, the overpass is likely to be built as two
parallel structures. Where the approach is a multi-lane, undivided roadway, or
has a flush median less than 1.2 m wide, a raised median is considered
unnecessary on short bridges up to 30 m in length, but is desirable on bridges of
120 m or more in length. On bridges between 30 and 120 m in length, local
conditions such as traffic volume, speed, sight distance, need for luminaire
supports, future improvement, approach cross section, number of lanes, and
whether the roadway is to be divided determine whether or not a median is
warranted. Where there are medians of narrow or moderate width on
approaches to long single structures, the structure should be wide enough to
accommodate the same type of median barrier as is used in the median of the
approach roadway.
Longitudinal Distance to Attain Grade Separation
The longitudinal distance needed for adequate design of a grade separation
depends on the design speed, the roadway gradient, and the amount of rise or fall
needed to achieve the separation.
4.2.9
Interchanges
General Considerations
The functions of interchanges are:

To provide grade separation between two or more traffic arteries.

To make possible the easy transfer of vehicles from open artery to the other
or between local streets and the expressway.
There are several basic interchange configurations to accommodate turning
movements at a grade separation. The type of configuration used at a particular
site is determined by the number of intersection legs, expected volumes of
through and turning movements, type of truck traffic, topography, design
controls, proper signing, and culture.
While interchanges are custom designed to fit specific site conditions, it is
desirable that the overall pattern of exists along a expressway have some degree
of uniformity. The need to simplify interchange design from the standpoint of
driver understanding and signing cannot be overstated.
Three-Leg Designs
An interchange with three intersecting legs consists of one or more highway
grade separations and one-way roads for all traffic movements. When two of the
three intersection legs form a through road and the angle of intersection is not
acute, the term ‘T-interchange’ applies. When all three intersection legs have a
through character or the intersection angle with the third intersection leg is
small, the interchange may be considered a Y-configuration.
Three-leg interchanges are very difficult to expand or modify in the future, and
should only be considered when future expansion to the unused quadrant is
highly unlikely.
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Four-Leg designs
Interchanges with four intersection legs may be grouped under six general
configurations: (1) ramps in one quadrant, (2) diamond interchanges, (3) double
roundabout interchange, (4) single-point diamond interchanges, (5) full or
partial cloverleafs, and (6) directional interchanges.
Interchanges with ramps in only one quadrant have application for an
intersection of roadways with low traffic volumes. Where a grade separation is
provided at an intersection because of topography, a single two-way ramp of
near-minimum design will usually suffice for all turning traffic. The ramp
terminals may be simple T intersections. A typical location would be at the
intersection of a scenic parkway and a rural two-lane highway where turning
movements are light, there is minimal truck traffic, and culture, and/or the
terrain, and/or the preservation of environment take precedence over providing
additional ramps. A high degree of channelization at the ramp terminals, at the
median, and at the left-turn lanes on the through facility is normally needed to
control turning movements. In some instances, a one-quadrant interchange may
be constructed as the first stage of a multi-stage construction program.
The simplest and most common interchange configuration is the diamond. A full
diamond interchange is formed when a one-way diagonal ramp is provided in
each quadrant. The ramps are aligned with free-flowing terminals on the major
highway, and the left turns at grade are confined to the crossroad. Advantages of
the diamond interchange are: all traffic can enter and leave the major road at
relatively high speed, left-turning maneuvers entail little extra travel, and a
relatively narrow band of right-of-way is needed. They are used in both rural and
urban areas. Roundabouts are the preferred solution for ramp and minor road
intersections. Where roundabouts are not used, a median should be provided on
the crossroad to prevent wrong-way entry on to one-way ramps, and
signalization is usually needed where the cross street carries moderate to large
traffic volumes. Where the cross street carries large traffic volumes, additional
structures can be added to provide directional ramps that replace signalized
intersections.
Double roundabout interchanges have roundabouts at each crossroad ramp
terminal, which allow free-flow movements on the cross street. Consideration
need to be given to the cross street traffic volumes and expressway ramp
volumes when analyzing the roundabout operations. Profile grades approaching
the roundabouts should be limited to 3% or less to avoid restricting driver sight
lines.
The single-point diamond interchange controls all four turning movements by a
single traffic signal. Typical characteristics include a narrow right-of-way, high
construction cost, and greater capacity than conventional tight diamond
interchanges. They are primarily suited to urban areas.
Cloverleafs are four-leg interchanges that employ loop ramps to accommodate
left-turning movements. Interchanges with loops in all four quadrants are
referred to as ‘full cloverleafs’ and all others are referred to as ‘partial
cloverleafs’. Cloverleafs are not recommended as they increase travel distance for
left-turning traffic, generate weaving maneuvers through the very short weaving
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length typically available, there are difficulties in placing signing for the exit, and
relatively large right-of-way areas are needed.
Directional ramp interchanges are used for important turning movements to
reduce travel distance, increase speed and capacity, eliminate weaving, and avoid
the need for out-of-direction travel. Rural areas rarely have the volume
justification required for direct connections. A semi-direct connection is defined
as a ramp where the driver exits to the right first, heading away from the
intended direction of travel, and then gradually swings left passing around other
interchange ramps before entering the intended road. These are usually confined
to major interchanges in urban areas.
Other Interchange Configurations
Offset Interchanges – This arrangement consists of a pair of trumpet
interchanges, one on each highway, which are connected to each other with a
ramp highway. The length of the connecting roadway depends on the distances
between each trumpet interchange and the crossing of the expressways.
Combination Interchanges – When one or two turning movements have very high
volumes with respect to other turning movements, a combination of two or more
for the above interchange options may be used.
General Design Considerations
Interchange configurations are considered under two categories – system
interchanges and service interchanges. The term system interchanges identifies
those interchanges that connect two or more expressways, whereas the term
‘service interchange’ applies to interchanges that connect an expressway to a
lesser facility.
Interchange configurations in rural areas are selected primarily on the basis of
service demand. When the intersecting roadways are expressways, directional
interchanges may be needed for high turning volumes. A combination of
directional, semi-directional, and loop ramps may be appropriate where turning
volumes are high for some movements and low for others. Theoretically a
cloverleaf interchange is the minimum design that can be used for the
intersection of two fully controlled access facilities, where right-of-way is not
prohibitive and weaving is minimal. However, a simple diamond interchange is
the most common configuration for the intersection of a major roadway with a
minor facility. Interchanges in rural areas are usually widely spaced and can be
designed on an individual basis.
Selection of appropriate interchange configuration in an urban environment
involves considerable analysis of prevailing conditions for the development and
comparison of configuration alternatives. Often interchanges are so closely
spaced that each one may be influenced directly by the preceding or following
interchange. On a continuous urban route, all interchanges should be integrated
into a system design rather than being considered on an individual basis. The
impact of a new interchange on existing crossroads is a major consideration.
Once several alternatives have been prepared for system design, they should be
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compared on the following principles: (1) capacity, (2) route continuity, (3)
uniformity of exit patterns, (4) single exits in advance of the separation structure,
(5) with or without weaving, (6) potential for signing, (7) cost, (8) availability of
right-of-way, (9) potential for staged construction, and (10) compatibility with
the environment. The most desirable alternatives can be selected for further
development.
Ramps
The term ‘ramp’ includes all types, arrangements, and sizes of turning roadways
that connect two or more legs at an interchange. The components of a ramp are a
terminal at each leg and a connecting road. The geometry of the connecting road
usually involves some curvature and a grade. Generally, the horizontal and
vertical alignment of ramps is based on lower design speeds than the intersecting
highways, but in some cases it may be equal.
Ramps are generally a one-way roadway, usually with both a left and right-turn
movement at the terminal on the minor intersecting road. Diamond interchanges
have four diagonal ramps. Full cloverleaf interchanges have four loop ramps that
require a turn through approximately 720 degrees to enter the other highway.
General ramp design considerations are:

Design speed – Desirably, ramp design speed should approximate the lowvolume running speed on the intersecting highways. This may not always be
practical, but design speeds should not be less than the low range presented
in Table 4-23.
Table 4-23
Guide Values for Ramp Design Speed as Related to Highway Design Speed
Highway Design
Speed (kph)
50
60
70
80
90
100
110
120
Ramp Design Speed (kph)
Upper range (85%)
40
50
60
70
80
90
100
110
Middle range (70%)
30
40
50
60
60
70
80
90
Lower range (50%)
20
30
40
40
50
50
60
70
Corresponding
minimum radius (m)
See Table 4-24
Source: Table 10-1 in AASHTO, 2011, A Policy on Geometric Design of Highways and Streets 6 th Edition.
Used by Permission.

Portion of ramp to which design speed is applicable – Values in Table 4-20
apply to the sharpest, or controlling ramp curve, usually on the ramp proper;
they do not apply to the ramp terminals which should be properly
transitioned and provided with speed-change facilities.

Ramps for right turn – A value between the upper and lower range value of
design speed is usually practical on ramps for right-turns. For right turns on
diagonal ramps of a diamond interchange, a value in the middle range is
usually practical.
4-76
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design

Loop ramps – minimum design speeds usually control, but for highway design
speeds above 80 kph, the loop design speed preferably should be no less than
40 kph.

Two-lane loop ramps – The radius of the inner edge of the travelled way of the
loop ramp normally should not be less than 55 to 60 m.

Semi-direct connections – Design speeds between the middle and upper
ranges shown in Table 4-20 should be used, and are typically 50 to 60 kph.

Direct connections – Design speeds between the middle and upper ranges
shown in Table 4-20 should be used, with a minimum of 60 kph.

Different design speeds on intersecting highways – The highway with the
greater design speed should be the control in selecting the design speed for
the ramp.

At-grade terminals – Where a ramp joins a major crossroad or street forming
an intersection at grade, Table 4-20 is not applicable to that portion of the
ramp near the intersection because a stop sign or signal control is normally
employed.

Curvature – Design guidelines for turning roadways at interchanges are
discussed in Section 3. The general shape of a ramp evolves from the type of
ramp selected, and is influenced by factors such as traffic pattern, traffic
volume, design speed, topography, intersection angle, culture, and type of
ramp terminal.

Sight distance – Sight distance along a ramp should be at least as great as the
design stopping sight distance.

Grade and profile design – The profile of a typical ramp should be as flat as
practical, but it usually consists of a central portion on an appreciable grade,
coupled with terminal vertical curves and connections to the profiles of the
intersection legs.

Vertical curves – usually ramp profiles assume the shape of the letter ‘S’ with
vertical curves at each end. Additional vertical curves may be needed where
ramps overpass or underpass other roadways.

Superelevation and cross slope – The following guideline should be used:
o
4-77
Superelevation rates, as related to curvature and design speed on ramps,
are provided in Tables 3-13 to 3-16 should apply.
o
The cross slope on portions of ramps on tangent should be sloped one way at a
practical rate ranging from 1.5 to 2% for high-type pavements.
o
In general, the rate of change in cross slope in the superelevation runoff section
should be based on the maximum relative gradients presented in Table 3-18.
o
Another important control in developing superelevation along the ramp terminal
is that of the crossover crown line at the edge of the through-traffic lane. The
maximum algebraic difference in cross slope between the auxiliary lane and the
adjacent through lane is shown in Table 4-25.
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Table 4-24
Minimum Radius Using Limiting Values of e and f
Metric
US Customary
Design
Speed
(km/h)
Maximum e
(%)
Maximum ∱
Total
(e/100 +
f)
Calculated
Radius
(m)
Rounded
Radius
(m)
Design
Speed
(mph)
Maximum e
(%)
Maximum
∱
Total
(e/100 +
f)
Calculated
Radius
(m)
Rounded
Radius
(ft)
15
4.0
0.40
0.44
4.0
4
10
4.0
0.38
0.42
15.9
16
20
4.0
0.35
0.39
8.1
8
15
4.0
0.32
0.36
41.7
42
30
4.0
0.28
0.32
22.1
22
20
4.0
0.27
0.31
86.0
86
40
4.0
0.23
0.27
46.7
47
25
4.0
0.23
0.27
154.3
154
50
4.0
0.19
0.23
85.6
86
30
4.0
0.20
0.24
250.0
250
60
4.0
0.17
0.21
135
135
35
4.0
0.18
0.22
371.2
371
70
4.0
0.15
0.19
203.1
203
40
4.0
0.16
0.20
533.3
533
80
4.0
0.14
0.18
280
280
45
4.0
0.15
0.19
710.5
711
90
4.0
0.13
0.17
375.2
375
50
4.0
0.14
0.18
925.9
926
100
4.0
0.12
0.16
492.1
492
55
4.0
0.13
0.17
1186.3
1190
60
4.0
0.12
0.16
1500.0
1500
15
6.0
0.40
0.46
3.9
4
10
6.0
0.38
0.44
15.2
15
20
6.0
0.35
0.41
7.7
8
15
6.0
0.32
0.38
39.5
40
30
6.0
0.28
0.34
20.8
21
20
6.0
0.27
0.33
80.8
81
40
6.0
0.23
0.29
43.4
43
25
6.0
0.23
0.29
143.7
144
50
6.0
0.19
0.25
78.7
79
30
6.0
0.20
0.26
230.8
231
60
6.0
0.17
0.23
123.2
123
35
6.0
0.18
0.24
340.3
340
70
6.0
0.15
0.21
183.7
184
40
6.0
0.16
0.22
484.8
485
80
6.0
0.14
0.20
252
252
45
6.0
0.15
0.21
642.9
643
90
6.0
0.13
0.19
335.7
336
50
6.0
0.14
0.20
833.3
833
100
6.0
0.12
0.18
437.4
437
55
6.0
0.13
0.19
1061.4
1061
110
6.0
0.11
0.17
560.4
560
60
6.0
0.12
0.18
1333.3
1333
120
6.0
0.09
0.15
755.9
756
65
6.0
0.11
0.17
1656.9
1657
130
6.0
0.08
0.14
950.5
951
70
6.0
0.10
0.16
2041.7
2042
75
6.0
0.09
0.15
2500.0
2500
80
6.0
0.08
0.14
3047.6
3048
15
8.0
0.40
0.48
3.7
4
10
8.0
0.38
0.46
14.5
15
20
8.0
0.35
0.43
7.3
7
15
8.0
0.32
0.40
37.5
38
30
8.0
0.28
0.36
19.7
20
20
8.0
0.27
0.35
76.2
76
40
8.0
0.23
0.31
40.6
41
25
8.0
0.23
0.31
134.4
134
50
8.0
0.19
0.27
72.9
73
30
8.0
0.20
0.28
214.3
214
60
8.0
0.17
0.25
113.4
113
35
8.0
0.18
0.26
314.1
314
70
8.0
0.15
0.23
167.8
168
40
8.0
0.16
0.24
444.4
444
80
8.0
0.14
0.22
229.1
229
45
8.0
0.15
0.23
587
587
90
8.0
0.13
0.21
303.7
304
50
8.0
0.14
0.22
757.6
758
100
8.0
0.12
0.20
393.7
394
55
8.0
0.13
0.21
960.3
960
110
8.0
0.11
0.19
501.5
501
60
8.0
0.12
0.20
1200.0
1200
120
8.0
0.09
0.17
667
667
65
8.0
0.11
0.19
1482.5
1483
130
8.0
0.08
0.16
831.7
832
70
8.0
0.10
0.18
1814.8
1815
75
8.0
0.09
0.17
2205.9
2206
80
8.0
0.08
0.16
2666.7
2667
4-78
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Metric
US Customary
Design
Speed
(km/h)
Maximum e
(%)
Maximum ∱
Total
(e/100 +
f)
Calculated
Radius
(m)
Rounded
Radius
(m)
Design
Speed
(mph)
Maximum e
(%)
Maximum
∱
Total
(e/100 +
f)
Calculated
Radius
(m)
Rounded
Radius
(ft)
15
10.0
0.40
0.50
3.5
4
10
10.0
0.38
0.48
13.9
14
20
10.0
0.35
0.45
7.0
7
15
10.0
0.32
0.42
35.7
36
30
10.0
0.28
0.38
18.6
19
20
10.0
0.27
0.37
72.1
72
40
10.0
0.23
0.33
38.2
38
25
10.0
0.23
0.33
126.3
126
50
10.0
0.19
0.29
67.9
68
30
10.0
0.20
0.30
200.0
200
60
10.0
0.17
0.27
105
105
35
10.0
0.18
0.28
291.7
292
70
10.0
0.15
0.25
154.3
154
40
10.0
0.16
0.26
410.3
410
80
10.0
0.14
0.24
210
210
45
10.0
0.15
0.25
540.0
540
90
10.0
0.13
0.23
277.3
277
50
10.0
0.14
0.24
694.4
694
100
10.0
0.12
0.22
357.9
358
55
10.0
0.13
0.23
876.8
877
110
10.0
0.11
0.21
453.7
454
60
10.0
0.12
0.22
1090.9
1091
120
10.0
0.09
0.19
596.8
597
65
10.0
0.11
0.21
1341.3
1341
130
10.0
0.08
0.18
739.3
739
70
10.0
0.10
0.20
1633.3
1633
75
10.0
0.09
0.19
1973.7
1974
80
10.0
0.08
0.18
2370.4
2370
15
12.0
0.40
0.52
3.4
3
10
12.0
0.38
0.50
13.3
13
20
12.0
0.35
0.47
6.7
7
15
12.0
0.32
0.44
34.1
34
30
12.0
0.28
0.4
17.7
18
20
12.0
0.27
0.39
68.4
68
40
12.0
0.23
0.35
36
36
25
12.0
0.23
0.35
119.0
119
50
12.0
0.19
0.31
63.5
64
30
12.0
0.20
0.32
187.5
188
60
12.0
0.17
0.29
97.7
98
35
12.0
0.18
0.30
272.2
272
70
12.0
0.15
0.27
142.9
143
40
12.0
0.16
0.28
381.0
381
80
12.0
0.14
0.26
193.8
194
45
12.0
0.15
0.27
500.0
500
90
12.0
0.13
0.25
255.1
255
50
12.0
0.14
0.26
641.0
641
100
12.0
0.12
0.24
328.1
328
55
12.0
0.13
0.25
806.7
807
110
12.0
0.11
0.23
414.2
414
60
12.0
0.12
0.24
1000.0
1000
120
12.0
0.09
0.21
539.9
540
65
12.0
0.11
0.23
1224.6
1225
130
12.0
0.08
0.20
665.4
665
70
12.0
0.10
0.22
1484.8
1485
75
12.0
0.09
0.21
1785.7
1786
80
12.0
0.08
0.20
2133.3
2133
Note: In recognition of safety considerations, use
𝑥
𝑒 =4.0%
should be limited to urban conditions.
Source: Table 3-7 AASHTO, 2011, A Policy on Geometric Design of Highway and Streets 6th Edition. Used
by Permission.
Table 4-25
Maximum Cross-Slope Difference at Crossover Crown
Design Speed of Exit or Entrance Curve (kph)
Maximum Algebraic Difference in Cross Slope
at Crossover Crown Line (5)
30 and under
5.0 to 8.0
40 and 50
5.0 to 6.0
60 and over
4.0 to 5.0
Source: Table 9-20 AASHTO, 2011, A Policy on Geometric Design of Highway and Streets 6th Edition. Used
by Permission
4-79
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Three segments of a ramp, the exit terminal, the ramp proper, and the entrance
terminal, should be analyzed to determine superelevation rates that would be
compatible with the design speed and the configuration of the ramp.
Gores
The term ‘gore’ indicates a neutral, usually triangular, area downstream from the
shoulder intersection points at the junction of a through roadway and an exit
ramp. The entire area should be striped to delineate the proper paths on each
side to assist the driver in identifying the gore area. The gore area, and the
unpaved area beyond, should be kept as free of obstructions as practical, and be
graded as nearly level as practical to provide a clear recovery area.
Yielding or breakaway supports should be employed for any exit sign in the gore
area. Where concrete footings are used, their surface should be flush with ground
level.
Ramp Terminal Design
The terminal of a ramp is that portion adjacent to the through traveled way,
including speed-change lanes, tapers, and islands. Ramp terminals may be the
STOP or GIVE WAY type, as in typical at the crossroad terminal of diamond or
partial cloverleaf interchanges, or the free-flow type where ramp traffic merges
with or diverges from high-speed through traffic at flat angles.
Terminals are further classified as either single or multilane, according to the
number of lanes on the ramp at the terminal, and as either a taper or parallel
type, according to the configuration of the speed change lane.
Profiles of ramp terminals should be designed in association with horizontal
curves to avoid sight restrictions that would adversely affect operations. At an
exit into a ramp on a descending grade, a horizontal curve ahead should not
appear suddenly to a driver. At an entrance terminal from a ramp on an
ascending grade, the portion of the ramp intended for acceleration and the ramp
terminal should closely parallel the through-lane profile to permit entering
drivers to have a clear view of the through road ahead, to the side, and to the
rear.
It is desirable that profiles of ramp terminals be provided with platforms of an
appropriate length that do not greatly differ from that of the adjacent throughtraffic lane or at-grade terminal.
Where off-ramps connect to a single carriageway local road it is important that
STOP or YIELD/GIVE WAY sign junction layouts (and preferably roundabouts)
are used in order to force the driver to adapt his speed and driving behavior to
the changed situation.
Other Interchange Design Features
The accommodation of pedestrians and bicycles through interchanges should be
considered early in the development of interchange configurations. The
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
movement of pedestrians and bicycles can be enhanced by providing appropriate
width sidewalks or paths, with the most direct route and minimal change in
vertical alignment, as far as practical from the vehicular traffic.
Grading and landscape development also requires due consideration, to provide
clear zones that assist in reducing crash severity, while also providing a low
maintenance landscape that help direct driver, pedestrian and cyclist attention to
their intended paths.
4-81
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Table 6-17
Joint Sealant Materials
Hot-pour sealants
Specification
Properties
AASHTO M 0173
Polymeric Asphalt-based
ASTM D 3405
SS-S-1401 C
ASTM D 1190
Polymeric
ASTM D 3405
Low Modulus
Modified
Elastomeric
SS-S-1614
Coal Tar, PVC
ASTM D 3406
Self Leveling
Cold-pour Sealants/Single Components
Silicone
ASTM D 5893
Self-leveling, non-sag, low
to ultra-low modulus
Nitrile Rubber
No specifications
Self-leveling, nonsag
Polysulfide
Self-leveling, low modulus
Preformed Polychloroprene Elastromeric (compression seals)
Preformed Compression Seals
ASTM D 2628
Lubricant Adhesive
ASTM D 2835
20 to 50% allowable strain
Source: American Concrete Institute ACI Committee Report 325-12R-12
In general, the joint sealants that are most effective in maintaining bond to the
face of the joint are those that are placed with a 1-to-1 width-to-height ratio, that
is, a shape factor of 1.0. Low-modulus sealants, however, can maintain good bond
strength even when placed at ratios of 1-to 2. With field-molded sealants, a stiff
self-adhering strip, coated paper, or metal foil is applied to the bottom of the
sealant space to prevent bond between the sealant and bottom of the reservoir
(Figure 6-13). The bond breaker also supports the sealant so that it does not sag
into the joint. Frequently, cord or rope is used as a bond breaker in the reservoir.
In that case, the reservoir should be deeper by an amount equal to the cord
diameter so the proper shape factor is maintained for the sealant (Figure 6-13).
The Joints should be filled to about 6 mm (0.25 in.) below flush with the
pavement surface.
Before sealing, the joint openings should be thoroughly cleaned of curing
compound, residue, laitance, and any other foreign material. Joint face cleanliness
directly affects the adhesion of the sealant to the concrete. Improper or poor
cleaning reduces the adhesion of the sealant to the joint interface, which
significantly decreases the life and effectiveness of the sealant. Cleaning can be
done with sandblasting, water, compressed air, wire brushing, or a number of
other ways, depending on the joint surface condition and sealant manufacturer’s
recommendations.
6-49
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Figure 6-13
Joint Sealant Reservoir
Source: American Concrete Institute ACI Committee Report 325-12R-12
Low-modulus Silicone Sealants
The newer low-modulus silicone sealants have properties that allow them to be
placed with a shape factor (depth-to-width) of 0.5 or slightly lower (twice as
wide as deep). This should be done only with the low-modulus silicones. They
should not be placed any thinner than half the width of the joint with a minimum
thickness of 13 mm (0.5 in.). These sealants have bonding strength in
combination with a low modulus, however, that allows them to be placed thinner
than the normal sealants. These recommendations should be cross-checked with
the sealant manufacturer to ensure proper performance. Usually, the supplier of
the sealant will provide minimum dimensions for width and depth for their
material. Silicone sealants require a separate operation to produce a uniform
surface and ensure bonding with the sidewall. They should be tooled by drawing
a specially shaped tool over the surface of the silicone sealant, which forces the
sealant into contact with the sidewall at the top of the sealant and forms the
correct shape for the sealant. If this s not done, the bond will be incomplete,
resulting in infiltration at the edge of the sealant and premature adhesive failure.
Recent studies have indicated improved bond of these types of sealant of sealants
to concretes containing limestone coarse aggregate when primers are used.
Polymer Sealants
Thermo-plastic polymer sealants are hot-poured and harden as they cool to
ambient temperature in the joint reservoir. Silicone sealants, cold-applied solvent
sealants, and the two-component polymer sealants require a curing period to
gain strength. Two-component polymer-type sealants require that two
components be thoroughly mixed in exact proportions as the material is being
placed in the joint. These sealants require special application equipment.
6-50
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Accurate temperature control for the polyvinyl chloride (PVC) type tar polymers
is critical for proper curing and development of beneficial properties.
Before sealing, the joint surfaces should be dry, clean, and free of curing
compound, residue, laitance, and any other foreign material. Cleaning can be
done by water or compressed air, wire brushing, sand blasting, or high-pressurewater blast, depending on the joint surface condition and sealant manufacturer’s
recommendation. Proper cleaning is essential to obtain a joint surface that will
not impair bond or adhesion with the field-molded sealant. The surfaces should
be dry when the sealant is placed in the joint well.
Compression Sealants
Preformed compression seals are compartmentalized or cellular elastomeric
devices that function between the joint faces in a compressed condition at all
times. The preformed compression seals should remain compressed
approximately 15% at maximum joint opening to maintain sufficient contact
pressure for a good joint seal and to resist displacement and generally not more
than 55% at maximum closing of the pavement joint to prevent
overcompression. A properly selected preformed seal takes into account the
specified compression range, installation temperature, width of the formed
opening, and expected slab movement. The seals should be installed about 6 mm
(0.25 in.) below the surface of the pavement. This dimension may vary in relation
to local environmental conditions and the service record of joints under similar
service conditions. For specific products, seal size recommendations and
availability should be obtained from the manufacturer or supplier.
Preformed compression seals require the application of a lubricant/ adhesive to
the reservoir side walls. While the lubricant/adhesive used during installation
has some adhesive qualities, its primary function is to provide lubrication during
installation. Its adhesive qualities should not be considered in design. The size of
the reservoir is chosen to ensure that the seal remains n compression at all times.
During installation, care should be taken to avoid twisting and to avoid stretching
the sealant more than 3%.
Hot-Applied, Field-Molded Sealants
When the sealant is hot-applied, the safe heating temperature should not be
exceeded, and the manufacturer’s instructions should be followed carefully.
Failure to follow to such instructions may result in a chemical breakdown of the
sealant and render the sealant useless. Because most of the hot-poured sealants
are asphalt-based, they are potential fire hazards, and safety precautions should
be taken. Proper melting units or kettles should be used to ensure proper control
of the sealant temperature. For liquid sealants, the surfaces should be dry and the
sealant should not be placed during cold weather. Good workmanship should
ensure that the sealant material is not spilled on the exposed surfaces of the
concrete.
Cold-Applied, Field-Molded Sealants
Most of the single-component cold-applied joint sealants are provided in small
cartridges and can be applied with a caulking gun. For a two-part or multipart
6-51
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
sealant, the components should be mixed in the proportion as specified by the
manufacturer. For these types of sealants, the mixing is an essential and
important part of the process and may require a specific type of mixer for large
projects. If the pot life (the maximum time after initial mixing when the sealant
can still be placed without adverse effects) of the sealant is long, the sealant can
be mixed at a placed other than the site. This is sometimes done to achieve more
complete mixing than can be done on-site.
6.7
Rigid Pavement Joint Design
Concrete pavements are subjected to expansion and contraction due to varying
environmental conditions and to minimize resultant cracking, joints are to be
provided at calculated intervals and to correct dimensions. There are three types
of joints in the construction of concrete pavements – contraction joints,
expansion joints and construction joints and they are discussed in detail below.
6.7.1
Contraction Joints
Contraction joints are provided to control cracking by relieving the stresses due
to temperature, moisture and friction. The provision of contraction joints at
calculated intervals effectively prevents random cracking that would have
otherwise occurred in concrete pavements. Contraction joints may be provided
both in transverse and longitudinal directions.
Joint movement in pavements is influenced by several factors such as the slab
length, volume change characteristics of concrete, and the temperature range.
Friction between subgrade and base provides some restraint to base movements
and this should be considered in determining the width of the joint.
The spacing of both transverse and longitudinal contraction joints depend on the
type of materials and environmental factors in relation to temperature
movements and moisture. Generally the spacing between contraction joints
decreases as the thermal coefficient, temperature change or sub-base frictional
resistance increases. The spacing may vary depending on the slab thickness and
the joint sealant capabilities.
The best method of determining the joint spacing for contraction joints is by
using the local knowledge and previous experience. However, the designer must
be careful to apply corrections for variations in construction material and other
parameters when determining the joint spacing.
In the absence of other information, as a rough guide, the contraction joints may
be provided so that the ratio of slab width to length does not exceed 1.25.
An approved joint sealant is used to fill the gap created by the joint. The sealant
must confirm to the DPWH Standard Specification for Concrete Joint Sealant
(Item 613, Department Order No. 11/2006).
For the joint sealant to work effectively, the sealant reservoir must be designed
to have a proper shape factor. The reservoir must be as nearly square as possible
and be recessed below the surface a minimum of 3 mm. The depth to width ratio
should be within the range of 1 to 1.5. For narrow joints with close joint spacing,
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
a cord or other material is to be inserted to a predetermined depth to define the
reservoir.
6.7.2
Expansion Joints
Concrete pavements when subjected to higher temperatures expand thus
inducing compressive stresses on concrete that may result in buckling of
pavements. Expansion joints are provided at calculated intervals to release those
compressive stresses and stop buckling of concrete pavements. The spacing of
expansion joints depend on the layout and construction limitations.
The use of expansion joints are usually introduced where pavement types
change, with pre-stressed pavements and at intersections. In general, the
provision of expansion joints is minimized for a given project due to cost,
complexity and performance problems that are specific to a given project.
6.7.3
Construction Joints
Construction joints are required to facilitate construction requirements
depending on the type and method of construction and the equipment used for
construction. The placement of construction joints is generally dictated by
construction method, sequence and the type of equipment used. For example,
longitudinal construction joints will have to be provided if the pavement is wider
than the width of the paving machine. The joints are to be provided in between
different passed of paving machine. Transverse construction joints may have to
be provided at the end of a day’s work or when recommencing work after a
machine breakdown.
6.7.4
Longitudinal Joints
Longitudinal construction joints are generally required when the width of the
pavement is more than the width of the paving machine.
6.7.5
Joint Layout
There are two major considerations that affect the design of joint layout:

The most common transverse joint is perpendicular to the road centerline
and across the full width of the carriageway.

Skewed transverse joints are preferred as against joints that are
perpendicular to the longitudinal axis. Skewed joints minimize the effect of
joint roughness and improve the pavement riding quality. The joint should be
skewed sufficiently so that wheel loads of each axle cross the joint one at a
time. Generally, the obtuse angle of the joint at the outside pavement edge
should be ahead of the joint in the direction of traffic. A skew angle of ten (10)
degree with respect to the line perpendicular to the centerline of the
pavement.
Figure 6-14 shows different types of PCCP Joints.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Despite the reduced deflections, skewed joints are no substitute for the use of
mechanical load transfer devices although there is some evidence that pavement
with skewed joints do have less faulting than those with perpendicular joints. It is
generally recommended that their use be limited to undoweled pavement on
low-volume routes.
Figure 6-14
Examples of Different Types of PCCP Joints
Source: American Concrete Pavement Association 1991, Design and Construction of Joints for Concrete
Highway
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
6.8
Material Properties and Specifications
6.8.1
Material Properties for Flexible Pavements
Sub-base, basecourse and asphalt surface course properties and AC structural
layer coefficients, modulus and structural number have been considered under
Section 6.5 Design Considerations.
Additional information on material properties is referenced in the following
DPWH Department Orders:
6.8.2

No. 66 dated 11 December 2007 on the subject ‘DPWH Standard
Specifications for Portland Cement Concrete Pavement (PCCP) using
Coralline Materials as Coarse Aggregates, Item 314’

No. 05 dated 29 January 2008 on the subject ‘DPWH Standard Specifications
for Instapave (Slurry Seal) System for Road Surface Treatment, Item 304A’

No. 30 dated 15 June 2010 on the subject ‘DPWH Standard Specifications on
the use of Lahar as Fine Aggregates in Hot Rolled Asphalt (HRA) and
Asphaltic Concrete (AC), Item 310A’

No. 13 dated 07 February 2013 on the subject ‘DPWH Standard Generic
Specification for Stone Mastic Asphalt (SMA), Item 734’.
Material Properties for Rigid Pavements
Refer to DPWH Orders:

No. 34 dated 12 February 1991 on the subject ‘Use of Fly Ash in Concrete
Mix’.

No. 11 dated 15 February 2006 on the subject ‘DPWH Standard Specifications
for Concrete Joint Sealant (Hot-Poured Elastic and Cold-Applied Types), Item
613’.

No. 18 dated 26 February 2006 on the subject ‘DPWH Standard Specification
for Portland Cement Concrete Pavement with Wire Mesh, Item 312’.
6.9
Design Procedure
6.9.1
Design Procedure for Flexible Pavements
The AASHTO method was chosen for determining the required pavement
structures. This method is described in the AASHTO ‘Guide for Design of
Pavement Structures, 1993.
The Structural Number (SN) were then determined from AASHTO Road Test
Equation for or Design Chart for flexible pavement based on using mean values
for each input.
The design procedure involves two major steps.
Step 1:
Solving for the value of SN from AASHTO test equation or Design Chart
shown in Figure 6-1
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
AASHTO Test Equation:
𝑙𝑙𝑙𝑙𝑙𝑙10 𝑊𝑊18 = 𝑍𝑍𝑅𝑅 ∗ 𝑆𝑆Ο + 9.36 ∗ 𝑙𝑙𝑙𝑙𝑙𝑙10 (𝑆𝑆𝑆𝑆 + 1) − 0.20 +
∗ 𝑙𝑙𝑙𝑙𝑙𝑙10 𝑀𝑀𝑅𝑅 − 8.07
where:
∆𝑃𝑃𝑃𝑃𝑃𝑃
𝑙𝑙𝑙𝑙𝑙𝑙10 [4.2−1.5]
1094
0.40 + (𝑆𝑆𝑆𝑆+1)5.19
+ 2.32
W18
=
predicted number of 18-kips equivalent single axle load
application for traffic study.
ZR
=
standard normal deviate from Table 6-11
𝑆𝑆Ο
=
combined standard error of the traffic prediction and
performance prediction from Table 6-11
=
MR
=
difference between the initial service ability index (𝑃𝑃Ο ) and the
design terminal service ability (𝑃𝑃𝑡𝑡 )
SN
=
Step 2:
Solving for thickness of pavement layers by the following equation:
where:
𝑆𝑆𝑆𝑆 = 𝑎𝑎1 𝐷𝐷1 + 𝑎𝑎2 𝐷𝐷2 𝑚𝑚2 + 𝑎𝑎3 𝐷𝐷3 𝑚𝑚3
ΔPSI
a1a2a3
Resilient modulus (psi)
Structural number indicative of the total pavement thickness
required
=
structural layer coefficient representative of surface, base and
sub-base course respectively from Table 6-3
D1D2D3 =
layer thickness of surface, base and sub-base course
respectively
m2m3
drainage coefficients of base and sub-base course respectively
from Table 6-12
=
Design Example for Flexible Pavement
Problem
:
Determine the thickness of AC, CAB and ASB for the singlelane entrance ramp of Zapote interchange based on the
following data:
1. 10 year analysis period
2. Bus AADT = 236 and Truck AADT = 393
3. Traffic growth rate = 7%
4. Subgrade CBR = 5.0
5. Bus LEF = 0.70 and Truck LEF = 1.60
6. m1 = m2 = 1.0
7. Layer coefficients : a1 = 0.38. a2 = 0.14 and a3 = 0.11
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Solution:
a) Solve for W18:
Bus Design Traffic = (236) [
(1+0.07)10 −1
](365) =
0.07
Truck Design Traffic = (393) [
For Single Lane Ramps
1.19*106
(1+0.07)10 −1
](365) =
0.07
1.982*106
DD = 1.00 and DL = 100%
100%
) (0.70) ∗ 106
100
Bus CESAL = 1.19 (1.00) (
100%
) (1.60) ∗
100
Truck CESAL = 1.982 (1.00) (
W18 = Total CESAL = 0.83 + 3.17
= 0.83 ∗ 106
106 = 3.17 * 106
= 4.00 * 106
b) Solve for R and ZR
From Section 6.5.8 see values from tables for expressway
R = 92% and
ZR = -1.405
c) Solve for 𝑆𝑆Ο
From Section 6.5.8 for flexible pavement the value of 𝑆𝑆Ο = 0.49
d) Solve for ∆𝑃𝑃𝑃𝑃𝑃𝑃
From Section 6.5.9, 𝑃𝑃Ο = 4.2 and 𝑃𝑃𝑡𝑡 = 2.5
∆𝑃𝑃𝑃𝑃𝑃𝑃 = 𝑃𝑃Ο − 𝑃𝑃𝑡𝑡
= 4.2 – 2.5
= 1.7
e) Solve for subgrade MR from Section 6.5.4
Convert CBR to MR
MR = 1500*5.0
= 7,500
f) Since SN is the only remaining unknown, it can be solved by assuming
any value, substitute to AASHTO Test Equation, compare the result to
the left side of the equation, adjust the assumed value and compared
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
again until the right side value is equal or slightly greater than the left
side of the equation.
Left Side = 𝑙𝑙𝑙𝑙𝑙𝑙10 𝑊𝑊18 = 𝑙𝑙𝑙𝑙𝑙𝑙10 (4.0 ∗ 106 )
= 6.602
Try SN = 4.00
Solve manually or by Excel spreadsheet formula.
Right
Side =
1.70
𝑙𝑙𝑙𝑙𝑙𝑙10 [
]
4.2−1.5
1094
0.40+
(4.0+1)5.19
=
(−1.405)0.49 + 9.36 ∗ 𝑙𝑙𝑙𝑙𝑙𝑙10 (4.00 + 1.0) − 0.20 +
+ 2.32 ∗ 𝑙𝑙𝑙𝑙𝑙𝑙10 (7,500) − 8.07
6.269 < 6.602 no good
Try again SN = 5.00
Right Side = 6.913 > 6.602 no good
Try again SN = 4.502
Right Side = 6.602 = 6.602 Left Side ok
Use :
SN = 4.502 = Required SN
g) Solve for thickness of AC, CAB and ASB.
By trial and error method using manual calculation or excel
spreadsheet formula:
Try D1 = 100 mm = 3.937 inches
D2 = 250 mm = 9.84 inches
D3 = 450 mm = 17.72 inches
Actual SN = 0.38 (3.937) + 0.14(9,84)(1.0) + 0.11(17.72)(1.0)
= 4.823 > 4.502 no good
Try D1 = 100 mm = 3.937 inches
D2 = 200 mm = 7.874 inches
D3 = 400 mm = 15.748 inches
Actual SN = 4.331 < 4.502 no good
Try D1 = 100 mm = 3.937 inches
D2 = 250 mm = 9.84 inches
D3 = 400 mm = 15.748 inches
Actual SN = 4.606 > 4.502 no good
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Try D1 = 100 mm = 3.937 inches
D2 = 200 mm = 7.874 inches
D3 = 450 mm = 17.72 inches
Actual SN = 4.547 ≈ 4.502 Ok
Use : D1 = 100 mm for AC
D2 = 200 mm for CAB
D3 = 450 mm for ASB
6.9.2
Design Procedure for Rigid Pavement
The design procedure discussed here is described in AASHTO Guide for Design of
Pavement Structures, 1993. The design method involves two major steps.
Step 1 : Determination of the value of modulus of subgrade reaction k by
assuming practical values of thickness of subbase course and using
AASHTO charts in Figure 6-3 and Figure 6-4.
Step 2 : Determination of thickness of PCCP slab using the following AASHTO
Road Test Equation for Rigid Pavement:
𝑙𝑙𝑙𝑙𝑙𝑙10 𝑊𝑊18 = 𝑍𝑍𝑅𝑅 ∗ 𝑆𝑆Ο + 7.35 ∗ 𝑙𝑙𝑙𝑙𝑙𝑙10 (𝐷𝐷 + 1) − 0.06 +
∗ 𝑙𝑙𝑙𝑙𝑙𝑙10
𝑆𝑆 ′ 𝑐𝑐 ∗ 𝐶𝐶𝑑𝑑 [𝐷𝐷0.75 − 1.132]
𝑙𝑙𝑙𝑙𝑙𝑙10 [
1+
∆𝑃𝑃𝑃𝑃𝑃𝑃
]
4.5−1.5
1.624∗107
(𝐷𝐷+1)8.46
+ (4.22 − 0.32𝑝𝑝𝑡𝑡 )
18.42
215.63 ∗ 𝐽𝐽 [𝐷𝐷0.75 − 𝐸𝐸𝑐𝑐 0.25]
( 𝑘𝑘 )
[
]
where:
W18
=
predicted number of 18-kips equivalent single axle load
application.
ZR
=
standard normal deviate from Table 6-11
𝑆𝑆Ο
=
combined standard error of the traffic prediction and
performance prediction from Table 6-11
D
=
thickness (inches) of pavement slab
∆𝑃𝑃𝑃𝑃𝑃𝑃
=
difference between the initial service ability index (𝑃𝑃Ο ) and the
design terminal service ability (𝑃𝑃𝑡𝑡 )
S’c
=
modulus of rupture (psi) for Portland cement concrete
Construction specifications usually require a characteristics
rigid pavement strength from which a mean target value for a
PCC Modulus of Rupture is established. To account for
variations in material characteristics and in the Modulus of
Rupture, and for the allowable percentage of strength (Ps)
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
distributions that normally fall below specified values, standard
deviations are calculated to determine the mean Modulus of
Rupture as follows:
S’c (mean) =
Sc + Z (Sds)
where:
S’c
=
estimated mean value for PCC modulus of Rupture, (psi);
Sc
=
construction specification for the Modulus of rupture, (psi);
Sds
Z
=
estimated standard Deviation of PCC modulus of rupture, (psi);
and
=
Standard normal deviation
=
0.841, for Ps = 20%
=
1.037, for Ps = 15%
=
1.282, for Ps = 10%
=
1.645, for Ps = 5%
=
2.327, for Ps = 1%
The average standard deviation for PCC flexural strength is 100
psi, which has been adopted for this project. A Ps of 10%, Sc of
600 psi, and an absolute minimum Sc of 510 psi will be allowed
in the construction specification at 14 days concrete strength.
These values yield and estimated S’c = 638 psi.
J
=
load transfer coefficient used to adjust for load transfer
characteristic of a specific design.
Cd
=
drainage coefficient from Table 6-13
Ec
=
modulus of elasticity (psi) for Portland cement concrete, and
k
=
effective modulus of subgrade reaction (pci).
=
composite modulus of subgrade reaction (pci)
k
͚
Design Example for Rigid Pavement
Design Example for Flexible Pavement
Problem
:
Determine the thickness of ASB and PCCP for a two-lane toll
plaza in Cavite as part of RT expressway extension based on
the following data:
1. 20 year analysis period
2. Bus AADT = 2466 and Truck AADT = 4110
3. Traffic growth rate = 7%
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
4. Subgrade CBR = 5.0, Aggregate subbase ESB = 15000, Loss
of Support = 1.0
5. Bus LEF = 0.70 and Truck LEF = 1.60
6. Terminal Serviceability Pt = 2.5
7. For concrete f’c = 4000 psi
8. Sc = 510, z = 1.282, Sds = 100
9. Dowelled pavement, J = 3.2
Solution:
a) Solve for W18:
Bus Design Traffic = (860) [
(1+0.07)20 −1
](365) =
0.07
Truck Design Traffic = (1,432) [
For Two Lane Toll Plaza
12.86*106
(1+0.07)20 −1
](365) =
0.07
21.43*106
DD = 1.00 and DL = 50%
50%
) (0.70) ∗
100
Bus CESAL = 12.86 (1.00) (
106 = 4.50 ∗ 106
50%
) (1.60) ∗
100
Truck CESAL = 21.43 (1.00) (
W18 = Total CESAL = 4.50 + 17.14
106 = 17.14 * 106
= 21.64 * 106
b) Solve for R and ZR
From Section 6.5.8 see values from tables for expressway
R = 92% and
ZR = -1.405 (based on Table 6-11)
c) Solve for 𝑆𝑆Ο
From Section 6.5.8 for rigid pavement the value of 𝑆𝑆Ο = 0.35
d) Solve for ∆𝑃𝑃𝑃𝑃𝑃𝑃
From Section 6.5.9, 𝑃𝑃Ο = 4.5 and 𝑃𝑃𝑡𝑡 = 2.5
∆𝑃𝑃𝑃𝑃𝑃𝑃 = 𝑃𝑃Ο − 𝑃𝑃𝑡𝑡
= 4.5 – 2.5
= 2.0
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
e) Solve for subgrade MR from Section 6.5.4
Convert CBR to MR
MR = 1500*5.0
= 7,500
f) Solve for S’c
S’c = Sc + z (Sds)
= 510 + 1.282 (100)
= 638
g) Solve for Ec
Ec = 57,000 (f’c)0.5
= 57,000 (4000)0.5
= 3.6 * 106
h) For drainage coefficient Cd
See values from Table 6-13
Cd = 1.00
i) Solve for k
Try subbase thickness DSB = 270mm = 10.63 inches
Use given values: ESB = 15,000
MR = 7,500
LS = 1.00
Use Figure 6-3 – Chart for Estimating Composite Modulus of
subgrade reaction, k
͚
͚
From Figure 6-3 chart the value of k = 425 pci
From Figure 6-4 chart the value of k = 150 pci
j) Solve for depth of slab D using AASHTO Road Test Equation for Rigid
Pavement. Compute for the values of left side and right side of the
equation separately. The only unknown value on the equation is D By
using excel spreadsheet assumed several practical values of D until the
right side of the equation is equal to or slightly greater than the left
side.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
𝑙𝑙𝑙𝑙𝑙𝑙10 (21.64) ∗ 106 = (−1.405) ∗ 0.35 + 7.35 ∗ 𝑙𝑙𝑙𝑙𝑙𝑙10 (𝐷𝐷 + 1) − 0,06 +
2.0
𝑙𝑙𝑙𝑙𝑙𝑙10 [4.5−1.5]
1.624∗107
1+
(𝐷𝐷+1)8.46
+ (4.22 − 0.32 ∗ 2.5) ∗ 𝑙𝑙𝑙𝑙𝑙𝑙10
For the left side:
638∗1.00[𝐷𝐷0.75 −1.132]
215.63∗𝐽𝐽[𝐷𝐷0.75 ]−
[
(
18.42
0.25
3.60∗106
150 )
]
log10 W18 = log10(21.64)*106
Left Side = 7.335
For the right side:
Try D = 250mm = 9,84 inches
Right Side = 6.89 < 7.335 no good
Try D = 300mm = 11.81
Right Side = 7.40 > 7.335 no good
Try D = 290mm = 11.417
Right Side = 7.304 < 7.335 no good
Try D = 293 mm = 11.545
Right Side = 7.335 = Left Side Ok for optimum value
But D = 293 mm is not practical
Also Subbase thickness of 270mm is also not practical
k) Try again using different thickness combination:
Try Subbase DSB = 180mm = 7.09 inches
͚
From Figure 6-3 Graph: k = 400
From Figure 6-4 Graph: k = 130
l) Re-calculate slab D using k = 130 to AASHTO Road Test Equation
Try D = 300mm = 11.811 inches
Right Side = 7.384 > 7.335 still higher
But 300mm is the most practical value for D.
Then use the Pavement Layer Thickness Combination of:
Subbase DSB = 180mm
PCCP
D = 300mm
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
6.9.3
Concrete Pavement
AASHTO Interim Guide Method – Selection of the serviceability index is based on
the lowest level of serviceability which will be tolerated before resurfacing or
reconstruction. For the conditions representative of a typical national highway,
an index of 2.5 is assumed.
The modulus of rupture (Sc) as determined by the test procedure specified in
AASHTO Designation T-97, using third-point loading is the basis for concrete
flexural strength. A working stress (ft) of 0.75 times the modulus of rupture is
recommended for use with these design charts. For this example, the modulus of
rupture (Sc) is assumed to have been determined to be 4.14 MPa.
Thus the working stress (ft x 0.75 Sc) for use in the design chart is 3.11 MPa.
Westergaard’s modulus of subgrade reaction (k) is used in this guide. It
represents the load in pounds per square inch on a loaded area, divided by the
deflection in inches of that loaded area. The scales for k in the design charts are
correlated with values obtained by plate loading tests performed in accordance
with AASHTO Designation T-222 using a 762 mm diameter plate. For this
example K is assumed to have been determined as 1.31 MPa.
The value assumed above for serviceability index is used to select the
appropriate design chart. Design Chart for Rigid Pavement, Pt= 2.5, is used for
this example. Two applications of a straight edge are required. First, the value of
total equivalent 80 kN single axle loads assumed for the 20-year analysis period
(2.8 x 106) on the left scale, and the assumed working stress (ft) in the concrete
3.11 MPa on the second scale, are used to locate a point on the pivot line. This
point on the pivot line and the assumed value for modulus of subgrade reaction,
K (190) on the right scale are used to determine the pavement slab thickness D,
on the third scale. For this example, the design thickness of the pavement slab is
8.3 inches, say 9 inches.
DPWH Department Order No. 22 dated 08 April 2011 on the subject ‘Minimum
Pavement Thickness and Width of National Roads’ specifies for Portland Cement
Concrete Pavement (PCCP):
6-64

Minimum thickness for new road construction, rehabilitation or upgrading
shall be 280 mm. However, a range of 230 mm - 280 mm may be used if
CESAL is not more than 7.0x106. Refer to DPWH Department Order No. 22,
Series 2011.

Minimum thickness for pavement rehabilitation using the crack and seal
method shall be 260 mm.

Thickness for pavement re-blocking shall be the same as the replaced blocks.

Minimum width for new road construction shall be 6.70 m.

Minimum width for rehabilitation or upgrading works involving a length of at
least 500 m shall be 6.70 m provided such works will not require right-ofway acquisition.
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
6.9.4
Flexible Pavement
AASHTO Interim Guide Method – Based on the design data, subgradeCBR equals
4.9% and using the chart for obtaining soil support value, the soil support value
is 4.6. Using the chart for structural number determination the weighted
structural number is found to be 3.7, refer Figure 6-15 Design Chart for Flexible
Pavement. Pt = 2.5 and W18 = 2.8 x 106 (10 year study).
Assuming that the flexible pavement structure is of asphalt concrete above a
crushed stone base course layer and with a granular sub-base (passing
specification) we have:
SN = 3.8 = 0.38 D1 + 0.15 D2 + 0.11 D3
Considering a minimum thickness of 100 mm of asphalt concrete:
SN of asphalt = (100/25.4) x 0.38 = 1.496
If the crushed stone base course is 200 mm,
SN of the base course = (200/25.4) x 0.15 = 1.181
SN of sub-base = 3.7 – (1.496 + 1.181) = 1.023
1.023 = 0.11 D3 / 25.4
D3 = (1.023 x 25.4) / 0.11 = 236.22 mm
Therefore the new pavement will be composed of:
100 mm Asphalt Concrete
200 mm Crushed Stone Base Course
250 mm Granular Sub-base (passing Specifications)
Road Note 29 Method – Using the same traffic data, the Total Cumulative Number
of standard axles as 2.8 x 106 (10 year study)
Subgrade CBR = 4.9%
Using Figure 6-16 and Figure 6-17, Thickness of Sub-base and Wet and Dry
Bound Macadam Roadbases, and Minimum Thickness of Surfacing and
Roadbases, the flexible pavement structure consists of the following:
Asphalt Concrete = 80 mm
Crushed Aggregate Base = 180 mm (175-graph)
Aggregate Sub-base = 180 mm
Group Index Method – This method of flexible pavement design has the
advantage of requiring only a very few simple tests for classification of soil and
calculation of an empirical number called the ‘Group Index’. Refer Figure 6-18.
Subgrade soil is classified into many groups which render itself an extremely
variable material without any possible exact solution of the required depth of
base and surfacing. However it is know from experience that certain soils are
more or less stable than others and their relative stabilities may be obtained
from such simple tests as liquid limit, plasticity index and grading (percentage of
fines). The ‘Group Index’ is determined from these tests by the formula or graph
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
shown in Figure 6-18. Group Index is shown in parenthesis after group
classification symbol as A-2-6(3), A-4(5), A-7-5(11).
The higher the ‘Group Index’ of the subgrade the lower its strength and the
greater the thickness of the sub-base required. The depths of base and surfacing
increases with the volume of commercial traffic expected and are determined
from Figure 6-19. The chart is based upon average highway experiences and
takes care of the subgrade characteristics, traffic and justifiable factor of safety
on the basis of availability of satisfactory sub-base materials. The design chart is
also based on the following assumptions with regard to climate, compaction and
drainage:
1. The overall thickness shown by the Chart takes care of most variations of
climatic conditions.
2. Compaction of the subgrade is not less than 95% of its maximum dry density
as determined by AASHTO method prior to placing of the sub-base and base
thereon, and compaction of the sub-base and base is not less than 100% of
such density.
3. The subgrade is sufficiently above the water table to permit the proper
compaction of the subgrade prior to placing base or sub-base and that under
drainage or sufficient embankment height is provided where necessary to
keep the group water table at least 1.00 meter below the road surface.
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Design Chart for Flexible Pavements Pt=2.5
Source: AASHTO, 1993, Design of Pavement Structure. Used by Permission.
Figure 6-15
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Wet-Mix and Dry-Bound Bituminous Roadbases: Minimum Thickness of Surfacing and Roadbase
Source: Road Note 29, TRRL
Figure 6-16
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Thickness of Subbase
Source: Road Note 29, TRRL
Figure 6-17
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Figure 6-18
Group Index – Chart No. 1
Source: DPWH DGCS Volume II, 1984
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Figure 6-20 shows the lowering of the water table by raising the grade of the
embankment to at least one meter from the surface. Figure 6-21 shows the
keeping of the water table at least 1.00 meter below the road surface by the
installation of under-drains.
Failure to comply with the required degree of construction compaction and / or
drainage conditions may be a cause of road failure.
Sub-base
The thickness of sub-base depends upon the subgrade characteristics expressed
in ‘Group Index’ number and not on daily volume of commercial traffic. The five
general classes of subgrade are as follows:
1. ‘Excellent’ subgrade classified as A-1-a, being of natural gravelly material
equivalent to high quality granular base. No sub-base and base are required.
2. ‘Good’ subgrade having a group index of zero with a tolerance of one does not
require sub-base under the base and surface course.
3. ‘Fair’ subgrade having a group index ranging from 2 to 4 should require about
100 mm of ‘good’ selected material or sub-base, or granular base thickness
should be increased by an amount approximately one-half of the sub-base or
50 mm.
4. ‘Poor’ subgrade has a group index ranging from 5 to 9 and requires about 200
mm of selected material sub-base, the lower of which should be of at least
‘Fair’ quality while the upper 100 mm should be at least ‘Good’ quality; or
granular base thickness should be increased by about 100 mm or one-half of
the sub-base thickness.
5. ‘Very Poor’ subgrade has a group index ranging from 10-20 and requires
about 300 mm of selected material sub-base, the lower 200 mm of which
should be at least ‘Fair’ quality and the upper 100 mm should be at least
‘Good’ quality; or the granular base thickness should be increased by about
150 mm or one-half the sub-base thickness required.
It is important to provide a seal layer of sand about 100 mm thick or have the
lower layer of sub-base or base placed contiguous to this class of subgrade
containing sufficient sand sizes to prevent infiltration of the clay soil into the subbase or base.
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Figure 6-19
Group Index – Chart No. 2
Source: DPWH DGCS Volume II, 1984
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Figure 6-20
The Lowering of the Watertable by Raising the Grade of the Embankment to at
Least One Meter from the Surface
Figure 6-21
The Keeping of the Water Table at Least 1.00 m Below the Road Surface by the
Installations of Under-Drains
Depth of Base Course and Surfacing
The maximum unit pressure applied to a subgrade under a given wheel loading
decreases as the depth of the base course or base and sub-base covering is
increased. To express it in a general way, the unit pressure caused by a wheel
loading at any point below the road surface decreases as the depth from the
surface increases. It is necessary, therefore, to place the strongest material at the
road surface where the unit pressure is greatest and progressively weaker
materials may be used as the depth increases.
The formula for the group index is:
Group Index = 0.2 a + 0.005 ac + 0.01 bd
where:
a
= that portion or percentage of subgrade soil passing No. 200 (0.074 mm
opening) sieve greater than 35 and not exceeding 75, expressed as a
positive whole number (1-40).
b
= that portion or percentage of subgrade soil passing No. 200 sieve greater
than 15 and not exceeding 55%, expressed as a positive whole number (140).
c
= that portion of the numerical liquid limit greater than 40 and not exceeding
60 expressed as a positive whole number (1-20).
d
= that portion of the numerical plasticity index greater than 10 and not
exceeding 30 expressed as a positive whole number (1-20).
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The group index of the soil can be more easily calculated from the charts given.
A Characteristic of Soils (Classification as per AASHTO Designation M-145) is
shown in Table 6-18.
Charts for graphical determination of group index are shown in the Figure 6-22.
The DPWH Department Order No. 22 dated 08 April 2011 on the subject
‘Minimum Pavement Thickness and Width of National Roads’ specifies for
Asphalt Pavement:
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
Minimum thickness for overlay works shall be 50 mm.

Pavement thickness of more than 50 mm shall be considered only if the cost
of the asphalt pavement is less than the cost of 230 mm thick PCCP.

Minimum width for new road construction shall be 6.70 m.

Minimum width for rehabilitation or upgrading works involving a length of at
least 500 m shall be 6.70 m provided such works will not require right-ofway acquisition.
Classification of Soil and Soil - Aggregate Mixtures (with Suggested Subgroups)
Source: DPWH DGCS Volume II, 1984
Table 6-18
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Group Index – Chart No. 3
Source: DPWH DGCS Volume II, 1984
Figure 6-22
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
6.10
Rigid Pavement Reinforcement Design
Steel reinforcement may be required in cement concrete pavements in some
cases. The purpose of providing steel reinforcement in pavement sections is not
to prevent cracking of concrete but to hold tightly closed any cracks that do occur
in such a manner that the load carrying capacity of the base is preserved.
The factors affecting the development of cracks in a concrete pavement section
are the changes in temperature and moisture levels and the frictional resistance
from underlying material. With the drop of temperature, the slab tends to
contract but the contraction is resisted by underlying material through friction
and shear between the two layers of material causing the tensile forces to
develop reaching a maximum at mid-slab. When the tensile stresses thus caused
exceed the tensile strength of concrete, a crack will develop transferring all the
stresses to the steel reinforcement. Therefore the purpose of the design of
reinforcements is to allow them to carry these stresses without any appreciable
elongation that would result in excessive crack width.
The following sections discuss the provision of reinforcement steel in Jointed
Reinforced Concrete Pavements and Continuously Reinforced Concrete
Pavements separately.
6.10.1
Jointed Reinforced Concrete Pavement
Jointed concrete pavements may be provided without reinforcement if the
probability of transverse cracking during pavement life is low as dictated by soil
movements and/or temperature and moisture variations. This type of pavement
is called Jointed Concrete Pavement (JCP). In this case, the spacing of joints
should be selected so that the tensile stresses developed do not produce
intermediate cracks. The maximum joint space may vary depending on the
temperature/moisture movements, sub-base types, coarse aggregate types and
other local conditions. However, reinforcements may still be required for odd
shaped slabs, mismatched joints and slabs containing pits or structures.
Following sections provide a discussion on the criteria required for the design of
Jointed Reinforced Concrete Pavements.
Slab Length
Slab length is a very important criterion in the design because the amount of
tensile stresses developed in the pavement heavily depends on the length of the
slab (spacing between joints) and consequently the amount of reinforcement to
be provided to minimize cracking. The selection of slab length depends on
various factors such as the site locality limitations, constructions method
adopted, ride quality requirements and other local requirements. Experience has
shown that the use of slab lengths between 8 and 12 m provide an optimum
balance of joint performance, cost and ride quality (Austroads 2008). The DPWHBOD do not allow a slab length to be more than 4.5 m. Refer the discussion in
Section 6.6.1.
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Steel Working Stress
This refers to the allowable working stress of steel (fs) and AASHTO recommends
a value equivalent to 75% of the steel yield strength. (Austroads 2008 more
conservatively recommends 60% of yield stress).
Friction Factor
The friction between the bottom of the slab and the top of the sub-base or
subgrade constrain the stresses developed due to temperature/moisture
variations. Recommended values from AASHTO design guide for natural
subgrade and other sub-base materials are given in Table 6-19.
Table 6-19
Recommended Values for Subgrade and Sub-base Materials
Type of Material Beneath Slab
Friction Factor
Surface Treatment
2.2
Lime Stabilization
1.8
Asphalt Stabilization
1.8
Cement Stabilization
1.8
River Gravel
1.5
Crushed Stone
1.5
Sandstone
1.2
Natural Subgrade
0.9
Source: AASHTO, 1993, Guide for Design of Pavement Structures Vol. 1. Used by Permission.
6.10.2
Continuously Reinforced Concrete Pavement
Continuously reinforced concrete pavements, as the name suggests, have
continuous longitudinal reinforcements for the entire length of the slab. The
action of longitudinal reinforcement is initially to induce transverse cracking by
providing restraint to shrinkage of concrete and then to tie the planned cracks
together to limit the width of the cracks formed within acceptable limits.
The reinforcing may be provided either in the form of bars or deformed wire
fabric. If the possibility of forming longitudinal cracks is low, the transverse
reinforcement may not be necessary. This decision has to be made after careful
consideration of the past experience with similar pavement performance,
aggregate types, and the soil characteristics.
If the likelihood of longitudinal cracks are high, transverse reinforcement should
be provided to restrain lateral movement and minimize the formation of cracks.
Transverse reinforcement is usually designed based on the same criteria and
methodology used for the design of jointed movements.
The following sections discuss the requirements for the design of longitudinal
steel reinforcement in CRC pavements.
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Concrete Tensile Strength
For the design of CRCP, two measures of concrete strength are used:

The Modulus of Rupture (or flexural strength) is used for the determination
of required slab thickness.

Tensile strength for the design of steel reinforcements.
For both values, the strength at 28 day strength should be used and the two
strengths should be consistent with each other. AASHTO recommends that the
indirect tensile strength should normally be about 86% of concrete modulus of
rupture.
Concrete Shrinkage
Concrete shrinkage is caused due to water loss in the concrete and is affected by
cement content, chemical admixtures, curing method, aggregates and curing
conditions. The value of shrinkage at 28 days is used as the design shrinkage
value for design purposes.
When more water is added to the concrete mix, the potential for shrinkage
increases and the strength reduces. Therefore shrinkage can be considered
inversely proportionate to the strength of concrete.
Concrete Thermal Coefficient
The thermal coefficient depends on the water cement ratio, the age of the
concrete, richness of the mix, relative humidity and the type of aggregates in the
mix. In face the most important factor affecting thermal coefficient is the type of
aggregate and therefore the recommended values for PCC thermal coefficient are
usually given as a function of aggregate type.
Bar or Wire Diameter
In general the diameter of longitudinal bars should be within the range of 12 mm
to 20 mm (NAASRA). The spacing between the bars should be at least twice the
nominal maximum aggregate size but never less than 100 mm. In order to
provide adequate load transfer and bond strength the spacing should not exceed
225 mm.
The size and spacing of transverse bars are related to the arrangement to support
the longitudinal reinforcement bars. In general, transverse bars are located at
spacing between 1 to 1.5 m. The spacing becomes greater with the placement of
larger bars for transverse reinforcements.
Steel Thermal Coefficient
The recommended value for steel thermal coefficient is 0.0000126 mm / degrees
Celsius in metric units.
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Design Temperature Drop
The design temperature drop used in the reinforcement design should be as per
the following formula:
Design temperature drop = Average concrete curing temperature – design
minimum temperature
The average concrete curing temperature is the average daily high temperature
during the month the pavement is expected to be constructed and the design
minimum temperature is the average daily low temperature for the coldest
month during the pavement life.
The temperature data may be obtained from published national weather records.
Friction Factor
The slab base friction factor for continuously reinforced pavements is the same
as that for jointed reinforced concrete pavements as given in Section 6.10.1.3.
Limiting Criteria
There are three limiting criteria that must be considered in addition to the inputs
required for the design of longitudinal steel which are outlined below:

Crack spacing where AASHTO recommends crack spacing between 1m and
2.4 m to minimize the potential for the development of punch-outs and crack
spalling.

The allowable crack width should not exceed 1 mm. The predicted crack
width can be reduced by selecting a higher steel percentage or smaller
diameter reinforcing bars.

To guard against possible steel fracture, the limiting stress should not be
taken more than 75% of the ultimate tensile strength. (AustRoads more
conservatively use a limit of 60%).
Design Procedure
For reinforcement design of jointed reinforced concrete pavements and
continuously reinforced concrete pavements, the method provided in AustRoads
Guide to Pavement Technology: Pavement Structural Design (2008) is used here.
Reinforcements in jointed reinforced concrete pavements can be calculated using
the following formula:
where:
𝐴𝐴𝑠𝑠 =
𝜇𝜇 × 𝐿𝐿 × 𝜌𝜌 × 𝑔𝑔 × 𝐷𝐷
𝑓𝑓𝑠𝑠
𝐴𝐴𝑠𝑠 = the required area of steel (mm2/m width of slab
𝑓𝑓𝑠𝑠 = the allowable tensile stress of the reinforcing steel (MPa), usually 0.6 times
characteristic yield strength fsy(AASHTO is less conservative and
recommends 0.75 of fsy
g
6-80
= acceleration due to gravity (m2/s)
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
D = thickness of the base (m)
L
= distance to untied joints or edges of the base (m)
Ρ
= mass per unit volume of the base (kg/m3)
µ
= coefficient of friction between the concrete base and the sub-base (Table
6-19 provides approximate values)
The DPWH Order No. 18/2006 allows the use of wire mesh instead of
reinforcement bars where appropriate. The design and construction must
conform to the requirements of DPWH Standard Specification for Item 312 –
Portland Cement Concrete Pavement with Wire Mesh which is provided as an
Annex to the above Departmental Order.
Longitudinal reinforcement for continuously reinforced concrete pavements can
be calculated using the following formula:
P=
where:
P
(f′t ÷ f′b ) × db × (€s − €t )
2W
= required proportion of longitudinal reinforcing steel – this is the ratio of
the cross–sectional area of the reinforcing steel to the gross area of the
cross – section of the base
f′t ÷ f′b = the ratio of the direct tensile strength of the immature concrete to the
average bond strength between the concrete and steel. The value of this
ratio may be assumed to be 1.0 for plain bars or 0.5 for deformed bars
db
= diameter of the longitudinal reinforcing bar (mm)
€t
= estimated maximum thermal strain from the peak hydration temperature
to the lowest likely seasonal temperature – a value of 300 µ€ may be
assumed, except where the average diurnal temperature at the time of
placing concrete is 10°C or less, when a value of 200 µ€ may be assumed
€s
W
= estimated shrinkage strain – the shrinkage strain may be considered to be
in the range of 200 to 300 µ€ for a concrete with a laboratory shrinkage
not exceeding 450 µ€ at 21 days when tested after three weeks air drying
= maximum allowable crack width (mm) – a value of 0.3 mm should be used
in normal conditions, with 0.2 mm for severe exposure situations, such as
adjacent to maritime environments
If deformed bars are used, the above equation can be simplified as follows:
𝑝𝑝 =
0.25 × db × (€s + €t )
𝑊𝑊
The theoretical spacing of cracks in continuously reinforced pavements may be
estimated by the following equation:
𝐿𝐿𝑐𝑐𝑐𝑐 =
𝑓𝑓𝑐𝑐𝑐𝑐2
𝑚𝑚𝑝𝑝2 𝑢𝑢𝑓𝑓𝑏𝑏 [(𝜀𝜀𝑠𝑠 − 𝜀𝜀𝑡𝑡 )𝐸𝐸𝑐𝑐 − 𝑓𝑓𝑐𝑐𝑐𝑐 ]
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where:
Lcr = theoretical spanning between cracks (m)
6.10.3
fct
= tensile strength of concrete (MPa)
m
= ration of the elastic moduli of steel to concrete Es/Ec – a value of 7.5 may
be assumed
p
= area of longitudinal steel per unit area of concrete (i.e. steel proportion)
u
= perimeter of bar per unit area of steel which may be simplified to 2
divided by radius of the bar (m-1)
fb
= bond stress (MPa) for mature concrete and when deformed bars are used
this may be assumed as 2 fct
€s
= estimated shrinkage strain – the shrinkage strain may be considered to be
in the range 200 to 300 µ€ for a concrete with a laboratory shrinkage not
exceeding 450 µ€ microstrain at 21 days when tested (after 3 weeks of air
drying).
Transverse Reinforcement
For calculation of transverse reinforcement, the formula used to calculate
longitudinal reinforcement for jointed reinforced concrete pavements (Section
6.10.2.9) may be used.
6.11
Pavement Overlay
6.11.1
Important Considerations in Overlay Design
Some important factors that should be given consideration in the design of
overlays are discussed below:
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
Pre-overlay repair: The designer should give careful consideration to the
extent of pre-overlay repair that should be done on existing pavement. Some
of the repair work may be visible while some of them may not be directly
visible in which situation the designer must organize further testing to
determine the extent of the pre-overlay repair to ensure that the
performance of the overlay will not be affected. The cost of carrying out the
pre-overlay repair should also be a consideration in choosing the most
appropriate alternative. In some cases the designer may have to choose an
expensive overlay rather than doing extensive pre-overlay repair.

Reflection crack control: This is an important consideration in overlay design
as the level of reflection crack control will directly affect the design life of the
overlay. The (AASHTO) thickness design procedure does not consider
provisions to minimize occurrence of reflection cracking. The designer may
have to take some additional steps to ensure due consideration is given to
control them in the design.

Traffic loading: The calculation of ESAL for rigid and flexible pavements was
discussed in Section 6.5.7. For overlays, similar estimation has to be done
using the appropriate flexible pavement or rigid pavement equivalency
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
factors. Table 6-20 from the AASHTO design guide provides the equivalency
factors for each overlay type and existing pavement type.
Table 6-20
Equivalency Factors for Overlay and Existing Pavement Types
Existing Pavement
Overlay Type
Equivalency Factors to Use
Flexible
AC
Flexible
Rubblized PCC
AC
Flexible
Break/Crack/Seat JRCP, JRCP
AC
Flexible
Jointed PCC
AC or PCC
Rigid
CRCP
AC or PCC
Rigid
Flexible
PCC
Rigid
Composite (AC/PCC)
AC or PCC
Rigid
Source: AASHTO, 1993, Guide for Design of Pavement Structures Vol. 1. Used by Permission.
AASHTO Guide provides that rigid pavement ESALs can be converted to flexible
pavement ESAL by multiplying the ridged pavement ESAL by 0.67. For example,
15 million rigid pavement ESALs equal to 10 million flexible pavement ESALs.

Sub-surface drainage: An evaluation of sub-surface drainage conditions of the
existing pavement should be conducted before the overlay design. It may be
necessary to improve sub-surface drainage conditions before attempting to
overlay to improve the performance of the overlay.

Rutting in AC pavements: If there is rutting present in the existing pavement,
the cause of rutting should be investigated. If there is extensive rutting due to
instability of any of the existing pavement layers, an overlay may not be
appropriate. In this situation, consideration should be given to mill and
remove the rutted surface and any underlying rutted asphalt layers.

Milling AC surface: In some cases it may be necessary to remove a portion of
an existing pavement to remove cracked and hardened AC material to
improve the performance of an overlay.

Recycling the existing pavement: Recycling of parts of an existing AC layer is
feasible in many situations and this has become a very common practice.
Therefore due consideration should be given in the design on the possibility
of recycling of parts of the existing pavement.

Structural versus functional overlays: AASHTO overlay design procedure
provides an overlay thickness to correct a structural deficiency. If there is no
structural deficiency, the theoretical overlay thickness may be less than or
equal to zero. However, this does not mean an overlay is not necessary.
Overlay may still be required to correct a functional deficiency. Therefore the
designer should be careful to allow for functional deficiencies in addition to
structural deficiencies to determine the final overall overlay thickness.

Overlay material: In the selection of overlay material, the designer should
consider the specific loading, climatic conditions and underlying pavement
deficiencies present and adjust the design accordingly.

Shoulders: Overlay design should include the design of shoulders and factors
such as the extent of deterioration of existing shoulders and the amount of
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traffic that will use the shoulder. The shoulder should be designed so that the
shoulder grade matches with the grade of the pavement.
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
Existing PCC slab durability: If an overlay is designed over an existing PCC
slab, due consideration should be given to the condition of the existing slab.
The designer must consider the possibility of progressive deterioration of
overlay and allowances should be made to minimize adverse impacts on the
new overlay.

PCC overlay joints: Reinforcements may have to be provided for jointed
reinforced or continuously reinforced overlays to hold cracks together.
Restraints due to the friction between base slab and the overlay slab should
be considered in the design of reinforcements.

PCC overlay bonding/separation layers: Depending on the circumstances, an
overlay slab may be designed as a bonded or separated from the existing slab.

Overall design reliability level and overall standard deviation: Design of an
overlay must incorporate a suitable level of reliability similar to the design of
new pavements as discussed earlier. The designer should be aware that some
sources of uncertainty may be different in the design of overlays. For
example, the uncertainties in the estimation of effective existing structural
capacity should be considered as an added factor in the design of overlays.
Therefore the overall standard deviation may vary by overall type. In the
absence of reliable data, at the present time, AASHTO recommends the use of
0.39 for any type of concrete overlays and 0.49 for any type of AC overlay.

Pavement widening: In case where pavement widening is required in the
overlay sections, it is important to ensure coordination between the overlay
design and new pavement design for the widened section to ensure both
sections are structurally adequate. Failure to do this may cause serious
longitudinal cracking along the joint between the overlay and new
pavements. Key considerations recommended in AASHTO design guide are:
-
Both the overlay and the new widening should be designed to have the
same design life.
-
The widened cross section should generally closely match with the
existing pavement in material type, thickness, reinforcement. The joint
spacing should match where possible but a shorter joint spacing is
permitted for the new section if it is justifiable.
-
The widened slab should be tied with the existing slab using appropriate
tie bars.
-
A reflection crack relief fabric may be placed along the longitudinal
widening joint to reduce the possibility of forming a longitudinal crack
along the joint.
-
The overlay should be the same thickness over the widening section as
over the rest of the traffic lane.
-
If required, a longitudinal sub-drainage should be provides to release
water ingress.
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design

6.11.2
Potential errors and possible adjustments to thickness design procedures:
There may a number of areas where potential errors may be caused by the
direct application of the design procedures suggested. Therefore the designer
should be aware of the local situation and the possibility of inaccurate
application of procedures and make necessary adjustment to reflect the
reality. Some of the areas where there is a potential for errors are:
-
Consideration of only structural deficiencies in the design of overlays
without considering the functional deficiencies causing the existing
pavement to deteriorate.
-
The overlay design inputs may need to be modified to suit local
conditions. Factors such as the overall reliability, overall standard
deviation, effective slab thickness and the structural number adjustment
factors, design subgrade resilient modulus and effective k-value, and
other design inputs may have to be adjusted to suit the local
requirements.
Approaches in the Design of Overlay Projects
There are two approaches generally used for the design of overlay projects. The
designer should evaluate the advantages and disadvantages of each approach and
select the best approach for a given situation. The two approaches are discussed
below:
1. Uniform Section Approach: The full length of the project is divided in to a
number of different sections so that each section has relatively uniform
conditions. Each uniform section is considered separately and design for each
section is carried out independently. Overlay design inputs are obtained from
each section that represents its average conditions (e.g. mean thickness,
mean number of transverse cracks per km, mean resilient modulus etc.) so
that a single overall thickness is obtained for the entire length of the section.
This method may be suitable if the conditions of existing pavement are
relatively uniform throughout the project length or within a design section
selected.
2. Point-by-point Approach: Overlay thickness is determined for specific
points at similar intervals within the uniform section and the thickness is
calculated at each point using design inputs at each point. Some factors that
may change from point to point include deflection, thickness, and condition.
Most of the other factors may remain unchanged throughout the uniform
design section. Although some additional field work may be required for this
method compared to the previous method, it may not be excessive. This
approach will produce design thicknesses for each point and the designer
may choose a one value of thickness for the entire project or different
thicknesses for different sections. This approach may be suitable if the
conditions of the existing pavement vary considerably along the length of the
project or the design section.
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6.11.3
Recommended Overlay Solution to Functional Problems
Functional problems are those problems that adversely affect the use of the
highway by intended users. The following sub-sections discuss some
recommended overlay solutions to some important functional problems such as
loss of surface friction, hydroplaning and surface roughness.
Surface Friction and Hydroplaning
During wet weather, certain parts of the pavement surface may lose friction grip
due to polishing of the surface at the point of touch with the wheel causing the
wheel to slip unexpectedly. On AC surfaced pavements, this may be due to
excessive bleeding of the surface or water collecting in wheel path ruts. Ponding
of water in ruts starts to become a problem when rutting reaches a depth of 10
mm. Rutting of 15 mm can produce ponding as much as 5 mm of water. Hence
rutting of 10-15 mm can be critical with regard to the risk of light vehicles
hydroplaning. Rutting of 20 mm will be capable of ponding up to 7 mm of water.
Even heavy vehicles will be at risk at this depth. In some cases a thin correctional
overlay reinstating surface texture and that is adequate for the traffic level, may
be used to remedy these problems.
Surface Roughness
Surface roughness on AC surfaces may be created due to long wavelength surface
distortions, deteriorated transverse cracks, longitudinal cracks or potholes. A
conventional overlay will correct the roughness temporarily until the cracks
reflect through the overlay. For a longer term solution, a full-depth repair of
deteriorated areas and a thicker AC overlay incorporating a reflection crack
control treatment may remedy this problem.
On PCC surfaced pavements, roughness may be caused due to spalling and
faulting of transverse and longitudinal joints and cracks and it can be repaired by
full or partial depth repairs consisting of rigid materials.
In some situations, a layer of ‘preventive overlay’ may be warranted to slow the
rate of deterioration as a precautionary measure.
The general overlay methodology applied to all types of overlay placed on any
type of pavement structure. This methodology employs the serviceability-traffic
(performance) relationship, which rely on life-cycle cost concepts to select a
minimum economic overlay strategy.
Figure 6-23 shows the key relationship and concepts of the general overlay
methodology. They are a) serviceability-traffic repetitious, b) structural capacitytraffic repetitious, and c) pavement condition-traffic repetitions.
In this figure, the overall pavement condition factor, c is related to the effective
capacity by the following;
𝑆𝑆𝑆𝑆𝑥𝑥𝑥𝑥𝑥𝑥𝑥𝑥 = 𝐶𝐶𝑥𝑥 . 𝑆𝑆𝑆𝑆𝑜𝑜
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
When the concept of remaining life is considered, the general overlay equation
becomes:
𝑛𝑛
𝑛𝑛
= 𝑆𝑆𝑆𝑆𝑦𝑦𝑛𝑛 − 𝐹𝐹𝑅𝑅𝑅𝑅 (𝑆𝑆𝑆𝑆𝑥𝑥𝑥𝑥𝑥𝑥𝑥𝑥 )
𝑆𝑆𝑆𝑆𝑂𝑂𝑂𝑂
where:
𝐹𝐹𝑅𝑅𝑅𝑅
= remaining life factor which account for damage of the existing
pavement as well as the desired degree of damage to the overlay at the
end pf the overlay traffic. It is always less than or equal to a value of 1.0
𝑆𝑆𝑆𝑆𝑂𝑂
= initial capacity of original pavement
𝑆𝑆𝑆𝑆𝑦𝑦
= total structural capacity required to support overlay traffic
𝑆𝑆𝑆𝑆𝑂𝑂𝑂𝑂
= additional structural capacity required from the overlay
𝑆𝑆𝑆𝑆𝑥𝑥𝑥𝑥𝑥𝑥𝑥𝑥 = effective structural capacity of existing pavement immediately prior
time of overlay
Figure 6-23
Relation Between Serviceability - Capacity Condition Factor and Traffic
Source: Guide for Pavement Rehabilitation, Volume II. F.S of the Road Improvement Project on Pan-Phil.
Highway, September 1987, JICA
6-87
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
There are seven (7) steps in the overlay design procedure, as shown in Figure 624..
Step 1: Analysis Unit Delineation
The first step in the overlay process is the clear delineation of basic
analysis units. The objective is to determine boundaries along the project
length that subdivide the rehabilitation project into statistically
homogenous pavement units possessing uniform pavement cross
sections, subgrade (foundation) support, construction histories, and
subsequent pavement condition.
Step 2: Traffic Analysis
The purpose of the traffic analysis step is to determine the cumulative 18
ESAL repetitions along a pavement length from the date the pavement
was originally opened to traffic through the end of the anticipated overlay
period.
Step 3: Materials and Environmental Study
Design values for the layer materials used in the rehabilitation process
may be categorized into three major groups:

Existing pavement layer properties

Existing pavement subgrade (foundation) properties

Design properties of overlay layers
AASHTO Guide 1986 recommends the Pavement Layer Moduli Prediction
Method of NDT (Nondestructive Testing).
Step 4: Effective Structural Capacity Analysis
The fourth step in an overlay analysis is to estimate effective (in situ)
structural capacity of the pavement to be overlaid. Information regarding
material properties derived in the previous step is used to arrive at this
parameter.
Rigid Pavements
Aside from two methods of NDT, AASHTO Guide 1986 recommends
approximate procedures to estimate effective structural capacity of the
pavement to be overlaid. These are:

Visual Condition Factor Approach
The relationship between Cv (Visual condition factor) and DxeH value
(Effective Thickness of PCC Slab) are proposed as shown in Table
6-21.

Normal Size of PCC Slab Fragments

Remaining Life Approach
Flexible Pavements
Only NDT method can be applied
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Step 5: Future Overlay Structural Capacity Analsis (SCy)
The major objective of this step is simply to determine the total structural
capacity of a new pavement required to carry repetitions in the overlay
period to a terminal serviceability of Pt2 , using the same existing subgrade
(foundation) support for the design value. The analysis assumes that the
existing pavement (SCxeff) does not exist over the foundation.
Consequently, this step in overlay process is simply a new pavement
design for either a flexible system or rigid system.
Step 6: Remaining Life Factor Determination
Determining the remaining life factor, FRL . FRL is an adjustment factor
applied to the effective capacity parameter (SNxeff or Dxeff ) to reflect a
more realistic assessment of the weighed effective capacity during the
overlay period. This factor is dependent upon the remaining life value of
the existing pavement prior to overlay (𝑅𝑅𝐿𝐿𝐿𝐿 ) and the remaining life of the
overlaid pavement system after the overlay traffic (and subsequent
serviceability) has been reached (R LV ). As a consequence, both of these
values (R LX and R Ly ) must be known.
ASSHTO Guide 1986 recommends the following methods aside from NDT
approach to determine the remaining life of the existing pavement, R LX .

Traffic Approach

Time Approach

Serviceability Approach

Visual Condition Survey Approach
Table 6-21 provides general guidance to estimate visual condition factors.
After 𝐶𝐶𝑥𝑥 Value has been determined Figure 6-25 can be used to estimate
the R LX value.
The remaining life of overlaid pavement, R Ly is directly set in selection of
the desired terminal serviceability for SCy, by the following equation.
where:
R Ly = (Nfy − y)/Nfy
Nfy = ultimate number of traffic repetitions to failure.
y
= design overlay traffic
Knowing estimate of both R LX and R Ly, the remaining life factor, FRL , can
be estimated from Figure 6-26.
Step 7: Overlay Design Analysis
The final step is overlay design analysis which are discussed in the
following sections, separately.
6-89
6-90



Step 3
Analysis Method
 NDT Method
 Traffic Method
 Time Approach
 Serviceability Approach
 Visual Condition Survey Approach
Overlay Design Analysis
 Flexible-Rigid (Ex)
 Rigid-Rigid (Ex)
 Flexible-Flexible (Ex)
Step 7
Survey Method; Non-destructive
Text (NDT)
Material and Environmental Study
 PCC Plastic Modules: EC
 Subgrade Elastic Modules: Esg
 Overlay Material Property: Eol
Remaining Factor
Determination; FRL
 Remaining Life of Existing
Pavement; R LX
 Remaining Life of Overlaid
Pavement; R LT
Step 6
Repetition for overlay
period
Traffic Analysis
 Cumulative ESAL
Step 2
Step 4
Analysis methods:
For PCC:
For AC:
 NDT Method
 NDT Method
 Visual Condition
Factor
 Normal Size of
PCC Slab
Fragments
 Remaining Life
Approach
Effective Structural Capacity; SC x eff
 Epcc
 Eac
 Esb
 Eb
 Esg
 Esb
 Esq
GENERAL OVERLAY EQUATION: SCOL n = SCy n − FRL (SC × eff)n
Overlay Design Procedure
Source: Guide for Pavement Rehabilitation, Volume II. F.S of the Road Improvement Project on Pan-Phil. Highway, September 1987, JICA
Future Overlay
Structural Capacity
Analysis; SCy
 Non-Pavement
Design for Overlay
Traffic
Step 5
Homogenous
unit
0.5 km
Analysis Unit
Determination
Step 1
Figure 6-24
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Table 6-21
Layer Type
Pavement Condition
Asphaltic
PCC
Pozzolanic
Subbase
Summary of Visual (Cv) and Structural (Cx) Condition Values
Base/
Granular Base/
Subbase
𝐶𝐶𝑦𝑦 Visual Condition
Factor Range
𝐶𝐶𝑥𝑥 Structural
Condition Factor
Value
1.
Asphalt layers that are sound, stable, uncracked and
have little to no deformation in the wheel paths.
0.9 - 1.0
0.95
2.
Asphalt layers that exhibit some intermittent cracking
with slight stable.
0.7 – 0.9
0.85
3.
Asphalt layers that exhibit some moderate to high
cracking, have raveling or aggregate degradation
and show moderate to high deformation in wheel
path.
0.5 – 0.7
0.70
4.
Asphalt layers that show very heavy (extensive)
cracking, considerable ravelling or degradation and
very appreciable wheel path deformations.
0.3 – 0.5
0.60
1.
PCC pavement that is uncracked, stable and
undersealed, exhibiting no evidence at pumping.
0.9 – 1.0
0.95
2.
PCC pavement that is uncracked, stable and
undersealed, exhibiting no evidence of pumping.
0.9 – 1.0
0.85
3.
PCC pavement that is appreciably cracked or faulted
with signs of progressive crack deterioration: slab
fragments may range in size from 1 to 4 sq. vds.
Pumping may be present.
0.5 – 0.7
0.70
1.
Chemically stabilized bases (CTB, LCf…) that are
relatively crack free, stable and show no evidence of
pumping.
0.9 – 1.0
0.95
2.
Chemically stabilizes bases (CTB, LCF…) that have
developed very strong pattern or fatigue cracking,
with wide and working cracks that are progressive in
nature: evidence of pumping or other causes of
instability may be present
0.3 – 0.5
0.60
1.
Unbound granular layers showing no evidence of
shear or densification distress, reasonably identical
physical properties as when constructed and
existing at the same “normal” moisture – density
conditions as when constructed.
0.9 – 1.0
0.95
2.
Visible evidence of significant distress within layers
(shear or densification), aggregate properties have
changed significantly due to abrasion, intrusion of
fines from subgrade or pumping, and/or significant
change in-situ moisture caused by surface infiltration
or other sources.
0.3 – 0.5
0.60
Special Notes:
1. The visual condition factor, Cv is related to the structural condition factor, Cx, by:
Cv = Cx 2
2. The structural condition factor, Cx, and not the Cv value, is the variable used in the structural overlay and
design equation (for all overlay-existing pavement types), it is defined by:
SCself = Cv SCD
Source: Guide for Pavement Rehabilitation, Volume II. F.S of the Road Improvement Project on Pan-Phil.
Highway, September 1987, JICA
6-91
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Figure 6-25
Remaining Life Estimate Predicted from Pavement Condition Factor
Source: Guide for Pavement Rehabilitation, Volume II. F.S of the Road Improvement Project on Pan-Phil.
Highway, September 1987, JICA
Figure 6-26
Remaining Life Factor as a Function of Remaining Life of Existing and Overlaid
Pavements
Source: Guide for Pavement Rehabilitation, Volume II. F.S of the Road Improvement Project on Pan-Phil.
Highway, September 1987, JICA
6-92
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Overlay Design Analysis for Rigid Overlay-Rigid Existing
Three potential types of rigid overlay over rigid existing are proposed: full bond,
partial bond and unbonded.
For the pavements with severe structural defect and extensive cracks, only
unbonded overlay method can be applied, which was, therefore, adopted in this
study.
Overlay equation for unbonded type is:
DOL 2 = Dy 2 − FLR (Dxeff )2
where:
DOL = required thickness of overlay
Dy = total structure capacity required for overlay traffic
FLR = remaining life factor
Dxeff = 2r. DO
Effective structural capacity of existing pavement
Overlay Design Analysis for Flexible-Rigid Existing
Flexible overlays over existing rigid pavement is a significant and often used
rehabilitation overlay strategy. Problems associated with this method is the
reflective cracking potential of the asphalt overlay over the existing pavement. At
present, there are several techniques which minimize/eliminate reflective
cracking distress. Use of thick AC overlays method is recommended.
Overlay equation to be used for normal structural approach based on visual
condition factor is expressed;
SNol = SNy − FRL (A2r . D0 + SNxeff − rp)
where:
D0
hol = SNol /Aol
= existing PCC layer thickness 23 cm
A2r
= structural layer coefficient of the existing cracked PCC pavement layer.
This value has been related to the value of the visual condition factor C
Aol
= structural layer coefficient of overlay material
SNxeff = effective (in situ) structural capacity of all remaining pavement layers
above the subgrade except for the existing PCC layer (Subbase)
hol
= required thickness of Asphalt Overlay
Reflective cracking of asphalt overlays over existing rigid pavement is a complex
phenomena. To account for the possibility of reflective cracking, the value of
overlay thickness must be compared to minimum asphalt overlay thicknesses
which, in general, have been effective in minimizing the effect of reflective
cracking. These minimum thickness are a function of the existing PCC slab length
and maximum temperature difference expected within a year. The Asphalt
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Institute recommends 4 inches (10 cm) which is also recommended for the
country.
Overlay Design Analysis for Flexible Overlay- Existing
The basic equation for determining the required SN value due to a flexible
overlay over an existing flexible pavement is:
SNol = SNy − FRL SNxeff
where:
hol = SNol /Aol
Aol = Structural layer coefficient of overlay material
6.11.4
hol = Required thickness of asphalt overlay
Overlay Design Methodology for Pavement with Structural Deficiency
When the pavements are exposed to continuous traffic loads, over time, the
structural capacity of them decreases. One way of increasing the structural
capacity of a pavement with structural deficiency is to overlay the existing
pavement with a suitable material or combination of materials. This section
provides a detailed procedure for the design of an overlay on a structurally
deficient existing pavement.
In order to design an overlay that would increase the structural capacity of a
pavement to a required level, there are two parameters that should be known.
They are the effective structural capacity of the existing pavement and final
expected structural capacity after the overlay. If both the parameters are known,
the required capacity of the overlay can be found from the difference between
the expected structural capacity after overlay and the effective structural
capacity of the existing pavement.
The expected structural capacity after overlay is relatively easy to find but the
difficult is with the determination of the structural capacity of existing pavement.
Three alternative evaluation methods are recommended in the AASHTO guide
which are discussed in brief below.
Because the uncertainties involved in the process of estimation, it is impossible
to expect all three methods to give one value for effective structural capacity.
Therefore the designer should use all three methods where possible and select
the best estimate based on his or her experience and informed judgment.
Refer to DPWH Department Orders:
6-94

No. 185 dated 14 August 1991 on the subject ‘Guidelines on Patching
Pavement Defects’.

No. 45 dated 06 September 2006 on the subject ‘Interim Guidelines for the
Maintenance and Rehabilitation of Unreinforced Concrete Roads’.
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Structural Capacity Based on Visual Survey and Materials Testing
This involves assessment of current condition based on distress and drainage
surveys, and usually some coring and testing of materials. In this method the
assessment is started with a visual survey that involves:

Review of all information available regarding the design, construction, and
maintenance history of the pavement;

Detailed site survey to identify the type, amount, severity, and location of
surface distresses.
Some of the distress types that are indicators of structural deficiencies are:


On AC surfaced pavements:
-
Fatigue or alligator cracking in the wheel paths.
-
Rutting in the wheel paths.
-
Transverse and longitudinal cracks that develop into pot holes.
-
Localized failing areas where the underlying layers are disintegrating and
causing a collapse of the AC surface.
On PCC surfaced pavements:
-
Deteriorating transverse or longitudinal cracks (spalling or faulting).
-
Corner breaks at transverse joints or cracks.
-
Localized failing areas where the PCC slab is disintegrating and causing
spalls and potholes.
-
Localized punch-outs, primarily in CRCP.
The next step of the assessment is to conduct a sub-surface drainage survey
which should be coupled with the distress survey. The objective of the drainage
survey is to identify problems related to moisture ingress and locations where
drainage improvements should be introduced to improve the drainage capacity
and to stop the influence of excess moisture causing problems.
A coring and materials testing program is conducted to identify the causes of
observed distress. The locations for coring should be identified in the visual
survey to assure that all significant pavement conditions are represented. Coring
tests will determine the thickness and condition of the pavement layers.
Further testing may be required to determine how the existing material compare
with similar materials that would be used in new pavement and how the material
may have changed since the pavement was constructed. The types of tests will
depend on the type of material and the nature of distress observed.
Structural Capacity Based on Non-Destructive Deflection Testing (NDT)
NDT is a rapidly developing area of technology and a useful way of determining
the condition of an existing pavement. Depending on the type of pavement, NDT
serves different functions. For rigid pavements, it may be applied to:

To examine load transfer efficiency at joints and cracks
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design

To estimate the effective modulus of subgrade reaction

To estimate the modulus of elasticity of the concrete
For flexible pavements, NDT provides the following information

Estimation of the roadbed soil resilient modulus

Direct estimation of SNeff of the pavement structure
In addition to the structural evaluation, NDT can provide other useful data such
as deflection data to quantify variability along the project and data to estimate
resilient modulus values for the various pavement layers.
6.11.5
Determination of Design Subgrade MR - Design CBR
The road section to constructed with a uniform pavement thickness is
determined based on the results of preliminary studies and the CBR test. The
design CBR is determined based on CBR values of individual locations within the
road section, with extreme values discarded, by the following formula:
Design CBR = Average value of CBR of individual locations − (
where C is a coefficient given in Table 6-22
Table 6-22
Max.CBR−Min CBR
)
C
Values of C for Calculating Design CBR
No. of Values
Available (n)
C
2
3
4
5
6
7
8
9
10 or
more
1.41
1.91
2.24
2.48
2.67
2.83
2.96
3.08
3.18
Source: Guide for Pavement Rehabilitation– Feasibility Study of the Road Improvement
Project on Pan-Philippine Highway – September 1987, JICA
The design subgrade MRcan be determined either in the field (dynamic cone
penetrometer) or by means of laboratory testing. Non-destructive testing (NDT)
back-calculation, estimation from resilient modulus correlation studies, or using
original design and construction data can be used.
The following formula should be used to back-calculate the subgrade MR value
from NDT data: (This formula is valid for English units and must be checked for
Metric units.)
where:
𝑀𝑀𝑅𝑅 =
0.24𝑃𝑃
𝑑𝑑𝑟𝑟 𝑟𝑟
MR = back-calculated subgrade resilient modulus (units)
6-96
P
= applied loads (units)
dr
= measured direction of radial distance r (units)
r
= radial distance at which the deflection is measured (units)
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
The selected resilient modulus value has a significant effect on the resulting
structural number determined and therefore caution should be exercised in
accurately determining the value. If a high resilient modulus values are used, the
resulting overlay thickness may be too thin.
6.11.6
Asphalt Concrete (AC) Overlay of AC Pavement
Construction tasks involved in the placement of an AC overlay on an existing AC
pavement are:

Repairing deteriorated areas and making sub-surface drainage improvements
if required.

Correcting surface rutting by milling or placing a leveling course.

Constructing widening if required.

Applying a tack coat.

Placing the AC overlay (including a reflective crack control treatment if
needed.
Feasibility
An AC overlay may be a feasible alternative for a deteriorating AC pavement,
except if excessive rutting exists. Excessive surface rutting indicates a lack of
sufficient stability, serious deterioration in the existing stabilized base, and a
weak granular base due to infiltration by a soft subgrade. Stripping of the existing
AC surface may also dictate that it should be removed and replaced.
Pre-Overlay Repair
In many situations, if the damage to the existing pavement is excessive, repair
work should be carried out prior to overlay of AC pavements to ensure the
expected service life of the overlay is maintained. Some of those situations are as
follows:

Alligator cracking: All areas of severe alligator cracking must be repaired.
Areas with medium damage should be repaired unless reflective crack
control is used in the overlay. Any soft sub-surface material must be removed
as a part of the repair work.

Linear cracking: Cracks with high severity damage should be patched. Linear
cracks with a width greater than 6 mm should be filled with a sand-asphalt
mixture or other suitable crack filler. If transverse cracks are experiencing
significant opening and closing, some method of reflective crack control is
recommended in the new overlay.

Rutting: Ruts should be removed by milling or placing a leveling course
before overlay. If rutting is too severe, the cause for it should be investigated.
Depending on the results of the investigation, a decision has to be made
whether overlay is feasible option or not.

Surface irregularities: Excessive depressions, humps and corrugations will
have to be investigated and they should be removed and replaced where
required.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Reflection Crack Control
Reflective cracking can have a considerable influence on the life of an AC overlay
and therefore it is important that the design includes measures to control them
to be within acceptable limits. Some of the recommended methods are:

Incorporation of synthetic fabrics and stress-absorbing interlayers for
controlling reflection of low and medium severity alligator cracking. When
they are used in combination with crack filling, they may be able to control
reflection of temperature cracks. However, they may not be able to contain
reflection of cracks subject to significant horizontal or vertical movements.

Crack relief layers composed of open graded coarse aggregate and a small
percentage of asphalt cement have been proven to be effective in controlling
reflection of cracks subject to larger movements.

Sawing and sealing joints in the AC overlay directly above straight cracks in
the underlying AC may be effective in controlling the deterioration of
reflection cracks.

Thicker overlays are more effective in delaying the occurrence and
deterioration of reflection cracks than thinner overlays. However the down
side is that thicker layers of overlay are more expensive and it may not be
economically feasible to use as a measure of controlling cracks.
Thickness Design
The following equation is specified in the AASHTO Guide to calculate the required
thickness to increase structural capacity to carry future traffic.
where:
𝑆𝑆𝑆𝑆𝑜𝑜𝑜𝑜 = 𝑎𝑎𝑜𝑜𝑜𝑜 ∗ 𝐷𝐷𝑜𝑜𝑜𝑜 = 𝑆𝑆𝑆𝑆𝑓𝑓 − 𝑆𝑆𝑆𝑆𝑒𝑒𝑒𝑒𝑒𝑒
SNol = required overlay structural number
aol
= structural coefficient for the AC overlay
Dol = required overlay thickness (mm)
SNf = structural number required to carry future traffic
SNeff = effective structural number of the existing pavement
Design Procedure
Step 1: Existing Pavement Design and Construction
Obtain the following parameters from the existing pavement design and
construction data:

Thickness and material type of each pavement layer.

Available subgrade soil information (from construction records, soil surveys
or any other reports or documents.
Step 2: Traffic Analysis
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Obtain or determine the following data about existing and future traffic:

Past cumulative ESALs in the design lane (Np), for use in the remaining life
method of SNeff determination only.

Predicted future ESALs in the design lane over the design period (Nr)
Step 3: Condition Survey
Obtain following information on the existing condition of the pavement (this is
usually done during a condition assessment survey):

Percentage of surface area with alligator cracking (low, medium and high
severities).

Number of transverse cracks per km (low, medium and high severities)

Mean rut depth.

Evidence of pumping at cracks and at pavement edges.
Step 4: Deflection Testing
Measurement of deflections in the outer wheel path at an interval sufficient to
adequately assess conditions -intervals of 30 to 300m are typical. Leave out the
areas that have been deteriorated and require repairs. Use of a Benkelman Beam
or a Falling Weight Deflectometer with a nominal 40 kN loading per half axle
Deflection should be measured at the center of the load and at least one other
distance from the load.
Using the deflection measurement away from the load magnitude, back-calculate
the subgrade resilient modulus using the following formula:
where:
𝑀𝑀𝑅𝑅 =
0.24𝑃𝑃
𝑑𝑑𝑟𝑟 𝑟𝑟
MR = back-calculated subgrade resilient modulus (units)
P
= applied loads (units)
dr
= measured direction of radial distance r (units)
r
= radial distance at which the deflection is measured (units)
The minimum distance to the point where deflection is measured away from the
point of load magnitude can be calculated using the following equation:
𝑟𝑟 ≥ 0.7𝑎𝑎𝑒𝑒
where:
2
𝐸𝐸𝑝𝑝
𝑎𝑎𝑒𝑒 = √[𝑎𝑎2 + (𝐷𝐷 √ )
𝑀𝑀𝑅𝑅
3
a
= NDT load plastic radius, mm
D
= total thickness of pavement layers above the subgrade, mm
Ep
= effective modulus of all pavement layers above the subgrade (unit)
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design

Temperature of AC mix during the deflection testing should be determined.
AC mix temperature may be measured directly or may be estimated from
surface or air temperatures.

If the subgrade resilient modulus and total thickness of all layers above the
subgrade are known or assumed, the effective modulus of the entire
pavement structure may be determined form the deflection measured at the
center of the load plate using the following equation:
𝑑𝑑𝑜𝑜 = 1.5𝑝𝑝𝑝𝑝
where:
1
2
𝐷𝐷 3 𝐸𝐸
𝑀𝑀𝑅𝑅 √1 + ( √ 𝑝𝑝 )
𝑎𝑎 𝑀𝑀𝑅𝑅
{
+
[1 −
1
√1+(𝐷𝐷)
𝐸𝐸𝑝𝑝
𝑎𝑎
2
]
}
d0
= deflection measured at the center of the load plate (and adjusted to a
standard temperature of 20°C.
p
= NDT load plate pressure (unit)
a
= NDT load plate radius (unit)
D
= total thickness of pavement layers above the subgrade (unit)
MR = subgrade resilient modulus (unit)
Ep
= effective modulus of all pavement layers above the subgrade (unit)
Step 5: Coring and Material Testing

If deflection test as discussed in Step 4 is not performed, laboratory testing of
samples of the subgrade may be conducted to determine its resilient
modulus.

Samples of AC layers and stabilized base should be visually examined to
assess asphalt stripping, degradation and erosion.

Samples of granular base and sub-base should be visually examined and a
gradation carried out to assess degradation and contamination by fines.

The thickness of all layers should be measured.
Step 6: Determination of Required Structural Number for Future Traffic (SNf)


6-100
Effective design subgrade resilient modulus should be calculated using one of
the following methods:
o
Laboratory testing described in Step 5.
o
Back-calculation from deflection data.
o
A very approximate estimate can be made using available soil
information and relationships developed from resilient modulus studies.
Design PSI loss by calculating the difference between PSI immediately after
overlay (P1) and the PSI at the time of next rehabilitation (P2).
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design

Determine Overlay design reliability R

Determine overall standard deviation S0 for flexible pavement.

Compute SNf value using the formula given in Section 6.11.3.4
Step 7: Determination of Effective Structural Number (SNeff) of the Existing
Pavement
Three methods are recommended for the calculation of SNeff as discussed in
Section 6.113.4. They are the NDT method, the condition survey method and the
remaining life method. The designer may use all three methods to evaluate the
pavement and then select a value for SNeff based on the results, using engineering
judgment and the past experience.
i.
Determination of SNeff from NDT for AC Pavement
The assumption in this method is that the structural capacity of the pavement is a
function of its total thickness and overall stiffness. The relationship between
SNeff, thickness and stiffness can be expressed as:
𝑆𝑆𝑆𝑆𝑒𝑒𝑒𝑒𝑒𝑒 = 0.0045𝐷𝐷 3√𝐸𝐸𝑝𝑝
where:
D
= total thickness of all pavement layers above the subgrade (units)
Ep
= effective modulus of pavement layers above the subgrade (units)
Ep may be back-calculated from deflection data as described in Step 4.
ii.
Determination of SNeff from Condition Survey of AC Pavements
The SNeff may be determined using the following formula:
where:
𝑆𝑆𝑆𝑆𝑒𝑒𝑒𝑒𝑒𝑒 = 𝑎𝑎1 𝐷𝐷1 + 𝑎𝑎2 𝐷𝐷2 𝑚𝑚2 + 𝑎𝑎3 𝐷𝐷3 𝑚𝑚3
D1, D2, D3 = thickness of existing pavement surface, and sub-base layers
a1,a2,a3
= corresponding structural layer coefficients
m2, m3
= drainage coefficients for granular base and sub-base
Determination of SNeff using Remaining Life for AC pavements
The remaining life of the pavement can be calculated using the following formula:
where:
𝑁𝑁𝑝𝑝
)]
𝑅𝑅𝑅𝑅 = 100 [1 − (
𝑁𝑁1.5
RL = remaining life, present
Np = total traffic to date, ESALs
N1.5 = total traffic to pavement failure, ESLAs
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
N1.5 may be determined from the new pavement design equations or
nomographs. The AASHTO guide recommends a failure PSI equal to 1.5 and a
reliability of 50%.
SNeff can now be determined using following formula:
where:
𝑆𝑆𝑆𝑆𝑒𝑒𝑒𝑒𝑒𝑒 = 𝐶𝐶𝐶𝐶 + 𝑆𝑆𝑆𝑆0
CF = condition factor
SN0 = structural number of the pavement if it were newly constructed.
Step 8: Determination of Overlay Thickness
The thickness of AC overlay is calculated using the following formula:
where:
𝐷𝐷𝑜𝑜𝑜𝑜 =
𝑆𝑆𝑆𝑆𝑜𝑜𝑜𝑜 (𝑆𝑆𝑆𝑆𝑓𝑓 − 𝑆𝑆𝑆𝑆𝑒𝑒𝑒𝑒𝑒𝑒 )
=
𝑎𝑎𝑜𝑜𝑜𝑜
𝑎𝑎𝑜𝑜𝑜𝑜
SNol = required overlay structural number
Aol = structural coefficient for the AC overlay
D0l = required overlay thickness, mm
SNf = structural number determined in Step 6
SNeff = effective structural number of the existing pavement from Step 7
6.11.7
AC Overlay of Fractured PCC Slab Pavement
Huang (1993) discusses the design of various types of overlays. A major problem
in the design of AC overlays on PCC pavements is reflection cracking, defined as
the fractures in an overlay or surface that reflect the crack or joint pattern in the
underlying layer. Such cracking must be prevented or controlled to provide a
smoother riding surface, maintain structural integrity of the overlay and prevent
intrusion of water into the pavement system. The primary mechanisms leading to
the development of reflection cracks in an AC overlay on a PCC pavement are the
horizontal movement due to temperature and moisture changes and the
differential vertical movement due to traffic loadings, both occurring at the joints
and cracks in the PCC pavement. Horizontal movement is considered the most
critical.
Several methods can be used to minimize or control reflection cracking in AC
pavements over PCC pavements.
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
Design a thicker AC overlay (if the thickness of overlay to alleviate reflection
cracking is less than 230 mm)

Crack and seat the existing PCC slab into smaller sections

Use a crack relief layer with drainage system

Saw and seal joints in an AC overlay
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design

Use a stress absorbing membrane interlayer (SAMI) with an overlay (consult
manufacturer)

Incorporate a geotextile fabric membrane interlayer with an overlay (consult
manufacturer)
Crack and Seat PCC Slab
The crack and seat procedure involves cracking the PCC slab into small segments,
seating the segments with heavy rollers to eliminate underlying voids, and
overlaying the PCC slab with AC.
Objective of Fracturing PCC Slab
The objective is to create small pieces of concrete so that slab movement by
thermal or other causes is minimal, thereby reducing the reflection cracking in
the AC overlay. The segments, usually between 0.37 and 0.56 m2 are still large
enough to have some structural integrity due to aggregate interlock. Cracking
and seating will generally delay, rather than eliminate refection cracking.
Thickness Design
The AASHTO 1993 Guide for Design of Pavement Structures contains the most
comprehensive procedure. The procedure is based on the remaining life concept
and can be applied to any type of overlay (Huang 1993). The process is similar to
that for asphalt overlays on asphalt pavements. Both effective thickness and
deflection based methods are available for asphalt overlays on rigid pavements of
all types.
6.12
Bonded Concrete Overlay of JPCP, JRCP, and CRCP
Bonded concrete ovelays have been placed on jointed plain, jointed reinforced
and continuously reinforced concrete pavements to improve both structural
capacity and functional condition. A bonded concrete overlay consists of the
following construction tasks:
6.12.1

Repairing deteriorated areas and making subdrainage improvements

Constructing widening

Preparing the existing surface to ensure a reliable bond

Placing the concrete overlay

Sawing and sealing the joints
Feasibility
A bonded overlay of JPCP, JRCP, or CRCP is a feasible rehabilitation alternative for
PCC pavements except when the conditions of the existing pavement dictate
substantial removal and replacement or when durability problems exist (28).
Conditions under which a PCC bonded overlay would not be feasible include:
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design

The amount of deteriorated slab cracking and joint spalling is so great that a
substantial amount of removal and replacement of the existing surface is
dictated.

Significant deterioration of the PCC slab has occurred due to durability
problems (e.g., “D” cracking or reactive aggregates). This will affect
performance of the overlay.

Vertical clearance at bridges is inadequate for required overlay thickness.
This is not usually a problem because bonded overlays are usually fairly thin.
If construction duration is critical, PCC overlays may utilize high-early-strength
PCC mixes. PCC overlays have been opened within 6 to 24 hours after placement
using these mixtures.
6.12.2
Pre-overlay Repair
The types of distress should be repaired prior to placement of the bonded PCC
overlay as shown in Table 6-23.
Table 6-23
Types of Distress
Distress Type
Repair Type
Working cracks
Full-depth repair or slab replacement
Punhouts
Full-depth repair
Spalled joints
Full- or partial-depth repair
Deteriorated patches
Full-depth repair
Pumping/faulting
Edge drains
Settlements/heaves
Slab jack or reconstruct area
Source: AASHTO, 1993, Guide for Design of Pavement Structures Vol. 1. Used by Permission.
Full-depth repairs and slab replacements in JPCP and JRCP should be PCC,
dowelled or tied to provide load transfer across repair joints. Full-depth repairs
in CRCP should be PCC and should be continuously reinforced with steel which is
tied or welded to reinforcing steel in the existing slab, to provide load transfer
across joints and slab continuity. Full-depth AC repairs should not be used prior
to placement of a bonded PCC overlay, and any existing AC patches should be
removed and replaced with PCC.
Installation of edge drains, maintenance of existing edge drains, or other
subdrainage be done prior to placement of the overlay if a subdrainage
evaluation indicates a need for such an improvement.
Pressure relief joints should be done only at fixed structures, and not at regular
intervals along the pavement. The only exception to this is where a reactive
aggregate has caused expansion of the slab. On heavily trafficked routes,
expansion joints should be of the heavy-duty type with dowels. If joints contain
significant incompressibles, they should be cleaned and resealed prior to overlay
placement.
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6.12.3
Reflection Crack Control
Any working (spalled) cracks in the existing JPCP, JRCP, or CRCP slab may reflect
through the bonded concrete overlay within one year. Reflection cracks can be
controlled in bonded overlays by full-depth repair of working cracks in the
existing pavement, and for JPCP or JRCP, sawing and sealing joints through the
overlay directly over the repair joints. Tight non-working cracks do not need to
be repaired because not all will reflect through the overlay and those that do will
usually remain tight. Tight cracks in CRCP will take several years to reflect
through, and even then, will remain tight.
6.12.4
Subdrainage
The subdrainage condition of an existing pavement usually has a great influence
on how well the overlay performs. A subdrainage evaluation of the existing
pavement should be conducted. Improving poor subdrainage conditions will
have a beneficial effect on the performance of an overlay. Removal of excess
water from the pavement cross-section will reduce erosion and increase the
strength of the base and subgrade, which in turn will reduce deflections. In
addition, stripping in AC pavement and “D” cracking in PCC pavement may be
slowed by improved subdrainage.
6.12.5
Thickness Design
If the overlay is being placed for some functional purpose such as roughness or
friction, a minimum thickness overlay that solves the functional problem should
be placed.
If the overlay is being placed for the purpose of structural improvement, the
required thickness of the overlay is a function of the structural capacity required
to meet future traffic demands and the structural capacity of the existing
pavement. The required overlay thickness to increase structural capacity to carry
future traffic is determined by the following equation.
where:
Dol = Df − Deff
Dol = required thickness of bonded PCC overlay, inches
Df = slab thickness to carry future traffic, inches
Deff = effective thickness of existing slab, inches
Bonded concrete overlays have been successfully constructed as thin as 2 inches
and as thick as 6 inches or more. The typical overlay is 3 to 4 inches for most
highway pavement overlays. If the bonded overlay is being placed only for a
functional purpose such as roughness or friction, a thickness of 3 inches should
be adequate.
The required overlay thickness may be determined through the following design
steps. These design steps provide a comprehensive design approach that
recommends testing the pavement to obtain valid design inputs. If it is not
possible to conduct this testing, an approximate overlay design may be
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
developed based upon visible distress observation by skipping Steps 4 and 5, and
by estimating other inputs.
The overlay design can be prepared for a uniform section or on a point-by-point.
Step 1: Existing Pavement Design
1. Existing slab thickness
2. Type of load transfer (mechanical devices, aggregate interlock, CRCP)
3. Type of shoulder (tied, PCC, other)
Step 2: Traffic Analysis
1. Past cumulative 18-kip ESALs in the design lane (Np ), for use in the
remaining life method of Deff determination only
2. Predicted future 18-kip ESALs in the design lane over the design period (Nf )
Step 3: Condition Survey
The following distresses are measured during the condition survey for JPCP,
JRCP, and CRCP. Sampling along the project may be used to estimate these
quantities in the most heavily trafficked lane.
JPCP/JRCP:
1. Number of deteriorated transverse joints per mile
2. Number of deteriorated transverse cracks per mile
3. Number of existing expansion joints, exceptionally wide joints (>1 inch) or
AC full depth patches
4. Presence and general severity of PCC durability problems

“D” cracking: low severity (cracks only), medium severity (some spalling),
high severity (sever spalling)

Reactive aggregate cracking: low, medium, high severity
5. Evidence of faulting, pumping of fines or water at joints, cracks and pavement
edge
CRCP:
1. Number of punchouts per mile
2. Number of deteriorated transverse cracks per mile
3. Number of existing expansion joints, exceptionally wide joints (>1 inch) or
AC full-depth patches
4. Number of existing and new repairs prior to overlay per mile
5. Presence and general severity of PCC durability problems (Note: surface
spalling of tight cracks where the underlying CRCP is sound should not be
considered a durability problem)

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“D” cracking: low severity (cracks only), medium severity (some spalling),
high severity (sever spalling)
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design

Reactive aggregate cracking: low, medium, high severity
6. Evidence of pumping of fines or water
Step 4: Deflection Testing (strongly recommended)
Measure slab deflection basins in the outer wheel path along the project at an
interval sufficient to adequately assess conditions. Intervals of 100 to 1,000 feet
are typical. Measure deflections with sensors located at 0, 12, 24, and 36 inches
from the center of load. A heavy-load deflection device (e.g., Falling Weight
Deflectometer) and a load magnitude of 9,000 pounds are recommended. ASTM
D 4694 and D 4695 provide additional guidance on deflection testing.
For each slab tested, backcalculate the effective k-value and the slab’s elastic
modulus using Figure 6-27 and Figure 6-28 or a backcalculation procedure. The
AREA of each deflection basin is computed as follows:
d12
d24
d36
AREA = 6 ∗ [1 + 2 ( ) + 2 ( ) + ( )]
d0
d0
d0
where:
d0 = deflection in center of loading plate, inches
di = deflections at 12, 24, and 36 inches from plate center, inches
AREA will typically range from 29 to 32 for sound concrete
1. Effective dynamic k-value. Enter Figure 6-27 with d0 and AREA to determine
the effective dynamic k-value beneath each slab for a circular load radius of
5.9 inches and magnitude of 9,000 pounds. Note that for laods within 2,000
pounds more or less, deflection may be scaled linearly to 9,000-pound
deflections.
If a single overlay thickness is being designed for a uniform section, compute
the mean effective dynamic k-value of the slabs tested in the uniform section
2. Effective static k-value
Effective static k—value = Effective dynamic k—value/2
3. Elastic modulus of PCC slab (E). Enter Figure 6-28 with AREA, proceed to the
effective dynamic k-value curves, and determine a value for ED3 , where D is
the slab thickness. Solve for E knowing the slab thickness, D. Typical slab E
values range from 3 to 8 million psi. If a slab E value is obtained that is out of
this range, an error may exist in the assumed slab thickness, the deflection
basin may have been measured over a crack, or the PCC may be significantly
deteriorated.
If a single overlay thickness is being designed for a uniform section, compute
the mean E value of the slabs tested in the uniform section.
Do not use any k-values or E values that appear to be significantly out of line
with the rest of the data.
4. Joint load transfer. For JPCP and JRCP, measure joint load transfer in the outer
wheel path at a representative transverse joints. Do not measure load
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transfer when the ambient temperature is greater than 80°F. Place the load
plate on one side of the joint with the edge of the plate touching the joint.
Measure the deflection at the center of the load plate and at 12 inches from
the center. Compute the deflection load transfer from the following equation.
Δul
ΔLT = 100 ∗ ( ) ∗ B
Δl
where:
ΔLT = deflection load transfer, percent
Δul = unloaded side deflection, inches
Δl
= slab bending correction factor
The slab bending correction factor, I is necessary because the deflections d0
and d12 , measured 12 inches apart, would not be equal even if measured in
the interior of a slab. An appropriate value for the correction factor may be
determined from the ratio of d0 to d12 for typical center slab deflection basin
measurements, as shown in the equation below. Typical values for B are
between 1.05 and 1.15.
B=
d0 center
d12 center
If a single overlay thickness is being designed for a uniform section, compute
the mean deflection load transfer value of the joints tested in the uniform
section.
For JPCP and JRCP, determine the Load Transfer Coefficient “J” from Table 624.
Table 6-24
Load Transfer Co-efficient
Percent Load Transfer
“J”
>70
Load 3.2
50-70
Transfer 3.5
<50
Coefficient 4.0
Source: AASHTO, 1993, Guide for Design of Pavement Structures Vol. 1. Used by Permission.
If the rehabilitation will include the addition of a tied concrete shoulder, a
lower J factor may be appropriate.
For CRCP, use J = 2.2 to 2.6 for overlay design, assuming that working cracks
and punchouts are repaired with continuously reinforced PCC.
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Effective Dynamic k-Value Determination from d0 and Area
Source: AASHTO, 1993, Guide for Design of Pavement Structures Vol. 1. Used by Permission.
Figure 6-27
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6-110
PCC Elastic Modulus Determination from k-Value, Area and Slab Thickness
Source: AASHTO, 1993, Guide for Design of Pavement Structures Vol. 1. Used by Permission.
Figure 6-28
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Step 5: Coring and Materials Testing (strongly recommended)
1. PCC modulus of rupture (S′c). Cut several 6-inch-diameter cores at midslab and test in indirect tension (ASTM C 496). Compute the indirect
tensile strength (psi) of the cores. Estimate the modulus of rupture with
the following equation:
S′c = 210 + 1.02IT
where:
S′c = modulus of rupture, psi
IT = indirect tensile strength of 6-inch diameter cores, psi
Step 6: Determination Of Required Slab Thickness for Future Traffic (Df )
The inputs to determine Df for bonded PCC overlays of PCC pavements are
representative of the existing slab and foundation properties. This is emphasized
because it is the properties of the existing slab (i.e., elastic modulus, modulus of
rupture, and load transfer) which will control the performance of the bonded
overlay.
1. Effective static k-value. Determine from one of the following methods.

Backcalculate the effective dynamic k-value from deflection basins as
described in Step 4. Divide the effective dynamic k-value by 2 to obtain
the effective static k-value.

Conduct plate load tests (ASTM D 1196) after slab removal at a few sites.
This alternative is very costly and time-consuming and not often used.

Estimate from soils data and base type and thickness, using Figure 6-29.
2. Design PSI loss. PSI immediately after overlay (P1) minus PSI at time of next
rehabilitation (P2).
3. J load transfer factor. See Step 4.
4. PCC modulus of rupture determine by one of the following methods:

Estimated from indirect tensile strength measured from 6-inch diameter
cores as described Step 5.

Estimated from the backcalculated E of slab using the following equation:
E
S′c = 43.5 ( 6 ) + 488.5
10
where:
S′c = modulus of rupture, psi
E
= backcalculated elastic modulus of PCC slab, psi
For CRCP, S′c may be determined from the backcalculated E values only at
points which have no cracks within the deflection basins.
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5. Elastic modulus of existing PCC slab, determined by one of the following
methods:

Backcalculate from deflection measurements as described in Step 4.

Estimate from indirect tensile strength.
6. Loss of support of existing slab. Joint corners that have loss of support may be
identified using FWD deflection testing. CRCP loss of support can be
determined by plotting a slab edge or wheel path deflection profile and
identifying locations with significantly high deflections. Existing loss of
support can be improved with slab stabilization. For thickness design,
assume a fully supported slab, LS = 0.
7. Overlay design reliability, R (percent).
8. Overall standard deviation (So ) for rigid pavement.
9. Subdrainage capability of existing slab, after subdrainage improvements, for
guidance in deterimining Cd . Pumping or faulting at joints and cracks
determined in Step 3 is evidence that a subdrainage problem exists. In
selecting this value, note that the poor subdrainage situation at the AASHTO
Road Test would be given a Cd of 1.0.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Figure 6-29
Chart for Estimating Composite Modulus of Subgrade Reaction, k, Assuming a
Semi-Infinite Subgrade Depth. (for Practical Purposes, a Semi-Infinite Depth is
Considered to be Greater than 10 Feet Below the Surface of the Subgrade)
Source: AASHTO, 1993, Guide for Design of Pavement Structures Vol. 1. Used by Permission.
Compute Df for the above design inputs using the rigid pavement design equation
or nomograph. When designing an overlay thickness for a uniform pavement
section, mean input values must be used, When designing an overlay thickness
for specific points along the project, the data for that point must be used. A
worksheet for determining Df is provided in Table 6-25. Typical values of inputs
are provided for guidance. Values outside these ranges should be used with
caution.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Step 7: Determination of Effective Slab Thickness (Deff) of Existing Pavement
The condition survey and remaining life procedures are presented.
𝐃𝐃𝐞𝐞𝐞𝐞𝐞𝐞 From Condition Survey for PCC Pavements
The effective thickness of the existing slab (Deff) is computed from the following
equation:
Deff = Fjc x Fdur x Ffat x D
where:
D = existing PCC slab thickness, inches
1. Joints and cracks adjustment factor (Deff). This factor adjust for the extra loss
in PSI caused by deteriorated reflection cracks in the overlay that will result
from any unrepaired deteriorated joints, cracks and other discontinuities in
the existing slab prior to overlay. A deteriorated joint or crack in the existing
slab will rapidly reflect through an AC overlay and contribute to loss of
serviceability. Therefore, it is recommended that all deteriorated joints and
cracks (for non-“D” cracked or reactive aggregate related distressed
pavements) and any other major discontinuities in the existing slab be fulldepth repaired with dowelled or tied PCC repairs prior to overlay, so that Fjc
= 1.00.
If it is not possible to repair all deteriorated areas, the following information
is needed to determine Fjc , to increase the overlay thickness to account for
the extra loss in PSI from deteriorated reflection cracks (per design lane):
Pavements with no “D” cracking or reactive aggregate distress:
Number of unrepaired deteriorated joints/mile
Number of unrepaired deteriorated cracks/mile
Number of unrepaired punchouts/mile
Number of expansion joints, exceptionally wide joints (greater than 1
inch), and full depth, full-lane-width AC patches/mile
Note that tight cracks held together by reinforcement in JRCP or CRCP are
not included. However, if a crack in JRcp or CRCP is spalled and faulted
the steel has probably ruptured, and the crack should be considered as
working. Surface spalling of CRCP cracks is not an indication that the
crack is working.
The total number of unrepaired deteriorated joints, cracks, punchouts,
and other discontinuities per mile is used to determine the Fjc from
Figure 6-27.
Pavements with “D” cracking or reactive aggregate deterioration:
These types of pavements often have deterioration at the joints and cracks
from durability problems. The Fdur factor is used to adjust the overlay
thickness for this problem. Therefore, when this is the case, the Fjc should be
determined from Figure 6-28 only using those unrepaired deteriorated joints
and cracks that are not caused by durability problems. If all of the
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
deteriorated joints and cracks are spalling due to “D” cracking or reactive
aggregate, then Fjc = 1.0. This will avoid adjusting twice with the Fjc and Fdur
factors.
2. Durability adjustment factor (Fdur ). This factor adjust for an extra loss in PSI
of the overlay when the existing slab has durability problems such as “D”
cracking or reactive aggregate distress. Using condition survey data from
Step 3, Fdur is determined as follows.
1.00:No sign of PCC durability problems
0.96-0.99:Durability cracking exists, but no spalling
0.80-0.95:Cracking and spalling exist (normally a bonded PCC overlay is not
recommended under these conditions)
Table 6-25
Worksheet for Determination of Df for JPCP, JRCP and CRCP
SLAB:
Existing PCC slab thickness
=
Type of load transfer system: mechanical device, aggregate interlock CRCP
Type of shoulder = tied PCC, other
PCC modulus of rupture (typically 600 to 800 psi)
=
PCC E modulus (3 to 8 million psi for sound PCC, <3 =
million for unsound PCC)
J load transfer factor (3.2 to 4.0 for JPCP, JRCP 2.2 to 2.6 =
for CRCP)
TRAFFIC:
Future 18=kip ESALs in design lane over the design period =
(Nf)
SUPPORT AND DRAINAGE:
Effective dynamic k-value
=
Effective static k-value = effective dynamic k-value/2 =
(typically 50 to 500 psi/inch)
Subdrainage coefficient, Cd
=
(typically 1.0 for poor subdrainage conditions)
SERVICEABILITY LOSS:
Design PSI loss (P1 – P2)
=
RELIABILITY:
Design Reliability, R (80 to 99 percent)
=
Overall standard deviation, So (typically 0.39)
FUTURE STRUCTURAL CAPACITY:
Required slab thickness for future traffic is determined
from rigid pavement design equation or nomograph in,
Figure 6.2
Df =
inches
inches
psi
psi
psi/inch
psi/inch
percent
Source: AASHTO, 1993, Guide for Design of Pavement Structures Vol. 1. Used by Permission.
3. Fatigue damage adjustment factor (𝐹𝐹𝑓𝑓𝑓𝑓𝑓𝑓 ). This factor adjusts for past fatigue
damage that may exist in the slab. It is determined by observing the extent of
transverse cracking (JPCP, JRCP) or punchourt (CRCP) that may be caused
primarily by repeated loading. Use condition survey data from Step 3 and the
following guidelines to estimate Ffat for the design lane.
0.97-1.00: Few transverse cracks/punchouts exist (none caused by “D”
cracking or reactive aggregate distress)
JPCP: <5% slabs are cracked
JRCP: <25 working crack per mile
CRCP: <4 punchouts per mile
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
0.94-0.96: A significant number of transverse cracks/punchouts exist
(none caused by “D” cracking or reactive aggregate distress)
JPCP: 5-15% slabs are cracked
JRCP: 25-75 working cracks per mile
CRCP: 4-12 punchouts per mile
0.90-0.93: A large number of transverse crack/punchouts exist (none
caused by “D” cracking or reactive aggregate distress)
JPCP: >15% slabs are cracked
JRCP: >75 working cracks per mile
CRCP: >2 punchouts per mile
𝐃𝐃𝐞𝐞𝐟𝐟𝐟𝐟 From remaining Life for PCC Pavements
The remaining life of the pavement is given by the following equation:
Np
RL = 100 [1 − (
)]
N1.5
where:
RL = remaining life, percent
Np = total traffic to date, ESALs
N1.5= total traffic to pavement “failure,” ESALs
N1.5 may be estimated using the new pavement design equations or
nomographs. To be consistent with the AASHO Road Test and the
development of these equations, a “failure” PSI equal to 1.5 and a reliability of
50% is recommended.
Deff is determined from the following equation:
Deff = CF ∗ D
where:
CF = condition factor determined from Figure 6-30
D = thickness of the existing slab, inches
The designer should recognize that Deff determined by this method does not
reflect any benefit for pre-overlay repair. The estimate of Deff obtained
should thus be considered a lower limit value. The Deff of the pavement will
be higher if pre-overlay repair of load-associated distress is done.
A worksheet for determination of Deff of JPCP, JRCP, and CRCP is provided in
Table 6-26.
Step 8: Determination of Overlay Thickness
The thickness of bonded PCC overlay is computed as follows:
Dol = Df + Deff
where:
Dol = Required thickness of bonded PCC overlay, inches
6-116
Df = Slab thickness determined in Step 6, inches
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Deff =
Effective thickness of existing slab determined in Step 7, inches
The thickness of overlay determined from the above relationship should be
reasonable when the overlay is required to correct a structural deficiency.
Table 6-26
Calculation of Deff for Bonded PCC Overlay of JRCP, and CRCP
Condition Survey Method:
Fjc
Fdur
Ffat
Number of unrepaired deteriorated joints/mile
=
Number of unrepaired deteriorated cracks/mile
=
Number of unrepaired punchouts/mile
=
Number of expansion joints, exceptionally wide joints
(>1 inch) or AC full-depth patches/mile
=
Total/mile
=
Fjc =
(Figure 6-31)
(Recommended value 1.0, repair all deteriorated areas)
1.00:
No sign of PCC durability problems
0.96-0.99:
Some durability cracking exists, but no spalling exists
0.88-0.95:
Cracking and spalling exist
Fdur =
0.97-1.00:
Very few transverse cracks/punchouts exist
0.94-0.96:
A significant number of transverse cracking/punchouts exist
0.90-0.93:
A large amount of transverse cracking/punchouts exist
Ffat =
Remaining Life Method:
Np
Deff = Fjc ∗ Fdur ∗ Ffat ∗ 𝐷𝐷 = _______________________
= Past design lane ESALs
=
N1.5 = Design lane ESALs to P2 of 1.5
CF =
=
Np
)] = _______________________
RL = 100 [1 − (
N1.5
Deff = CF ∗ D = _______________________
Source: AASHTO, 1993, Guide for Design of Pavement Structures Vol. 1. Used by Permission.
6-117
6-118
Relationship Between Condition Factor and Remaining Life
Source: AASHTO, 1993, Guide for Design of Pavement Structures Vol. 1. Used by Permission.
Figure 6-30
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Fjc Adjustment Factor
Source: AASHTO, 1993, Guide for Design of Pavement Structures Vol. 1. Used by Permission.
Figure 6-31
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
6.12.6
Shoulders
Overlaying traffic lanes generally requires that the shoulders be overlaid to
match the grade line of the traffic lanes. In selecting an overlay material and
thickness for the shoulder, the designer should consider the extent to which the
existing shoulder is deteriorated and the amount of traffic that will use the
shoulder. For example, if trucks tend to park on the shoulder at certain locations,
this should be considered in the shoulder overlay design.
If an existing shoulder is in good condition, any deteriorated areas should be
patched. An overlay may then be placed to match the shoulder grade to that of
the traffic lanes. If an existing shoulder is in such poor condition that it cannot be
patched economically, it should be removed and replaced.
6.12.7
Joints
Existing JPCP and JRCP. Transverse and longitudinal joint should be saw cut
completely through the overlay thickness (plus 0.5-inch depth) as soon as
curving allows after overlay placement. Failure to saw joints soon after
placement may result in debonding and cracking at the joints. No dowels or
reinforcing steel should be placed in these joints. An appropriate sealant
reservoir should be sawed and sealant should be placed as soon as possible.
Existing CRCP. Transverse joints must no be cut in the bonded overlay, as they
are not needed. Transverse joints are also not needed for the end joints for fulldepth reinforced tied concrete patches. Longitudinal joints should be sawed in
the same manner as for JPCP and JRCP.
6.12.8
Bonding Procedures and Material
The successful performance of the bonded overlay depends on a reliable bond
with the existing surface. The following guidelines are provided:
1. The existing surface must be cleaned and roughened, through a mechanical
process that removes a thin layer of concrete, but does not damage (crack)
the surface. Shot blasting is the most used system. Cold milling has been used,
but may cause damage to the surface and thus requires sand blasting
afterward to remove loose particles.
2. A bonding agent is recommended to help achieve a more reliable bond.
Water, cement, and sand mortar; water and cement slurry; and low-viscosity
epoxy have been used for this purpose. Bonded overlays constructed without
a bonding agent have performed well in some instances.
6.13
Unbonded JPCP, JRCP, and CRCP Overlay of JPCP, JRCP, CRCP and AC/PCC
An unbounded JPCP, JRCP, or CRCP overlay of an existing JPCP, JRCP, CRCP, or
composite (AC/PCC) pavement can be placed to improve both structural capacity
and functional condition. An unbounded concrete overlay consists of the
following construction tasks:
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6.13.1

Repairing only badly deteriorated areas and making subdrainage
improvements (if needed)

Constructing widening (if needed)

Placing a separation layer (this layer may also serve as a leveling course)

Placing the concrete overlay

Sawing and sealing the joints
Feasibility
An unbounded overlay is a feasible rehabilitation alternative for PCC pavements
for practically all conditions. They are most cost-effective when the existing
pavement is badly deteriorated because of reduced need for pre-overlay repair.
Conditions under which a PCC unbounded overlay would not be feasible include:

The amount of deteriorated slab cracking and joint spalling is not large and
other alternatives would be much more economical.

Vertical clearance at bridges is inadequate for required overlay thickness.
This may be addressed by reconstructing the pavement under the overhead
bridges or by raising the bridges. Thicker unbounded overlays may also
necessitate raising signs and guardrails, as well as increasing side slopes and
extending culverts. Sufficient right-of-way must be available or obtainable to
permit these activities.

The existing pavement is susceptible to large heaves or settlements.
If construction duration is critical, PCC overlays may utilize high-early strength
PCC mixes. PCC overlays have been opened within 6 to 24 hours after placement
using these mixtures.
6.13.2
Pre-overlay Repair
One major advantage of an unbounded overlay is that the amount of repairs to
the existing pavement are greatly reduced. However, unbounded overlays are not
intended to bridge localized areas of nonuniform support. The following types
distress (on the next page) should be repaired prior to placement of the overlay
to prevent reflection cracks that may reduce its service life.
Other forms of pre-overlay treatment for badly deteriorated pavements include
slab fracturing (break/seat, crack/seat, or rubblizing) the existing PCC slab prior
to placement of the separation layer. Fracturing and seating the existing slab may
provide more uniform support for the overlay.
6.13.3
Reflection Crack Control
When an AC separation layer of 1 to 2 inches is used, there should be no problem
with reflection of cracks through unbounded overlays. However, this separation
layer thickness may not be adequate for an unbounded overlay when the existing
pavement has poor load transfer and high differential deflection across
transverse crack or joints. Repairs for various distress types and overlay type are
provided in Table 6-27.
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Table 6-27
Distress Type
Working crack
Repair of Reflection Cracks
Overlay Type
Repair
JPCP or JRCP
No repair needed
CRCP
Full-depth dowelled repair if differential deflection is
significant
Punchout
JPCP, JRCP, CRCP
Full-depth repair
Spalled joint
JPCP or JRCP
No repair needed
CRCP
Full-depth repair of severely deteriorated joints
Pumping
JPCP, JRCP, CRCP
Edge drains (if needed)
Settlement
JPCP, JRCP, CRCP
Level-up with AC
Poor joint/crack
load transfer
JPCP, JRCP, CRCP
No repair needed; if pavement has many joints or
cracks with poor load transfer, consider a thicker AC
separation layer
Source: AASHTO, 1993, Guide for Design of Pavement Structures Vol. 1. Used by Permission.
6.13.4
Thickness Design
The required thickness of the unbounded overlay is a function of the structural
capacity required to meet future traffic demands and the structural capacity of
the existing pavement. The required overlay thickness to increase structural
capacity to carry future traffic is determined by the following equation.
where:
Dol = √D2f − D2eff
Dol = required thickness of unbounded PCC overlay, inches
Df
= slab thickness to carry future traffic, inches
Deff = effective thickness of existing slab, inches
Unbonded concrete overlays have been successfully constructed as thin as 5
inches and as thick as 12 inches or more. Thicknesses of seven to 10 inches have
been typical for most highway pavement unbounded overlays.
The required overlay thickness may be determined through the following design
steps. These design steps provide a comprehensive design approach that
recommends testing the pavement to obtain valid design inputs. If it is not
possible to conduct this testing, an approximate overlay design may be
developed based upon visible distress observations by skipping Steps 4 and 5,
and by estimating other inputs.
The overlay design can be prepared for a uniform section or point-by-point basis.
Step 1: Existing Pavement Design
1. Existing slab thickness
2. Type of load transfer (mechanical devices, aggregate interlock CRCP)
3. Type of shoulder (tied, PCC, other)
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Step 2: Traffic Analysis
1. Past cumulative 18-kip ESALs in the design lane (Np ), for use in the
remaining life method of Deff determination only
2. Predicted future 18-kip ESALs in the design lane over the design period (Nf )
Step 3: Condition Survey
The following distresses are measured during the condition survey for JPCP,
JRCP, and CRCP. Sampling along the project may be used to estimate these
quantities in the most heavily trafficked lane.
JPCP/JRCP:
1. Number of deteriorated transverse joints per mile
2. Number of deteriorated transverse cracks per mile
3. Number of existing expansion joints, exceptionally wide joints (more than 1
inch) or full-depth, full-lane-width AC patches
4. Presence and general severity of PCC durability problems

“D” cracking: low severity (cracks only), medium severity (some spalling),
high severity (severe spalling)

Reactive aggregate cracking: low
5. Evidence of faulting, pumping of fines or water at joints, cracks and pavement
edge
CRCP:
1. Number of punchouts per mile
2. Number of deteriorated transverse cracks per mile
3. Number of existing expansion joints, exceptionally wide joints (>1 inch) or
full-depth, full-lane-width AC patches
4. Number of existing and new repairs prior to overlay per mile
5. Presence and general severity of PCC durability problems (Note: Surface
spalling of tight cracks where the underlying CRCP is sound should not be
considered a durability problem)

“D” cracking: low severity (cracks only), medium severity (some spalling),
high severity (sever spalling)

Reactive aggregate cracking: low, medium, high severity
6. Evidence of pumping of fines or water
Step 4: Deflection Testing (strongly recommended)
When designing an unbounded overlay for existing JPCP, JRCP, or CRCP, follow
the guidelines given below for deflection testing and determination of the
effective static k-value. When designing an unbonded overlay for existing
AC/PCC, follow the guidelines given in Section 6.12.5, Step 4, for deflection
testing and determination of the effective static k-value.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Measure slab deflection basins in the outer wheel path along the project at an
interval sufficient to adequately assess conditions. Intervals of 100 to 1,000 feet
are typical. Measure deflections with sensors located at 0, 12, 24, and 36 inches
from the center of load. A heavy-load deflection device (e.g., Falling Weight
Deflectometer) and a load magnitude of 9,000 pounds are recommended. ASTM
D 4694 and D 4695 provide additional guidance on deflection testing.
For each slab tested, backcalculate the effective k-value using Figure 6-32 or
backcalculation procedure. The AREA of each deflection basin is computed from
the following equation.
where:
AREA = 6 x [1 + 2 (
d12
d36
d12
) + 2 ( ) + ( )]
d0
d0
d0
d0 = deflection in center of loading plate, inches
d𝑖𝑖 = deflections at 12, 24, and 36 inches from plate center, inches
AREA will typically range from 29 to 32 for sound concrete.
1) Effective dynamic k-value. Enter Figure 6-32 with d0 and AREA to determine
the effective dynamic k-value beneath each slab for a circular load radius of
5.9 inches and magnitude of 9,000 pounds. Note that for loads within 2,000
pounds more or less, deflections may be scaled linearly to 9,000-pound
deflections.
If a single overlay thickness is being designed for a uniform section, compute
the mean effective dynamic k-value of the slabs tested in the uniform section.
2) Effective static k-value.
Effective static k-value = Effective dynamic k-value/2
The effective static k-value may need to be adjusted for seasonal effects using
the approach presented in Part II, AASHTO, Design of Pavement Structure,
1993, Section 6.5.4. However, the k-value can change substantially and have
only a small effect on overlay thickness.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Figure 6-32
Effective Dynamic K-value Determination from d0 and Area
Source: AASHTO, 1993, Guide for Design of Pavement Structures Vol. 1. Used by Permission.
Step 5: Coring and Materials Testing.
When designing an unbounded overlay for existing JPCP, JRCP, or CRCP, coring
and materials testing of the existing PCC slab are not needed for overlay
thickness design. When designing an unbonded overlay for existing AC/PCC, Step
5, for determination of the AC modulus by coring and materials testing.
Step 6: Determination of Required Slab Thickness for Future Traffic (Df ).
The elastic modulus, modulus of rupture, and load transfer inputs to determine
Df for unbounded PCC overlays of PCC and AC/PCC pavements are representative
of the new PCC overlay to be placed rather than of the existing slab. This is
emphasized because it is the properties of the overlay slab (i.e., elastic modulus,
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
modulus of rupture, and load transfer), which will control the performance of the
unbounded overlay.
1. Effective static k-value beneath the existing pavement. Determine from one of
the following methods.

Backcalculate the effective dynamic k-value from deflection basins as
described in Step 4. Divide the effective dynamic k-value by 2 to obtain
the effective static k-value. The static k-value obtained may need to be
adjusted for seasonal effects.

Conduct plate load tests (ASTM D 1196) after slab removal at a few sites.
This alternative is very costly and time-consuming and not often used.
The static k-value obtainted may need to be adjusted for seasonal effects.
See effective Modulus of Subgrade Reaction.

Estimate from soils data and base type and thickness, using Figure 6-29.
This alternative is simple, but the static k-value obtained must be
recognized as a rough estimate. The static k-value obtained may need to
be adjusted for seasonal effects. See effective Modulus of Subgrade
Reaction.
2. Design PSI loss. PSI immediately after overlay (P1) minus PSI at a time of next
rehabilitation (P2).
3. J, load transfer factor for joint design of the unbonded PCC overlay.
4. PCC modulus of rupture of unbounded PCC overlay.
5. Elastic modulus of unbounded PCC overlay.
6. Loss of support. Use LS = 0 for unbonded PCC overlay.
7. Overlay design reliability, R (percent).
8. Overall standard deviation (S0 ) for rigid pavement.
9. Subdrainage capability of existing slab, after subdrainage improvements.
Pumping or faulting at joints and cracks determined in Step 3 is evidence that
a determined in Step 3 is evidence that a subdrainage problem exists. In
selecting this value, note that the poor drainage situation at the AASHO Road
Test would be given a Cd of 1.0.
Compute Df for the above design inputs using the rigid pavement design
equation. A worksheet for determining Df is provided in Table 6-28.
Step 7: Determination of Effective Pavement
The condition survey and remaining life procedures are presented.
𝐃𝐃𝐞𝐞𝐞𝐞𝐞𝐞 From Condition Survey
The effective thickness (Deff) of an existing PCC or AC/PCC pavement is
computed from the following equation:
Deff = Fjcu x D
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
where:
D = existing PCC slab thickness, inches (Note: maximum D for use in unbounded
concrete overlay design is 10 inches even if the existing D is greater than 10
inches
Fjcu = joints and cracks adjustment factor for unbonded concrete overlays
Note that the existing AC surface is neglected determining the effective slab
thickness of an existing AC/PCC pavement.
Field surveys of unbounded jointed concrete overlays have shown very little
evidence of reflection cracking or other problems caused by the existing slab.
Therefore, the Fdur and Ffat are not used for unbounded concrete overlays. The
Fjcu factor is modified to show a reduced effect of deteriorated cracks and joints
in the existing slab, and is given in Figure 6-33.
1) Joints and cracks adjustment factor (Fjcu ). This factor adjust for the extra loss
in PSI caused by deteriorated reflection cracks or punchouts in the overlay
that result from any unrepaired deteriorated joints, cracks and other
discontinuities in the existing slab prior to overlay. Very little such loss in PSI
has been observed for JPCP or JRCP unbonded overlays.
The following information is needed to determine Fjcu to adjust overlay
thickness for the extra loss in PSI from deteriorated reflection cracks that are
not repaired:
Number of unrepaired deteriorated joints/mile
Number of unrepaired deteriorated cracks/mile
Number of expansion joints, exceptionally wide joints (greater than 1 inch) or
full depth full-lane-width AC patches/mile
The total number of unrepaired deteriorated joints/cracks and other
discontinuities per mile prior to overlay is used to determine the Fjcu from
Figure 6-31 for the appropriate type of PCC overlay. As an alternative to
extensive full-depth repair for an unbonded overlay to be placed on a badly
deteriorated pavement, a thicker AC interlayer should eliminate any
reflection cracking problem, so that Fjcu = 1.0.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Table 6-28
Worksheet for Determination of Df for Unbonded PCC Overlay
SLAB:
Type of load transfer system: mechanical device, aggregate interlock CRCP
Type of shoulder = tied PCC, other
PCC modulus of rupture of unbonded overlay (typically 600 to
800 psi)
=
PCC E modulus of unbounded overlay (3 to 5 million psi)
=
J load transfer factor of unbonded overlay (2.5 to 4.4 for jointed
PCC, 2.3 to 3.2 for CRCP)
=
TRAFFIC:
Future 18=kip ESALs in design lane over the design period (Nf) =
SUPPORT AND DRAINAGE:
Effective dynamic k-value
=
Effective static k-value = effective dynamic k-value/2 (typically
50 to 500 psi/inch)
=
Subdrainage coefficient, Cd
(typically 1.0 for poor subdrainage conditions)
=
SERVICEABILITY LOSS:
Design PSI loss (P1 – P2)
=
RELIABILITY:
Design Reliability, R (80 to 99 percent)
=
Overall standard deviation, So (typically 0.39)
=
FUTURE STRUCTURAL CAPACITY:
Required slab thickness for future traffic is determined from
rigid pavement design equation or nomograph in Figure 6-2, Df
=
inches
psi
psi
psi/inch
psi/inch
percent
Source: AASHTO, 1993, Guide for Design of Pavement Structures Vol. 1. Used by Permission.
6-128
Fjcu Factor for Unbonded JPCP, JRCP, and CRCP Overlays
Source: AASHTO, 1993, Guide for Design of Pavement Structures Vol. 1. Used by Permission.
Figure 6-33
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
𝐃𝐃𝐞𝐞𝐞𝐞𝐟𝐟 From Remaining Life for PCC Pavements
The remaining life of the pavement is given by the following equation:
where:
Np
RL = 100 [1 − (
)]
N1.5
RL = remaining life, percent
Np = total traffic to date, ESALs
N1.5 = total traffic to pavement “failure,” ESALs
N1.5 may be estimated using the new pavement design equations or nomographs
in Figure 6-2. To be consistent with the AASHO Road Test and the development of
these equations, a “failure” PSI equal to 1.5 and reliability of 50% are
recommended.
Deff is determined from the following equation:
where:
Deff = CFxD
F
= condition factor determined from Figure 6-34
D
= thickness of the existing slab inches (Note: marimum D for use in
unbonded concrete overlay design is 10 inches even if the existing D is
greater than 10 inches)
The designer should recognize that Deff determined by this method does not
reflect any benefit for preoverlay repair. The estimate of Deff obtained should
thus be considered a lower limit value. The Deff of the pavement will be higher if
preoverlay repair of load-associated distress is done. It is also emphasized that
this method of determining Deff is not applicable to AC/PCC pavements.
A worksheet for determination of Deff is provided in Table 6-29.
Step 8: Determination of Overlay Thickness
The thickness of unbonded PCC overlay is computed as follows:
where:
Dol = √D2f − D2eff
Dol = required thickness of unbonded PCC overlay, inches
Df
= slab thickness determined in Step 6, inches
Deff = effective thickness of existing slab determined in Step 7, inches
The thickness of overlay determined from the above relationship should be
reasonable when the overlay is required to correct a structural deficiency.
6-130
Fjcu Adjustment Factor for Unbonded JRCP and CRCP Overlays
Source: AASHTO, 1993, Guide for Design of Pavement Structures Vol. 1. Used by Permission.
Figure 6-34
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
6.13.5
Shoulders
See Section 6.11.6 for guidelines.
6.13.6
Joints
Transverse and longitudinal joints must be provided in the same manner as for
new pavement construction, except for the following joint spacing guidelines for
JPCP overlays. Due to the unusually stiff support beneath the slab, it is advisable
to limit joint spacing to the following to control thermal gradient curling stress:
Maximum joint spacing (feet) = 1.75 x Slab thickness (inches)
Example: slab thickness = 8 inches
Joint spacing = 8 x 1.75 = 14 feet
Table 6-29
Calculation of Deff for Unbonded FCC Overlay of JPCP, JRCP, CRCP, and AC/PCC
Condition Survey Method:
JPCP, JRCP, or CRCP Overlay:
Fjcu Number of unrepaired deteriorated joints/mile
=
Number of unrepaired deteriorated cracks/mile
=
Number of unrepaired deteriorated punchouts/mile
=
Number of expansion joints, exceptionally wide joints
(>1 inch) or full-depth, full-lane-width AC patches/mile
=
Total/mile
=
Fjcu =
(Figure 6-33)
Effective Slab Thickness:
Notes:
Deff = Fjcu ∗ 𝐷𝐷 = _______________________
Maximum D allowed is 10 inches for use in calculating Deff for unbonded overlays. Existing AC surface is
neglected in calculating Deff for existing AC/PCC pavement when designing an unbonded PCC overlay.
Remaining Life Method:
Np
= Past design lane ESALs
=
N1.5 = Design lane ESALs to P2 of 1.5
=
CF =
Np
RL = 100 [1 − (
)] = _______________________
N1.5
Note:
Deff = CF ∗ D = _______________________
(Figure 6-28)
Maximum D allowed is 10 inches for use in calculating Deff for unbonded overlays
Source: AASHTO, 1993, Guide for Design of Pavement Structures Vol. 1. Used by Permission.
6.13.7
Reinforcement
Unbonded JRCP and CRCP overlays must contain reinforcement to hold crack
tightly together. The design of the reinforcement would follow the guidelines
given for new pavement construction, except that the friction factor would be
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high (e.g., 2 to 4) due to bonding between the AC separation layer and the new
PCC overlay. Refer Section 6.5.2.3.
6.13.8
Separation Interlayer
A separation interlayer is needed between the unbonded PCC overlay and the
existing slab to isolate the overlay from the cracks and other deterioration in the
existing slab. The most common and successfully used separation interlayer
material is an AC mixture placed one inch thick. If a level-up is needed, the AC
interlayer may also be used for the purpose. Refer reference No. 29 and 30,
AASHTO Guide for Design of Pavement Structures, 1993.
Some thin materials that have been used as bondbreakers have not performed
well. Other thin layers have been used successfully, including surface treatments,
slurry seals, and asphalt with sand cover for existing pavements without a large
amount of faulting or slab breakup. For heavily trafficked highways, the potential
problem of erosion of the interlayer must be considered. A thin surface treatment
may erode faster than an AC material. There is no reason that a permeable opengraded interlayer cannot be used, provided a drainage system is designed to
collect the water from this layer. This type of interlayer would provide excellent
reflective crack control as well as preventing pumping and erosion of the
interlayer.
6.13.9
JPCP, JRCP, and CRCP Overlay of AC Pavement
JPCP, JRCP and CRCP overlays of AC pavement can be placed to improve both
structural capacity and functional conditions. This type of overlay consists of the
following major construction tasks:
1. Repairing deteriorated areas and making subdrainage improvements (if
needed)
2. Constructing widening (if needed)
3. Milling the existing surface if major distortion or inadequate cross-slope
exists
4. Placing an AC leveling course (if needed)
5. Placing the concrete overlay
6. Sawing and sealing the joints
Feasibility
A PCC overlay is a feasible rehabilitation alternative for AC pavements for
practically all conditions. They are most cost-effective when existing pavement is
badly deteriorated. Conditions under which a PCC overlay would not be feasible
include:
1. The amount of deterioration is not large and other alternatives would be
much more economical.
2. Vertical clearance at bridges is inadequate for required overlay thickness.
This may be addressed by reconstructing the pavement under the overhead
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bridges or by raising the bridges. Thicker PCC overlays may also necessitate
raising signs and guardrails, as well as increasing side slopes and extending
culverts. Sufficient right-of-way must be available or obtainable to permit
these activities.
3. The existing pavement is susceptible to large heaves or settlements.
If construction duration is critical, PCC overlays may utilize high-early-strength
PCC mixes. PCC overlays have been opened within 6 to 24 hours after placement
using these mixtures.
Pre-overlay Repair
One major advantage of a JPCP, JRCP, or CRCP overlay over AC pavement is that
the amount of repair required for the existing pavement is greatly reduced.
However the following types of distress (Refer Section 2.4.1) should be repaired
prior to placement of the overlay to prevent reflection cracks that may reduce its
service life. Guidelines on repairs are provided in References 1 and 3, AASHTO
Guide for Design of Pavement Structures, 1993.
Reflection Crack Control
Reflection cracking is generally not a problem for JPCP, JRCP, or CRCP overlays of
AC pavement. However, if the existing AC pavements has severe transverse
thermal cracks, it may be desirable to place some type of separation layer over
the transverse cracks to reduce the potential for reflection cracking. Table 6-30
provides overlay types and repairs for various distress types.
Table 6-30
Overlays and Repair Methods
Distress Type
Alligator cracking
Overlay Type
Repair
JPCP or JRCP
No repair needed
CRCP
Patch areas with high deflections
Transverse cracks
JPCP, JRCP, CRCP
No repair needed
Pumping, stripping
JPCP, JRCP, CRCP
Edge drains (if needed)
Remove stripping layer if severe
Settlement/heave
JPCP, JRCP, CRCP
Level-up with AC
Source: AASHTO, 1993, Guide for Design of Pavement Structures Vol. 1. Used by Permission.
Thickness Design
The required thickness of the PCC overlay is a function of the structural capacity
required to meet future traffic demands and the support provided by the
underlying AC pavement. The required overlay thickness to increase structural
capacity to carry future traffic is determined by the following equation.
where:
Dol = Df
Dol = required thickness of PCC overlay, inches
Df = slab thickness to carry future traffic, inches
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PCC overlays of AC pavement have been successfully constructed as thin as 5
inches and as thick as 12 inches or more. Seven to 10 inches has been typical for
most highway pavement overlays.
The required overlay thickness may be determined through the following design
steps. These design steps provide a comprehensive design approach that
recommends testing the pavement to obtain valid design inputs. If it is not
possible to conduct this testing, an approximate overlay design may be
developed based upon visible distress observation by skipping Steps 4 and 5 and
by estimating other inputs.
The overlay design can be done for a uniform section or on a point-by-point basis
as described in Part III, AASHTO Guide for Design of Pavement Structure, 1993.
Step 1: Existing Pavement Design.
1. Existing material types and layer thickness.
Step 2: Traffic Analysis.
1. Predicted future 18-kip ESALs in the design lane over the design period (Nf ).
Step 3: Condition Survey.
A detailed survey of distress conditions is not required. Only a general survey
that identifies any of the following distresses that may affect the performance of a
PCC overlay is needed:
1. Heaves and swells.
2. Signs of stripping of the AC. This could become even more serious under a
PCC overlay.
3. Large transverse cracks that, without a new separation layer, may reflect
through the PCC overlay.
Step 4: Deflection Testing (strongly recommended)
Measure deflection basins in the outer wheel path along the project at an interval
sufficient to adequately assess conditions. Intervals of 100 to 1,000 feet are
typical. A heavy-load deflection device (e.g., Falling Weight Defectormeter) and a
load magnitude of 9,000 pounds are recommended. ASTM D 4694 and D 4695
provide additional guidance on deflection testing. Deflections should be
measured at the center of the load and at least one other distance from the load,
as described in Section 6.11.6.5, Step 4.
For each point tested, backcalculate the subgrade modulus (MR ) and the effective
pavement modulus (Ep ) according to the procedures described in Part III,
AASHTO, Design of Pavement Structure.
1. Effective dynamic k-value. Estimate the effective dynamic k-value from
Figure 6-29 in using backcalculated subgrade resilient modulus (MR ), the
effective modulus of the pavement layers above the subgrade (Ep ), and the
total thickness of the pavement layers above the subgrade (D). It is
emphasized that the backcalculated subgrade resilient modulus value used to
estimate the effective dynamic k-value should not be adjusted by the C factor
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(e.g., 0.33) which pertains to establishing the design MR for AC overlays of AC
pavements.
If a single overlay thickness is being designed for a uniform section, compute
the mean effective dynamic k-value of the uniform section.
Step 5: Coring and Materials Testing.
Unless some unusual distress condition exists coring and materials testing are
not required.
Step 6: Determination of Required Slab Thickness for Future Traffic (Df ).
1. Effective static k-value (at bottom of PCC overlay over an existing AC
pavement). Determine from one of the following methods.

Determine the effective dynamic k-value from the backcalculated
subgrade modulus MR , pavement modulus Ep , and pavement thickness D
as described in Step 4. Divide the effective dynamic k-value by 2 to obtain
the static k-value. The static k-value may need to be adjusted for seasonal.

Estimate from soils data and pavement layer types and thicknesses, using
Figure 6-29. The static k-value obtained may need to be adjusted for
seasonal effects (see Development of Modulus of Subgrade Reaction).
2. Design PSI loss. PSI immediately after overlay (P1) minus PSI at time of next
rehabilitation (P2).
3. J, load transfer factor for joint design of the PCC overlay.
4. Modulus of rupture of PCC overlay. Use mean 28-day, third-point-loading
modulus of rupture of the overlay PCC.
5. Elastic modulus of PCC overlay. Use mean 28-day modulus of elasticity of
overlay PCC.
6. Loss of support.
7. Overlay design reliability, R (percent).
8. Overall standard deviation (S0 ) for rigid pavement.
9. Subdrainage capability of existing AC pavement, after subdrainage
improvements, if any.
Refer Part II, Table 2.5, AASHTO, Guide for Design of Pavement Structure) for
guidance in determining Cd . In selecting this value, note that the poor
drainage situation at the AASHO Road Test would be given a Cd of 1.0.
Compute Df for the above design inputs using the rigid pavement design equation
or nomograph in Figure 6-2. When designing an overlay thickness for a uniform
pavement section, mean input values must be used. When designing an overlay
thickness for specific points along the project, the data for that point must be
used. A worksheet for determining Df is provided in Table 6-31.
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Table 6-31
Worksheet for Determination of Df for PCC Overlay of AC Pavement
SLAB:
Type of load transfer system: mechanical device, aggregate interlock CRCP
Type of shoulder = tied PCC, other
PCC modulus of rupture of unbonded overlay (typically 600 to 800
psi)
=
PCC E modulus of unbounded overlay (3 to 5 million psi)
=
J load transfer factor of unbonded overlay (2.5 to 4.4 for jointed PCC,
2.3 to 3.2 for CRCP)
=
TRAFFIC:
Future 18=kip ESALs in design lane over the design period (Nf)
=
SUPPORT AND DRAINAGE:
Effective dynamic k-value
=
Effective static k-value = effective dynamic k-value/2 (typically 50 to
500 psi/inch)
=
Subdrainage coefficient, Cd
(typically 1.0 for poor subdrainage conditions)
=
SERVICEABILITY LOSS:
Design PSI loss (P1 – P2)
=
RELIABILITY:
Design Reliability, R (80 to 99 percent)
=
Overall standard deviation, So (typically 0.39)
=
FUTURE STRUCTURAL CAPACITY:
Required slab thickness for future traffic is determined from rigid
pavement design equation or nomograph in Figure 6-2
Df =
inches
psi
psi
psi/inch
psi/inch
percent
Source: AASHTO, 1993, Guide for Design of Pavement Structures Vol. 1. Used by Permission.
Step 7: Determination of Overlay Thickness.
The PCC overlay thickness is computed as follows:
Dol = Df
The thickness of overlay determined from the above relationship should be
reasonable when the overlay is required to correct a structural deficiency. See
Section 6.11.6 for discussion of factors which may result in unreasonable overlay
thickness.
6.14
Overlay Planning Guidelines
It is important that the rehabilitation program planners and rehabilitation
designers understand the locations where this specific overlay design method
can be best applied. The pavement engineers should pay special attention to the
following characteristics of the existing roadway cross section and condition:
1. Review of "as built" plans. The performance of this type of overlay depends
heavily on the materials and condition of the underlying layers at the time of
the overlay. This review will help the pavement engineer understand the
visual survey performance indicators in a field review.
a. The materials used in the concrete layer at the bottom of the existing
pavement should be durable. Gain an understanding of the existence of
reinforcement and its layout. Is it continuous mesh, load transfer, or a bar
configuration in portions of the slab?
b. Identify the transverse joint spacing and the depth of the PCC slabs. Is this
a uniform depth section or thickened edge pavement, and what were the
cross slopes on the surface of that layer?
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c. How many ACC overlays have been placed on this slab, what was the
overlay depth and date of placement, what was the material makeup, and
what surface preparations were done prior to each overlay? Are the
overlay materials subject to stripping or deformation?
d. What type, amount, and location of drainage layers or systems are
present along the roadway? A review of existing drainage ways adjacent
to the project can identify farm tile and drainage obstructions.
2. Traffic Estimate. Determine the historical mix of the traffic and the number of
axle loadings it has experienced from original construction to the present
time. What will be the mix and volumes of various vehicle types (especially
trucks or farm equipment) expected in the design period?
3. Field review of the project. This activity should involve the combined efforts
of the pavement engineer, soils engineer, planner, designer, and the local
maintenance supervisor at a minimum and any pavement
history/management records review for the project area. This is an
important step in collecting knowledge of the pavement that does not appear
in written records.
a. Identify each pavement distress, frequency, and severity over the length
of the project. Is the distress found throughout the project or in isolated
portions? Take note of field entrances of heavy farm equipment or
crossroads locations such as a quarry location near the roadway.
b. Try to determine whether the distress is load, environmental, or material
related. Examples may include excessive numbers of transverse cracks
that are "tented" upwards. If these cracks are associated with joints in the
underlying PCC pavement, it may mean that the concrete is deteriorated
to a point where the new overlay will not bridge this problem. It may also
mean there is no load transfer or the reinforcement is deteriorated to a
point of no practical use. If there are many transverse cracks between
PCC joints, one should consider the existence of transverse steel in the
concrete layer acting as another deteriorated joint.
c. Look for longitudinal working cracks at a joint between the original
pavement and the widening unit. A working crack here must be
considered in the jointing plan for the overlay.
d. Determine the location of other working cracks (longitudinal, transverse,
or diagonal) that will require reinforcement in the overlay.
e. Identify the location of existing full depth patches and their condition. If
they have failed, this is the time to replace them. If more than 20% of the
transverse joints in the project have been patched in this manner and
more are needed, thin concrete overlay rehabilitation should not be
considered. An unbonded overlay of significant thickness (6" or greater)
should be considered along with the option of removal and replacement
of the entire roadway structure.
f.
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Identify areas of potential delamination in the ACC or ACC/PCC
underlying pavement layers due to moisture or pavement longitudinal
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
growth and blowups. If blowups have occurred in the past, what material
was used to patch them and were moisture conditions dealt with at the
site? How much additional drainage work will be required and how will it
be placed relative to the existing pavement cross section?
g. Identify special needs for additional widening to accommodate trucks
entering and leaving the pavement and navigating horizontal curves.
h. Identify special needs of the surrounding property owners that must be
met during construction. Is daily access essential or can special
arrangements for access be provided during construction?
i.
Identify the drainage structures that must be lengthened or widened. Will
bridge decks be overlaid or will the overlay be transitioned into the
existing decks? Are the drainage structures in need of replacement? If so,
they should be replaced and patched with a full depth before the overlay
is placed.
j.
Identify the need and location for any shoulder widening. The new
pavement should have at least 3 foot of earth or granular shoulder
outside the paved area to protect the pavement section.
k. A review of detour potential routes should be considered at this time. It is
not necessary, but could speed up construction and opening to traffic.
l.
Field review data collection should include the following: random coring
of layers at selected distressed and good performing transverse joints,
wheel paths and mid panel/quarter points to determine the stability of
each layer, and potential delamination areas between various layers.
m. FWD deflection testing should be performed at each 0.2-mile location in
the outer wheel path in each direction of travel on the project roadway.
This amount may be reduced over time by the design agency when they
develop a level of confidence in the variability along the roadway with
their results. Testing should be done with a load and with sensors placed
between surface cracks and not across a reflective crack.
6.15
Overlay Design Guidelines
The following items should be considered in the development of the PCC
overlays.
1. Depth and condition of the various layers of the existing pavement. If the
underlying PCC layer contains large amounts of steel reinforcement that is
causing reflective cracking in the surface of the pavement, this pavement
should not be considered for a thin PCC overlay. Treat this pavement as an inplace base and design a conventional unbonded PCC overlay of 6 inches or
greater on the pavement.
If the cores taken in the field indicate debonding or delamination between the
ACC layers or the ACC/PCC, interface consideration should be given to
removal of the ACC to that interface. In the case the interface between the
ACC/PCC is the problem, complete removal of the ACC and replacement with
a one-inch ACC bond breaker should be considered to insure good
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performance of the new PCC overlay and to reduce the potential for reflective
cracking.
2. Overlay depth should be determined from the results of the coring, deflection
testing, and anticipated traffic mix and volume. It should consider the amount
of ACC depth being removed in the milling procedure and make allowance for
the reduced composite section depth.
3. The entire ACC surface should be milled to a minimal depth that allows for
the road surface crown to be restored while removing any high spots across
the road surface. It is not necessary or advised to remove the ACC to the
bottom of the ruts that may be present in the ACC surface. Millings can be
used for shoulder materials in most cases.
Termini at bridges or ends of the project should be milled to a depth of 6
inches or greater at the junction with the existing pavement. Mill across the
slab at this point to get a vertical edge at the junction of the new and old
surfaces. If this means removal of some of the existing PCC, treat that section
as a bonded section in terms of joint patterns. This provides a thickened end
where vehicle impact loads are prevalent.
In the event the existing widening unit longitudinal joint is open and working,
it is suggested that the widening unit be milled to a depth of the design
widening unit and a new joint established at this point. Expansion joint
material shall be replaced near bridge termini according to highway agency
standards.
4. Joint patterns in the surface PCC overlay should be determined in
relationship to the slab width of the underlying PCC pavement. Divide the
lane width into segments that are nearly square in nature and allow for the
retention of a lane or centerline joint. Either joint can become a reflective
crack if not considered in the design. Joint spacing of the PCC overlay in feet
should not exceed twice the depth in inches, unless fibers are used. In this
case, do not exceed the instructions of the fiber manufacturer. The fibers will
hold cracks tight and to some degree assist in bonding, but they will not stop
the crack from forming. The object of joint design is to allow for the
differences in cement chemistry of today and allow for curl and warp to occur
in the thin surface without causing cracks. Care should be taken to eliminate
longitudinal joints in the wheel paths wherever possible.
Evaluate the condition of the existing widening joint. If geotextile or metal
reinforcement was used across the original joints in the overlay and the joint
is tight, the designer can neglect this joint as a potential reflective crack in the
overlay.
5. The use of fibers is suggested in PCC overlays of less than 4 inches as an
insurance policy against loss of surface due to a crack. They will not stop
cracking, but will retard loss of material around the crack. Research is
ongoing to evaluate the use of the fibers to allow for larger joint patterns in
the overlay that could match the underlying PCC layer joint pattern.
6. Tie bars across the longitudinal joints should be used in PCC depths of 6
inches or greater due to placement problems. In the case of the longitudinal
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joint between a new widening unit and the existing pavement, where the PCC
overlay thickness is less than 3 inches to 6 inches, tie bars, nailed to the
existing ACC surface at 36 inch spacings should be considered.
7. Concrete quantities should be bid in two bid items. The first should be for
volume delivered in cubic yards. Be aware that this reduces the risk to the
contractor, fills the rutted areas and the overlay volume, and will often
overrun between 10%-20% due to irregularities in the slab wheelpaths. The
second bid item is for concrete placement by the square yard that covers the
placement and finishing, curing, and texturing of the concrete as will be used
in a conventional PCC new pavement construction.
8. Construction survey for the overlay should involve cross sections on 25 foot
intervals and points across the slab that involve the two edges of slab,
centerline of roadway, center of wheelpath (rut), and lane centerpoints as a
minimum. This minimizes the potential for unexpected overrun identified in
item 7. It allows the engineer to identify high points in the cross section that
will affect the minimum depth in a given cross section and the longitudinal
grades vs. the volume of concrete required to meet a good profile and
minimum depth of concrete requirements. This also requires the paving
contractor to utilize two string lines to guide his machine for each edge of the
roadway.
6.16
Overlay Construction Guidelines
The construction of this type of overlay creates its own problems and solutions.
With consideration of the following items, the project should move smoothly
forward with little or no inconvenience for the traveling public. The goal of this
work is to treat the construction process in the same manner as a conventional
ACC overlay construction project. This requires that the designer and
construction industry consider new paradigms in portland cement concrete
pavement construction.
1. Scheduling of work items on the contract should direct the contractor to close
portions of the road only when paving is ongoing. All removal items, drainage
construction, and shouldering work should be done under traffic.
One of the critical considerations in this process is the location of the
subdrains and their timing in the construction process. Consideration of the
location relative to the edge of the existing vs. finished pavement edge is
critical to the sequencing and traffic control.
2. Paving can be accomplished by either one-lane or two-lane construction
techniques. The use of maturity testing for determination of strength gain
and pilot car operations will allow for continuous use of the highway during
one-lane construction. The rapid placement of this overlay proceeds at a pace
that will allow multiple miles to be placed in one week in one direction and in
the following week in the opposite direction. This process will require new
ideas to protect the drop-off at the edge of the pavement for access points by
public traffic during construction.
Two-lane construction can proceed at an equally fast pace if a detour is
available. This method allows for a better performing longitudinal centerline
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joint. The maturity method of estimating concrete strength can be used with
proper concrete mix selection to reduce property owner inconvenience to
less than 20 hours (car traffic) and construction traffic to approximately 30
hours (truck traffic) being off the placed slab.
This type of paving moves very quickly and the contractor must be prepared
to not only take care of the surface area in terms of finishing, but also
maintain a good surface profile. This will be the controlling activity in this
type of construction.
3. The use of fibers and current texturing methods using burlap, Astroturf, and
tining machines will bring some of the fibers to the surface behind the paving
operation. These will not damage the pavement performance and can be
removed by the first set of highway traffic or snowplow operations of the
winter season.
4. Traffic control can be the same as that used on other overlay projects. It is
essential that the contractor and the businesses along the project understand
the construction process to be used. A preconstruction public meeting can
alleviate many of the potential conflicts with deliveries and access along the
project.
5. Concrete placement for this type of overlay requires that the pavement be
clean of all foreign matter in front of the paving operation. This will insure
the proper opportunity for bonding between the overlay and the existing
surface. Temperature monitoring of the existing ACC surface should be
employed to keep the surface temperature below 100 degrees Fahrenheit. In
the event that temperature is exceeded, the surface should be sprayed with
water to cool the surface. Excess water should be removed prior to the
placement of the concrete.
6. In the event fibers are selected for introduction into the mix, the contractor
should demonstrate that the introduction method will eliminate the potential
for balling. The use of agitating haul units allows for the addition of the fibers
at the plant and mixing during hauling. If dump trucks are used for hauling,
means of blowing the fibers into the mixing drum or alternative but workersafe modes should be demonstrated prior to beginning the work.
7. Curing and surface texturing for this type of work must remain close to the
slipform paving operation due to the depth of the overlay and the changing
weather conditions over the course of the day. It is recommended that
multiple maturity locations be considered for each day's placement. Research
indicates that the changes in existing pavement surface temperature over the
course of the day may dictate that some areas be skipped by the saw in the
morning in order to meet the rapid strength gain potential that could have
occurred in the middle of the afternoon.
8. Due to the small distance between joints, the joint formation should be done
with early entry saws and 1/8-inch-wide blades. Joint formation in both
directions should be initiated as soon as concrete strengths are in the 100 to
120 psi flexural strength range or when raveling of the joint does not occur
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behind the saw. Depth of joint formation should be accomplished to the same
relative depth as that in full-depth pavement specifications.
The process of first sawing a transverse joint in the 15 to 20 foot range of
spacing and then allowing other saws to follow with the intermediate joints
can prevent premature cracks on hot weather days. In the case of thin
overlays, it is important to keep the longitudinal sawed joint formation closer
to the slipform than on full depth pavements. It is probable and possible to
keep all the joint formation operations within 1,000 to 1,500 feet of the
slipform paver for mixes that do not contain slag or similar retarding type
materials.
Existing Iowa research indicates no difference in joint performance between
joints that were air blasted after sawing and joints that were not cleaned. The
only caution is the buildup of sawing dust and its effect on profile
measurements. This can be eliminated with the use of a poer broom. If water
is used to cool the saw blades, then water flushing of the joints should be
done.
9. The owners and contactors must change their perspective on opening this
road to traffic. Shouldering should be done as soon as the pavement strength
allows for construction traffic. This normally will be within two days after
paving and can be monitored with the maturity method of estimating
concrete strength.
6.17
Shoulder Design
The roadside shoulder is described by AASHTO in 1968 and referenced by Huang
(1993) as that portion of roadway contiguous with the travelled way for
accommodation of stopped vehicles for emergency use and for lateral support of
base and surface courses. Shoulders also provide emergency parking space for
vehicles, lateral clearance for signs and guardrails, improved sight distance in
cuts, areas for maintenance operations and additional lane capacity for detoured
traffic or even peak hour traffic. A major consideration is the volume of traffic
that is expected to travel on the shoulder. A commonly used assumption for
normally trafficked roads is to design the shoulder for 10% of the traffic in the
most heavily trafficked lane. This must however be compared to actual usage. In
many cases of congested highways the shoulders can carry as much traffic as the
main lanes. Where this is a possibility the shoulders must be designed as full
traffic lanes
The design process is as per Chapter 1.9 of the AASHTO 1993 Guide for Design of
Pavement Structures.
6.18
References
AASHTO (1993), Guide for Design of Pavement Structures. American Association
of State Highway and Transportation Officials, Washington DC, 1993.
Austroads (2003). Guide to the Selection of Road Surfacings Revised Edition. Part 2
Properties of Road Surfacings. Austroads publication AP-G63/03, Austroads
Sydney 2003.
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Austroads (2008), Guide to Pavement Technology, Part 2: Pavement Structural
Design. Austroads Sydney, 2008.
Croney, D. & Croney P.(1998). Design and Performance of Road Pavements.
Chapter 17, The AASHO and WASHO road tests. McGraw-Hill, New York 1998.
FHWA (2013) ‘Geotechnical Aspects of Pavements Reference Manual. Chapter 5:
Geotechnical Inputs for Pavement Design.
(on-line site at fhwa.dot.gov/engineering/geotech/pubs/05c.cfm)
Huang, Y. H. (1993). Pavement Analysis and Design (Chapter 1 Introduction
pavement types, Chapter 13 Design of overlays). Prentice-Hall, New Jersey.
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7
Earthworks
7.1
Introduction
Traffic is very frequently disturbed by damage resulting from earthquakes and
heavy rains. In particular, traffic disturbances are largely caused by the failure of
slopes. The stability of a slope is maintained mainly by a balance between the
integrity of the ground subject to sliding probability and the sliding force by
slope gravity. However, the stability of a slope is greatly disturbed by (1)
decreases in the strength of the ground due to groundwater seepage or heavy
rainfall, (2) changes in the balance of gravity due to artificial cuts and fills, (3)
increase of the pore pressure due to heavy rainfall or movement of groundwater,
or (4) increases in the gravity acceleration during earthquakes. The failures of
slopes are classified into ‘Landslides’ and ‘Failures’. Prior survey and
measurement are of great importance since the areas where failures are expected
to frequently occur are determined by geological conditions and can be
topographically predicted. Many causes of ‘failures’ exist, and sometimes several
causes compound in one failure, making the prediction of the location, scale and
so forth of a ‘failure’ more difficult to perform when compared to the case of
‘landslides’.
Appropriate slope protection works are required in order to prevent slope
failures. Sodding is the generally preferred method in view of its construction
cost and aesthetic appearance combined with slope flattening. However, slope
protection works using structures are sometimes employed as an alternative
where sodding or slope flattening is difficult to perform because of
meteorological, topographical, agronomical, gradient, construction limitations or
spring water conditions. In principle, areas where landslides are likely to occur
should be avoided at the route selection stage, but appropriate countermeasure
works will become necessary if road construction in such areas is unavoidable.
The existence of water is one of the greatest causes of slope failures and
landslides and so full precautions should be taken for the action of water.
Permanent drainage facilities as well as the temporary drainage facilities during
construction should be very carefully planned.
Slope protection measures deteriorate after many years and their functions are
also gradually degraded. In addition, external forces not taken into account at the
time of construction may begin to act and result in deformation of the slope. Also,
changes in the terrain due to nearby land development may sometimes increase
the likelihood of instability of slopes. The detection of changes that may cause
failures and the appropriate countermeasures are of great importance in the
daily maintenance of slopes.
This chapter is a compilation of considerations to be taken at the stages of
survey, design, construction, and maintenance and is based upon experience and
the results of studies accumulated for the purpose of indicating guidelines for
stabilizing slopes and for securing the safety of road traffic.
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7.2
Factors Affecting Design
7.2.1
Height
Due to gravitational forces, high embankments generally pose serious threat. For
high embankments, it is sometimes necessary to provide lateral support such as
retaining structures or other structural measures. Standard slope gradients may
only be applicable up to a certain height, beyond which expert opinion/provision
must be conducted to ensure safety.
Safety is improved if embankment heights are kept as low as possible, as well as
the provision of berms, or other stepped measures, to reduce the effective height
of the slope.
7.2.2
Slopes
The gradient of the slope plays an important role that can produce tremendous
risk if left unchecked. The gradient of side slopes, provision for drainage, seismic
consideration, and erosion must be seriously addressed to prevent problems for
embankments.
7.2.3
Foundation
The highway embankment may be sufficient in terms of slope specifications, but
it will not be stable if the foundation it is placed on is soft ground or problematic
soils. It can result in problems such as soil liquefaction, settlement (both
differential and total), and deep-seated slope failure. The bearing capacity of the
foundation must be considered for suitability in undertaking highway designs.
7.2.4
Loading
The loading at the top and base of a slope can have both positive and negative
effects on the stability of a slope. A load at the base of the slope can improve toe
stability while a load at the top of a slope can increase the mobilization potential
of the slope. Construction loading from earthmoving equipment as well as
loading from traffic or structures must be taken into account in the design of
slopes.
7.2.5
Selection of Embankment Materials
The materials used for embankment play an important role for slopes. Materials
used in the construction of fill slopes and embankments shall be used if it is
suitable, and as far as possible be those excavated from adjacent cuts. Rock
excavated from the cuts may be used as material for fills if crushed to acceptable
grading envelopes. Drying out of borrow material during hauling and handling
from cut to position of placing should be allowed for.
The following DPWH Orders allow for the use of stabilization additives to
improve upper layers of embankment where considered necessary:

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No. 12 dated 05 February 2006 on the subject ‘Use of Terrazyme as Soil
Stabilizer to Aggregates and Soil Materials’
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design

7.2.6
No. 27 dated 01 May 2009 on the subject ‘DPWH Standard Specifications for
SOM Top Seal, Item 316’ approves the use of polymer based SOM Top Seal
soil stabilizer.
Groundwater
The groundwater table and saturated soil is of particular importance in design.
Landslide or slip risk can be affected by an increase in the soak-away drainage or
the construction of retaining walls which inhibit groundwater flow, or during
heavy rain or human activities that upset the natural balance.
These conditions can lead to a reduction in beneficial soil suction, increased
static water pressures below the water table, increased hydraulic pressures, loss
of strength (softening) of clay, loss of cementing in some soils/rock and
transportation of soil particles, all of which contribute to slope failure.
Suitable drainage measures at the top and base of slopes, behind and in front of
retaining structures and sub soil drainage for roads and embankments need to be
considered to reduce the effect of the groundwater table (or changes within it)
and their effect on constructed works. These can include surface water drains,
surface protection, sub-soil drains, deep drainage or other methods.
Vegetation and trees draw large quantities of water out of the ground on a daily
basis which lowers the water table and increases suctions, both of which reduce
the likelihood of a slip or landslide occurring.
7.3
Survey on the Stability of Cut Slopes
Natural ground is extremely complicated and not uniform in its properties, and
cut slopes tend to gradually become unstable after the completion of work due to
the effect of upsetting the natural balance of the ground. An overall judgment
and slope stability risk analysis should be made by fully taking account of the
requirements for stability by undertaking an assessment of the cut slope
including geological, hydrological and external pressures and establishing a
suitable factor of safety for the intended works.
Standard tables have been used in the past as an estimate for potential “safe” cut
slopes however these are intended as a guide only and in practice there is more
to consider than the material type. The slope batters in Table 7-1 can provide an
initial indication of generally regarded suitable slopes for different material
types. The design slope, height, berms and retaining structures (if required)
should be analyzed by a geotechnical engineer in order to determine the safe
working slope batter and retaining structures (if required) for cut slopes.
There are many factors influencing the stability of cut slopes and the
consistency/relative density of the ground needs to be considered for soil slopes
as well as the cohesive nature of the ground, external pressures (such as a
construction or structural load at the top of the slope) and groundwater.
Unsaturated clay slopes, for instance, can maintain near vertical cuts however on
saturation can lead to collapse due to the softening of the clay particles. Similarly,
slopes of non-cohesive material and silt can have high dry strength for dense
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
material but on saturation can lead to failure due to the effect of particle
transportation.
For cuts in rock, the weathering, strength, defects, discontinuities, dips, strikes,
infill and other pertinent geological information is assessed in order to determine
the most efficient and safe rock cut slope. For example, a Fresh weathered rock
may be of extremely low strength so needs a flatter slope, whereas an extremely
weathered rock may be of medium strength with few discontinuities and can be
steeper. It must be understood that there is a difference in weathering of a rock
and its potential for failure at certain cut angles based on the rock mass
orientation. Defects, infill and bedding plane orientation can have a major effect
on the stable cut slope and as such a generic cut slope cannot be defined for all
rock types for the degree of weathering. The values given in Table 7-1 are
conservative estimates. Geological mapping of a rock mass with oriented core
boreholes or similar effective drilling allows the nature of the rock mass to be
established for design of cut slopes.
Geotechnical design parameters should be established for the material of the cut
slope and the cut slope be analyzed by slope stability calculations or specialized
design software (such as Geostudio Slope/W or similar) that can establish the
likelihood of slope failure for the inherent geotechnical conditions.
Groundwater needs to be addressed in the analysis of cut slopes and drainage
measures to reduce the effect of groundwater on the instability of the slope
should be designed.
Slopes must be protected by means of retaining walls, soil nails, micropiles,
cribwork, or other stabilization options when it is unavoidable to form a cut
slope with a gradient steeper than the safe gradient determined by slope stability
analysis. An estimate of allowable gradients for some common soil types
encountered and rocks is provided in Table 7-1.
7.4
Fill Survey
7.4.1
Points of Survey
Fill survey can be roughly divided into foundation ground surveys and fill
material surveys. The foundation ground should be able to support the weight of
fill and associated structures without causing harmful settlement. Thus, the
ground should be fully surveyed and reviewed with respect to stability and
settlement.
Fill material surveys are performed to determine the suitability of soils for fill
materials and to obtain the soil constants required for studying the stabilization
of fill. The following items should be tested:

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Suitability of soil as fill material (this is mainly judged from classification tests
including particle size distribution, Atterberg limits and compaction density),
and
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design

7.4.2
Strength of compacted soils (CBR test for subgrade, unconfined compressive
strength test, direct shear test, or triaxial shear test, for reviewing the
stability of high fill slope, etc.)
Survey on the Stability of Cut and Fill Slopes
The gradient of a fill slope is normally determined by the material type. Table 7-1
provides an indicative guide to applicable slope gradients considering the
materials and height of fill. These values should be verified by a practicing
Geotechnical Engineer prior to construction. All material in a fill slope must be
compacted to a minimum of 98% maximum dry density in layers of maximum
300 mm thickness (loose lift).
Table 7-1
Stability of Cut and Fill Slopes for Different Material Types
Nature of
Material
Height of
Cut/ Fill (m)
Slope Ratio
(H:V)***
Soil
Less than 5
1.5:1 to 2.0:1
5 to 15
1.8:1 to 2.5:1
Poorly Graded Sand (SP)
Less than 10
1.8:1 to 2.5:1
Silty Sand (SM)
Less than 5
1.5:1 to 2.0:1
Hard clayey soils and clay
of alluvium, loam (CL)
5 to 10
2.0:1 to 2.5:1
Soft Clay of high plasticity
(CH), Silts (ML, MH)
0 to 5
2.5:1 to 3.0:1
Less than 10
0.5:1 to 1:1
10 to 15
0.75:1 to 1.2:1
Very Low to Medium
Strength Rock, Extremely
to Distinctly Weathered
Less than 5
0.75:1 to 1.2:1
5 to 10
1.0:1 to 1.5:1
Residual Soil to Extremely
Low Strength Rock,
Extremely Weathered
Less than 5
1.0:1 to 1.5:1
5 to 10
1.5:1 to 2:1
Filling Material*
Well graded sand (SW)
Gravel with Silt (GM)
Gravel with Clay (GC)
Well Graded Gravel (GW)
Poorly Graded Gravel (GP)
Remarks
Applied to fills with
sufficient bearing capacity
at foundation ground,
which are not affected by
inundation (assumed
drained and unsaturated).
Consistency assumed to
be medium dense (noncohesive) or stiff
(cohesive) or better.
Clayey Sand (SC)
Medium to High Strength
Rock, Slightly Weathered
to Fresh
Rock**
Assess all rock slopes in
cut in accordance with
Section 7.3.
* Table 7-1 not applicable for soil and rock types not included
** Refer Section 7.3 for rock slopes for cuts
*** All slope ratios assume that berms are in place at regular intervals and slope
protection such as nets, catch drains and other protective measures are in place, as
required.
The stability of fill in the above cases should be examined by stability calculations
or slope stability software (such as Slope/W) using the results of classification
tests, unconfined compression tests, direct shear tests, or tri-axial tests using fill
material which has been compacted to the prescribed degree with geotechnical
design parameters determined for each material type, including effects of
groundwater and saturation. Additionally, engineering judgment and experience
on material type and slope batters for construction and permanent works should
be sought.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Standard tables have been used in the past as an estimate for potential “safe” cut
slopes however these are intended as a guide only and in practice there is more
to consider than the material type. The slope batters in Table 7-1 can provide an
initial indication of generally regarded suitable slopes for different material
types. The design slope, height, berms and retaining structures (if required)
should be analyzed by a geotechnical engineer in order to determine the safe
working slope batter and retaining structures (if required) for cut slopes.
7.4.3
Survey on Fill Requiring Extra Precautions
The stability of not only the slope but also the main body of embankment should
be carefully examined when planning fills. The stability of the slope and the main
body is mainly affected by spring water and rainfall and extra precautions should
be taken.
7.4.4
Survey on Fill with Construction Loads
Construction loads from earthmoving equipment as well as traffic loading is to be
taken into account for slope instability calculations. These external pressures can
increase the sliding force at the top of the slope leading to instability.
Fill on Inclined Ground
Spring water from ground frequently permeates into fill to make fill slopes
unstable. Therefore, the real conditions of groundwater should be fully
investigated particularly for the fills on inclined ground, fills in valleys, partial
cuts and fills or transitions of cuts and fills. The main items to be clarified with
respect to groundwater are:

Distribution of groundwater or groundwater pressure.

Extent of permeable layer or water-bearing stratum, or extent of
impermeable layer.

Direction of groundwater flow, water vein, or water source.
These items and groundwater conditions cannot be determined from the results
of a single survey and should thus be comprehensively determined from the
results of many surveys including field surveys, borings, soundings, and so forth.
In addition and with respect to the relation between water and road, it is
desirable to perform a wide range of surveys based on the fact that roads are
affected by water and vice-versa over a wide area.
7.5
Slope Failures
7.5.1
Cut Slope Failures
Shallow Surface Failures
Where the cut slopes are formed with easily eroded sediments, non-cohesive
sand, or volcanic ash or sand, the slopes collapse locally by surface water or
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
seepage water. Slope failures also occur where rock formation has been
decomposed by weathering.
When considerably fractured rocks, rocks with fissures, or friable rocks are to be
excavated, partial slope falls sometimes occur due to vibration during work,
removal of load by cut works, or by weather.
Deep Cut Failures
Cut slopes often collapse at deep parts in the slope where the bedding or joints of
layers runs regularly in the direction of the slope in alternative layers of
crystalline schist, sandstone or shale, and where the faults accompanying
fractures zones, large fissures or seams are located in the middle of cut slopes
and dip to the slope. A considerably large-scale slope failure may occur in many
cases where ground thickly covered with talus cone type sediment is excavated.
Sediment also sometimes falls along the bedrock in areas where clay which has
developed along the fissures is excavated.
The signs of these failures cannot be easily distinguished and failures occur
suddenly in many cases, to occasionally result in disaster and loss of life.
Failures Reaching Foundation Ground
Deep large-scale slope failures or landslides may occur across a wide range of
slopes if they consist of fault-fractured zones, considerably transformed tuff, or
semi-solidified siltstone or mudstone. Also, large-scale sliding failures may occur
upon excavation, due to a rise in the groundwater after rainfall on diluvial ground
consisting of the alternating layers of clayey and sandy soil dipping to the slope.
The slope failures described above advance slowly along very clear sliding
surfaces in many cases, and the range and directions of failures can be predicted
at early stages from cracks occurring in the slopes. Thus, there is usually
sufficient time to examine countermeasures against these types of failures.
7.5.2
Fill Slope Failures
Shallow Surface Failures
The slope surface is scoured by concentrated water, and the surface failures
gradually advance in many cases when the slopes are formed with erosive soils
and when drainage facilities are not properly settled.
The widened portion of the embankment may sometimes collapse after rainfall
when soil which is easily weakened by seepage water is used for widening an
embankment, or when covered soil easily falls due to rainfall immediately after
the completion of work. Surface failures of this kind rarely affect the functions of
the main fill body but generally occur over a wide are of the fill body.
Deep Fill Failures
The pore water pressure within the fill may increase and failures may sometimes
occur from deep parts of the fill when high fill is quickly constructed with
cohesive soil having high water content.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Also, when embankments are made on a slope, the groundwater level in the
embankment may rise not only due to the rainfall but also to the seepage water
from the ground, sometimes resulting in failures of the entire embankment.
The scale of this type of failure is such that the functions of the embankments are
fully lost in many cases.
Failures Reaching Foundation Ground
A sliding surface is created in the foundation ground and a large-scale failure
reaching foundation ground may sometimes occur when an embankment is
constructed on a steep slope with a considerably weathered surface or on a slope
inter-bedded with an easily slippery layer.
Typical examples of this are failures which occur in embankments constructed on
soft ground. A new landslide will be induced, thereby resulting in a large failure
of the embankment and natural slope when an embankment is built on the head
of an old landslide area. Special attention must be directed to these failures since
they are all caused by embankments built upon unstable natural slopes with a
high potential of landslides and generally tend to cause great disasters causing
damage over an extensive area.
7.6
Basic Stability Design Considerations
7.6.1
Gradients of Fill Slope
Standard Gradients of Slope
Empirically determined standard values of gradients shown in Table 7-2 are
normally used for fill slopes. The standard gradient of slopes shown in Table 7-2
are the maximum slope ratios required for securing the stability of such fill
slopes so that the bearing capacity of the foundation is sufficient and so that
there is no danger of water inflow from foundation when such are made with the
earth thinly laid and compacted. Fills whose slopes are protected by anti-erosion
measures (such as blanket soil, sodding, protection works by simple slope cribworks, block pitching, etc.) may use these standard values as maximum values for
their slope gradients.
Generally, when low fill slopes are properly designed with a slope ratio of 1.5:1
and slope works are properly executed, large slope failures will hardly occur as
long as there are no soil problems or slopes are not extremely large. However,
the compaction of slopes with such a slope ratio as 1.5:1 tends to become
insufficient. There is a possible danger of occurrence of scour or spalling near the
surface. For these reasons, it is prescribed to apply a slope ratio of 1.8:1 as
required as standard gradient of slope and so as to enable mechanical
compaction work.
Also, if a slope of road facility is to be used commonly as a levee for river or coast,
then attention must be directed to the gradient of slope and the occurrence of
erosion by water action.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Forms of Slope and Fill Structures
Fill structure should be rationally designed by taking account of the surrounding
conditions such as ground conditions, materials available, weather condition,
stability of fill, and execution of work for each site. Also, it is recommended to use
a single gradient at least for the portions of slope located between berms.
Also, for a high fill made of more than two different kinds of materials, a standard
gradient suited to each material should be applied to each slope.
Where work is to be carried out using more than two different kinds of materials,
these materials should be used differently as follows by taking account of the
stability of fills and influence upon the pavement.

Where the height of fill is small and there are no stability problems:

Use of gravelly soil or sand is desirable up to a height which affects the
pavement structure (about 1m from the top of subgrade).

Where there is a fill stability problem.

Where there is the possibility of inflow of spring water into a fill on soft
ground, sloped ground or in swamp, the sand or gravelly soils with a smaller
amount of fine-grained portion should be used as much as possible at the
bottom of the fill so as to prevent any rise in water pressure inside fill and
minimize the occurrence of failure.
Since gravel (G), sand (S), and silt (M) are poor cohesive and easily subject to
erosion, special measures should be taken for the slope protection such as the
installation of ditches on berms if the height of a slope exceeds 3 to 5 m.
It is generally difficult to protect fill slopes made of pit-run gravel or sand with
vegetation. Thus, in order to protect these slopes against erosion, they should be
blanketed, if necessary, with gravelly soils (GF) or fine-grained soils (F) excluding
silt (M) as shown in Figure 7-1 to protect from erosion. The thickness of such
blanket soil is generally required to be greater than 30 cm if measured normal to
the face of the slope.
In this case, design precautions should be taken to drain inside seepage water to
the outside of fill body as shown in Figure 7-1.
Figure 7-1
Covering for Fill Slopes
Clayey Soil
1 :1
Pit-Run Gravel
.8
Sand, Etc.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Also, if sand with a poor grading is used as filling material, it is sometimes
becomes difficult to secure the surface traffic ability for heavy construction
machines, and thus a fill structure functioning as transportation road as well as
slope protection measures as shown in Figure 7-2 is occasionally used.
Figure 7-2
Example of Fill Using Sand with Poor Grading
Lower Subgrade
Upper Subgrade
Sand
Private Road
Pit-Run
Gravel
Layer for Draining Coin
Water During Work.
Berms
It is recommended to provide berms about 1.0 to 2.0 m wide every 3.0 to 5.0 m of
height, starting from the top of fill slope. Basically, the following considerations
should be taken for the berms:

Except low fills, ditches are frequently provided on fill berms to prevent
erosion due to rains during and after the works, and these berms will be
sometimes used as inspection galleries.

Earth structures are always required to be built while making corrections to
the original design, and the berms are able to provide allowances for making
such corrections (such as extra space for foundation for slope protection
structure).

Berms function as temporally work yards, if necessary, for maintenance and
repair work (such as restoration work after disasters, partial reinforcement
of slope).
When filling across a narrow valley, the location of berms may be determined
based upon the mean fill height instead of the maximum fill height (almost at the
center of valley).
Where a drainage layer is installed inside the fill, the positional relation between
the berms and drainage layer should be considered in the design stage. Generally,
the drainage layer should be designed in such a manner that it will be located 1 to
2 m above the berm.
7.6.2
Examination of Stability of Fills
Generally, the standard gradients are applied for fill slopes but their stability
should be checked by calculations or by other means in the cases described
below. However, instead of determining a gradient based upon only the result of
stability calculations, a comprehensive judgment should be made after fully
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
reviewing the records of slope works in adjacent areas or examples of disasters
in the past under similar soil conditions.
Fills Requiring Stability Investigations
Where the standard gradients in Section 7.4.2 cannot be applied under the
conditions stated below, stability investigations including stability calculations
should be performed, and then the fill structure including the selection of fill
material, groundwater drainage facility, gradient of slope and protection work
should be properly designed.
Conditions of Fill Itself
Stability investigations are required where:

The height of fill exceeds the standard values shown in Section 7.4.2.

The water content in the filling material is high, and the fill consists of soil
with a low shearing strength (such as volcanic ashes with a high water
content).

The filling material consists of soils such as silt in which pore water pressure
tends to increase easily with surrounding conditions.
External Conditions
Stability investigations are also required where:
7.6.3

The fill is easily affected by spring water from the ground (such as a partial
cut and fill section, a widened-embankment, fill on a slope, or fill across a
valley).

The fill slope may be inundated or the toe of the slope may be eroded during
floods (such as for fill on ponds).

Serious damage may occur to adjacent structures in the event of failure.

The bedrock of fill is instable, such as soft ground or a landslide area.

A long time is needed for restoration work and the function of the road may
be considerably disturbed in the event of failure (such as fill on slanted
ground like on a mountainous road and where there is no other alternate
road).
Stability Calculations
When examining the stability in advance for the fills, the smallest factor of safety
should be determined and the measures should then be formulated.
When designing the fill, it is desirable to determine the section of fill in such a
manner that a minimum factor of safety of 1.5 can be obtained from the results of
stability calculation.
In the stability calculations, the method of slices assuming a circular sliding
surface, as represented in Figure 7-3, based on the effective stress method can be
used.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
By this method, a mass on the sliding surface is divided into several slices with
appropriate width, the shearing forces and resisting forces of slices are totaled
respectively, and then the factor of safety is determined from the ratio between
them. Normally, the number of slices is greater than 6.
The shearing strength of soil is normally determined from the triaxial
compression, direct shear and unconfined compression tests, but the tests must
be carefully performed since the results may vary depending upon the test
method or skill of test. The pore water pressure should be determined by the
method described in Section 7.6.3.3.
Figure 7-3
Calculation for the Stability of Circular Sliding Surface
Center of Circle
0
Vertical Line
OC
Center of Gravity
W
0
Precautions should be taken since the test method in the survey stage is different
when making calculations by the effective stress and total stress method.
Calculating formulas:
With effective stress method
𝐹𝐹𝑆𝑆 =
∑(𝑐𝑐 ′ 𝑙𝑙 + (𝑊𝑊 cos 𝜗𝜗 − 𝑢𝑢𝑢𝑢) tan 𝜑𝜑′)
∑ 𝑊𝑊 sin 𝜗𝜗
where the shearing strength is given by:
With total stress method
𝑠𝑠 = 𝑐𝑐 ′ + (𝜎𝜎 − 𝑢𝑢) tan 𝜑𝜑′
𝐹𝐹𝑆𝑆 =
∑(𝑐𝑐𝑐𝑐 + (𝑊𝑊 cos 𝜗𝜗) tan 𝜑𝜑)
∑ 𝑊𝑊 sin 𝜗𝜗
where the shearing strength is given by:
where:
Fs = factor of safety
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𝑠𝑠 = 𝑐𝑐 + 𝜎𝜎 tan 𝜑𝜑
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
σ = normal stress
W = weight of slice
l = length of arc of sliding surface cut by each slice
c = cohesion
φ = angle of internal friction
u = pore water pressure
c’ = cohesion of soil for effective stress
φ’ = angle of internal friction for effective stress
Method of Shear Test
Since the shearing strength of soil varies depending upon the density, water
content in percent of dry weight, and extent of disturbance of samples, the
specimens for the test must have conditions the same as those of soil
representing the fill under consideration.

The shearing strength of soil should be determined by the drained shear test,
or consolidated un-drained shear test associated with the measurement of
pore water pressure, during the triaxial compression test when making
stability calculations by the effective stress method.

The unsaturated soil should be handled the same as saturated soil by taking
account of the pore water pressure. However, since the water content in
percent of dry weight will considerably affect the shearing strength in the
case of unsaturated soil, it is required to confirm the shearing strength
corresponding to the change in the water content in percent of dry weight.

Analysis based on the effective stress method should be used as a rule, but the
stability during immediately after the execution of work may be checked by
the total stress method when quickly building an embankment with finegrained soil (F). In this case, the shearing strength is generally determined by
performing the consolidated un-drained shear test but the shearing strength
obtained through confined compression test is sometimes used.
The prepared specimens should have their water content expressed as a
percentage of dry weight and degree of compaction close to those on the site and
then tested.
Method of Determination of Shearing Strength
When using the effective stress method, the shearing constants are generally
determined by the consolidated un-drained shear test. Mohr’s circle drawn by
the total stress is shifted by the magnitude of pore water pressure on the stress
axis, and then c’ and φ’ are determined from its envelope line.
In the case of the un-drained shear test of unsaturated soil, the envelope curve of
Mohr’s circle arranged by the total stress as shown in Figure 7-4 will become a
curve. In this case, a straight line is drawn as shown in the figure within the range
of stress to be calculated in order to determine cu and φu.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
When using the total stress method, the shearing constants are determined by
using the results of the un-drained shear test. For the cohesive soils, φ = 0 and c=
qu/2 may be used from an unconfined compression test.
Figure 7-4
Example of Results of Triaxial Un-drained Shear Test of Unsaturated Fine-Grained
Soil and Design Shear Strength Parameters, Cu and Φu
Determination of Pore Water Pressure
There are several kinds of pore water pressure in fills.

Excess pore water pressure generated from execution of filling work.

Pore pressure due to groundwater created by rain water or seepage water
from the bottom or sides of fill.
The excess pore water pressure generated from execution of filling work is used
for examining the stability of slope during or immediately after the execution of a
quickly filled embankment of fine-grained soil. It is the best way to determine the
pore water pressure to be used for the stability calculations by the actual
measurement on the site, but a pressure as shown in the Figure 7-5 may be used.
In addition, the pore water pressure from main water or seepage due to the rise
of ground-water, varies depending upon the soil and the shape of fill and the
conditions of the original ground. Water pressure should be determined by
drawing a flow net in accordance with a graphic solution method. Also, the pore
water pressure created by the infiltration of rain water sometimes becomes
considerably high depending upon the conditions of fill and thus, a seepage flow
should be assumed for the analysis as require in the fill.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Figure 7-5
Assumption of Pore Water Pressure Due to Load of Fill
H
Impermeable Layer
9.894
H
H
Impermeable Layer
9.978
Impermeable Layer
Impermeable Layer
An Example of Stability Calculations
For stability calculations, the shape of the actual fill are frequently simplified as
shown in Figure 7-6. An example of stability calculations where there is seepage
water in fill 20 m high with a slope ratio of 1.5:1 for the upper portion of the fill
slope and of 1.8:1 for the lower portion is indicated in Figure 7-7 and Figure 7-8.
The pore water pressure due to the seepage water is determined by drawing the
flow net in accordance with a graphic solution method. For the pore water
pressure on the sliding surface, point a is determined in such a manner that h = a
can be obtained in the normal direction of sliding surface at the intersection
between the sliding surface and equipotential line. If the same procedure is taken
for b, c, i, an envelope can be obtained as a pressure head curve, and the portion
surrounded by the sliding surface and pressure head curve becomes the pore
water pressure for the whole.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
7-16
Figure 7-6
Simplification of Fill Slope for Stability Calculations
Figure 7-7
Example of Stability Calculations
1.85
(5.9 + 7.8) x 4.0 x ½ = 27.40
⑨
⑧
⑦
⑥
⑤
④
1.85
(7.8 + 7.1) x 3.5 x ½ = 26.08
③
1.9
1.85
0.8 x 0.3 x ½ = 0.12
2.0 x 3.5 x ½ = 3.50
1.90
1.85
(0.3 + 1.5) x 4.8 x ½ = 4.32
(2.0 + 3.1) x 4.8 x ½ = 12.24
1.90
1.85
(1.5 + 2.6) x 4.8 x ½ = 9.84
(3.1 + 3.6) x 7.8 x ½ = 16.08
1.90
1.85
(2.6 + 4.0) x 4.4 x ½ = 14.52
(3.6 + 3.1) x 4.4 x ½ = 14.74
1.90
1.85
(4.9 + 5.9) x 4.0 x ½ = 19.80
(3.1 + 2.0) x 4.0 x ½ = 10.20
1.90
2.0 x 4.0 x ½ = 4.00
1.85
(7.1 + 5.5) x 3.5 x ½ = 22.05
②
1.85
3.4 x 5.5 x ½ = 9.35
(5)
0.99847
0.99813
0.95107
0.94646
0.89879
0.83549
0.75661
0.66044
0.52992
cos θ
∑ 𝑊𝑊 sin 𝜃𝜃
- 3° 10'
3° 30'
11° 10'
18° 50'
26° 00'
33° 20'
40° 50'
48° 40'
58° 00'
θ
(4)
sin θ
(6)
=
162.12
6.86
31.19
47.83
51.93
50.34
48.7
36.51
26.94
9.17
Wcos θ
(7)
27.59+134.30
-0.0552
0.0610
0.11366
0.32282
0.43879
0.54951
0.65386
0.75088
0.87805
∑[𝑐𝑐∙𝑙𝑙+(𝑊𝑊 cos 𝜃𝜃−𝑢𝑢∙𝑙𝑙) tan 𝜃𝜃]
31.25
31.25
48.75
54.87
56.01
58.29
𝐹𝐹. 𝑆𝑆. =
6.65
0.22
23.26
7.99
30.55
18.20
28.01
26.86
19.38
36.63
7.60
50.69
48.25
40.79
17.30
W = γA (t)
γ
(t/m3)
A (m )
①
Number
(3)
(2)
2
Example Slope Stability Sample Calculation
(1)
Figure 7-8
= 0.999
1.8 x 3.5 x ½ =
3/15
(4.8 + 3.1) x
4.9 x ½ =
12.01
(3.1 + 3.6) x
4.8 x ½ =
16.08
(3.1 + 3.6) x
4.7 x ½ =
15.75
(2.0 + 3.1) x
4.4 x ½ =
11.22
2.0 x 4.9 x ½ =
4.90
u·l
(t/m)
(8)
3.71
19.18
31.75
36.18
39.12
43.8
36.51
26.94
9.17
Wcos θ u·l
(9)
1.97
10.2
16.88
19.24
20.8
23.29
21.08
15.55
5.29
(Wcos θ
- u·l)
tan θ
(10)
-0.37
1.97
9.44
17.71
24.55
32.03
31.55
30.63
14.67
Wsin θ
(11)
3.46
4.92
4.82
4.71
4.40
4.92
4.71
5.34
6.39
l
(m)
(13)
1.73
1.73
2.41
2.36
2.20
2.46
4.00
4.54
5.43
c·l
(t/m)
(14)
7-17
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Stability Calculations for Collapsed Slopes
It is possible to examine slopes which have collapsed during or after the
execution of work by stability calculations in order to plan countermeasures.
That is, where the sliding surface can be easily assumed and the stability of
collapsed slope can be examined by stability calculations, the shearing strength
of the collapsed slope may be determined by the method stated above so that
countermeasures can be studied with this shearing force taken into account.
In this method, the sliding surface of the failure is to be assumed first, then the
mean values of shearing strength can be determined reversely from the equation
shown in Figure 7-3. That is, since a factor of safety can be assumed to be F =1.0
at the time of failure, the following formula can be derived from equation in
Figure 7-3.
where:
𝑐𝑐′ =
∑ 𝑊𝑊 sin 𝜗𝜗 − ∑(𝑊𝑊 cos 𝜗𝜗 − 𝑢𝑢𝑢𝑢) tan 𝜑𝜑′
∑ 𝑙𝑙
c’ = mean cohesion
φ’ = mean value of the angle of internal friction
The several combinations of c’ and φ’ values should be calculated using the above
equation, and then reasonable combination of c’ and φ’ should be determined
after comprehensively reviewing the soil data obtained through soil test on the
similar existing soils.
Values of c’ and φ’ thus derived are then used in the stability calculations when
selecting the restoration countermeasures.
7.6.4
Fills Requiring Extra Precautions
The stability of not only fill slope but also the main body (filled-up ground) must
be considered when reviewing the stability of fill. The stability of slope is mainly
affected by spring water and rainfall, and thus, the appropriate treatment of this
water is very important.
Fills on Inclined Ground
In the case of fill on inclined ground, fill in a valley, cut and embankment, and
boundary of cut-embankment, the spring water will frequently permeate into the
fill, thereby making the fill slope unstable. In this case, groundwater drainage
facilities should be designed to prevent groundwater from permeating into the
fill body. A drainage layer is often installed to reduce the water pressure in the fill
body. Refer Figure 7-9.
7-18
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Figure 7-9
Groundwater Drainage Facilities and Drainage Layer for Fill on Inclined Ground
Underground Drainage
Permeable layer
Water level without Drainage layer
Water level with
Drainage layer
In valleys developed on maintains, hills or plateaus, the nearby groundwater
flows into the poorly drainage paddy fields and irrigation canals, and fill failures
are very frequently caused by the inflow of groundwater from these places to
adjacent fills. Thus, it is desirable to dispose of the seepage water due to rain
water or snowmelt water by means or underground drainage works by taking
account of the underground impermeable and permeable layers.
Lowering the groundwater level in a fill body is effective for not only reducing
disasters due to rains but also for earthquake resistance.
Fills on Soft Ground
In the case of a fill on soft ground, a settlement occurs as embankment
progresses, making the slope gradient unfavorably smaller. In order to avoid this
problem, it is desirable to predict the amount of settlement in advance, to
provide a gradient steeper to a certain extent than the value actually needed, and
to make corrections to the designed gradient in response to the settlement of fill
during the execution of work. Where the settlement occurs even upon
completion of earthworks, the crown of fill is sometimes raised and a gradient
steeper than design gradient is employed for finishing. A schematic diagram of fill
on soft ground is provided in Figure 7-10.
Figure 7-10
Schematic Diagram of Fill on Soft Ground
Executed fill Slope
Slope after Settlement
Settlement of Original
Ground
Possible settlement of drainage facilities should be taken into account in their
design and execution. Particularly, the location of longitudinal drainage facilities
7-19
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
should be determined based upon the place where the ground settlement is the
greatest.
Damage to Slopes Due to Rain
It is known that failures of fill slopes occur more frequently with cohesive soils
than sandy soils and that their causes are decrease in strength with water
content, erosion, and the occurrence of pore water pressure due to the nonuniformity in compaction and materials.
For reducing the number of failures, it is important to perform the work by
means of so-called ‘horizontal thin layer compaction’ in which uniform horizontal
thin layer is first laid for easily draining rain water. The layer is then fully
compacted to reduce the coefficient of permeability. It is important especially for
large slopes to employ a fill structure capable of fully draining seepage water
(installation of horizontal drainage layer, etc.) and to perform temporary
drainage during execution of work.
For a large slope, it is desirable to prevent erosion by protecting the slope by
vegetation starting from the finished portion of the slope on a step by step basis.
During execution of work, it is required to find the place where the road surface
water concentrates on basing upon the horizontal and vertical alignments and to
make corrections of the details of design.
The conditions of finished fill slope should be investigated during works, and any
portion of slope surface which is always subjected to seepage water and likely to
be collapsed should be partially reinforced.
Soils and conditions of spring water are not necessarily uniform throughout a
slope in many cases, so that reasonable construction methods applicable to these
conditions should be selected. In this case, the schemes for drainage seepage
water in the fill slope should be basically considered.
Slopes with seepage water are frequently reinforced with slope protection
structures.
7.6.5
Execution of Fill Slope Work
General Precautions in Compaction of Slopes
Fill slope failures are frequently caused by water (from rains, groundwater, etc.).
One of the causes of these failures is the free water surface (i.e. pore water
pressure) due to the infiltration of rain water. This is mainly caused by the nonuniformity of the compaction near the slope, and this is considered to be
prevented by the horizontal thin layer compaction method described previously.
Slopes can be effectively compacted by direct compaction using the compaction
equipment illustrated in Figure 7-11.
7-20
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Figure 7-11
Winch
Compaction by Vibrating Roller and Bulldozer
Bulldozer
Vibrating Roller
Layer Compacted
Horizontally
That is, where the slope ratio is gentler than 1.8:1, a tire roller or vibrating roller
which is connected to and towed by a bulldozer is used to compact the main body
of fill. The fill is compacted by driving this heavy equipment up and down the
slope. Where the slope ratio is about 1.8:1, every layer in the main-fall body is
first compacted, the surface of slope is roughly finished according to finishing
stakes, and the slope is then compacted with a vibrating roller heavier than 3
tons pulled by a bulldozer on the top of fill as shown in Figure 7-11. If the roller is
lowered along the surface of slope while it is vibrating, the surface may
sometimes become loose and therefore it is more desirable to compact the earth
by vibrating while the roller is being pulled upward.
However, if the slope ratio is about 1.5:1, compaction by ordinary roller will
become difficult to perform and so special slope rollers such as vibrator or
vibration-type slope compacting equipment are normally used in this case. These
are used for compacting each layer of main fill body and simultaneously the
surface of slope, but the maximum slope ratio for the compaction with this
machine is 1:1.
The vibration roller specially designed for the slope is possible in the direction
transverse or parallel to the center line of the road. Compaction up to a slope
ratio of 1.2:1 is possible by the machine.
For Fine-Grained Soil (such as cohesive soils)
Slopes to be made with materials such as clay (CH) or volcanic ash type cohesive
soil (VH2) with high water contents, which cannot be completely compacted,
should be very carefully worked out by paying special attention to the stability of
the whole slope. Any deformation of finishing stakes or swelling of the slope
should be carefully observed during work. If any indications are found, their
causes and future stability should be examined.
Earth should be replaced or slope or the drainage work such as horizontal
drainage layer in fill slope should be provided as needed. In some cases, slope
protection with pile driving should be provided.
For Coarse-Grained Soil (such as sandy soils)
Where the main body of fill is to be made with the coarse-grained soils such as
gravel (G) or sandy soil (S) and therefore any problem is anticipated such as
erosion or difficulties in vegetation, slope is often covered with blanket soil. In
this case, the boundary between blanket soil and already executed fill body
should be formed by properly mixing them together and be compacted without
leaving a clear boundary. Refer Figure 7-12.
7-21
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Figure 7-12
Compaction of Slope Made of Coarse-Grained Soils
If possible, it is desirable to have a gentle slope ratio of about 1.8:1 for the fill
slope and to design a thickness of about 2 to 3 m for blanket soil in order to make
machine execution possible.
Protection Works for Temporarily Finished Slopes
Temporarily finished slopes are most unstable until the protection work is
performed, and will be easily eroded by rain water or others. Therefore, the slope
protection by vegetation or longitudinal drainage facilities should be provided as
quickly as possible. However, as temporary measures until the slope will be fully
protected, it is recommended to employ filling work shown in Figure 7-13 (a)
and (b). Also, if the executed fill is left for a long time after completion until
paving work, it is desired to install a temporary ditch using a soil cement mixture
in order to prevent the occurrence of any problem due to concentrated flow of
rain water.
Figure 7-13
Disposal of Surface Water During Work
Figure 7-14 shows an example of temporary drainage used for decomposed
granite which is easily eroded in heavy rain areas (where water cannot be
drained to the surface of slope it this case).
7-22
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Figure 7-14
Example of Temporary Drainage in a Fill Made of Decomposed Granite
Also shown in Figure 7-15 is an example of temporary drainage adopted in a high
fill slope made of volcanic ash which is likewise eroded very easily.
Figure 7-15
Example of Central Drain Pipe System in a Fill Slope Made of Volcanic Ash Under
Construction
7.7
Slope Protection Works
7.7.1
Selection Criteria for Slope Protection Works
Slope protection works are performed to protect the slopes from erosion or
weathering by covering them with vegetation or structures and also to stabilize
the slopes by means of drainage works or retaining structures.
A flowchart in Figure 7-16 provides a basis for selecting countermeasures for a
natural slope failure. A classification of slope failure countermeasures is provided
in Table 7-2 and provides a range of protection works and their features.
By Non-Structural Method
Vegetation works are performed to prevent erosion due to rain water by growing
plants on the faces of the slope and by firmly binding the faces with roots of
plants and to ease the temperature change on the ground surface and the provide
aesthetically pleasing views created by greening. Vegetation is frequently used in
places where the vegetation is possible, since the costs for vegetation are
relatively low in most cases. Typical types of slope protection by vegetation are
shown in Table 7-3.
7-23
7-24
Control works
Classification
Control works (1)
Protecting the slope
from the action of rain
Principal Goal
Table 7-2
Grating
Crib
works
Pitching
Work
Spraying
Slope Protection
work using
vegetation
Drainage works
Work Category
Cast-in-place
concrete grating
crib works
Pre-cast grating
crib works
Cast-in-place concrete or pre-cast grating crib
works are assembled on the slope and either
vegetation is planted or concrete poured inside
them to prevent weathering/erosion of the slope.
Pre-cast grating crib works that have been
developed provide preventive effects.
Cast-in-place concrete grating crib works also
have a preventive effect. The cast-in-place
grating crib work methods include spraying crib
works
Preventing weathering, erosion, and fine
separation or failure etc. of slopes.
Stone pitching,
block pitching,
concrete slab
pitching
Concrete pitching
Preventing erosion of the slope along with
weathering of the slope and a decline in the
strength of the ground that forms the slopes by
blocking it from the atmosphere, rainwater, etc.
Sprayed mortar or
shotcrete
It includes spreading seeds, soil dressing, thick
layer spraying method, vegetation network, sand
bag works, sodding, vegetation pots, and
transplanting. It is done to prevent rainwater
erosion, reduce surface temperature and
beautify slopes by reforestation.
Stabilizing the slope by draining the
underground water seepage to lower the pore
water pressure. It includes culvert work,
impervious wall works, collection well, etc.
Underground
water drainage
work
Sodding work
Preventing surface water from flowing on the
slope by rapidly collecting and draining surface
water outside the slope. It includes drainage
channels at the top of the slope, berm drainage
works, slope toe drainage channels, longitudinal
drainage channels, permeation prevention work,
and check dams
Purpose or Details of the Work
Surface water
drainage work
Work Subcategory
Classification of Slope Failure Countermeasures
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Slope protection works by structures
It is used at locations where landslide type
failure is predicted or where there is a lot of
underground water. It is often used for work
smaller than landslide prevention work.
It is used in almost all works. Its cost is low
and it is very cost-effective. This method
includes drainage channels that drain the
collected water out of the slope area.
Pre-cast crib works are used on slopes with a gradient
gentler than 1:1.0 and cast-in-place grating crib works are
used on steep slopes. The vertical height of pre-cast
grating crib works is, in principle, no higher than 5 m, and if
it is greater than this, separation walls are installed at
intervals of 10 m in the vertical direction. But where berms
cannot be formed, the cast-in-place method is used.
Stone pitching or block pitching is pitching is used on slopes
with a gradient less than 1:1.0 that are suitable for
vegetation work, and on slopes of non-cohesive sand or
hard plan pan and fragile clay. Concrete pitching is used on
rock slopes or slopes of compacted soil with developed
joints and a steep gradient greater than 1:1.0 that would
presumably be unstable with spraying or pre-cast grating
crib works.
It is suitable for rock with little spring water, which has only a
few cracks, and where a large failure has not occurred. Its
use is premised on a full study of its durability and of its
impact on the environment.
It is superior because it harmonizes the slope with its
surrounding environment.
When the principal method is vegetation, it is a cut slope
with little spring water, where in principle, a standard slope
gradient can be guaranteed.
One of the
most basic
methods, it
is rarely
used
alone, but
almost
always
with
another
method.
Application Range and Special Features
Restraint works
Classification
Control
works (2)
In addition to directly preventing failure,
effectively protects the slope from erosion and
weathering.
Directly preventing failure, stabilizing
counterweight fill, and providing a foundation for
slope protection works.
Preventing small failures and stabilizing slopes
with a lot of spring water and relatively soft
ground.
Stone masonry or
block masonry
retaining wall
Leaning concrete
wall
Gravity concrete
retaining wall
Concrete crib
retaining wall
Retaining wall
Anchor works
Preventing small failure at the bottom of the
slope.
Cutting works (B)
Cutting work that
improves the
shape of the
slope.
Ground anchor
work and rock bolt
work
It is used along with cast-in-place concrete
grating crib work, concrete retaining wall work,
concrete pitching work, or other
countermeasures to stabilize these works in
order to prevent failure and sliding of severely
weathered rock, rock with many cracks, and
surface soil. It also anchors rock that is cracked,
has joints, or bedding stratification to rock that is
internally stable to prevent its failure and
separation.
Cutting the slope to a gradient or height
necessary to maintain its safety even under the
action of rainfall
Overhangs are cut, unstable surface soil layers
are cut, and unfixed stones removed, to
eliminate soil layers or rock mass at risk of
collapse.
Balancing 0 forces to
prevent failure even
under the action of
rainfall
Cutting work (A)
Cutting unstable
soil mass
These include plastic soil cement works, net
works, fluid synthetic resin spray works, matcovering works, asphalt slope works, etc., and
are intended to prevent erosion.
Purpose or Details of the Work
Excluding slopes
where there is a high
probability of failure
under the action of
rainfall
Other slope
protection works
Work Subcategory
Others
Work Category
Protecting the slope
from the action of rain
Principal Goal
7-25
It is appropriate for cases where there are dwellings at the
top or bottom of the slope, if cutting work, passive retaining
wall work, etc. cannot be done, if the slope gradient is steep
and the slope is long, and cast-in-place grating crib work,
concrete grating crib work, concrete pitching work, etc. are
not stable enough. It is particularly appropriate when the
ground or rock to which the anchor is fixed is relatively solid
and shallower than the slope surface.
Because of its good permeability and its flexibility, it is suited
for places where there is a lot of spring water and the
ground is soft, or to prevent landslide type failure.
It is used to stabilize the bottom (toe) of slopes, and to
prevent failure. It is used in the middle parts of slopes.
It can be used on ground with inadequate solidity: less than
of gravelly ground. Even in a narrow space, it is compatible
with changing topography without taking space.
When it is a soil slope with gradient steeper than 1:1.0
(normally 0.3:1 to 0.5:1), and the earth pressure is low
because the ground behind it is firm.
It is one of the most basic countermeasures, and one of the
most reliable methods when it is executed safely. It is often
combined with drainage works, vegetation works, or slope
protection works based on structures. It is often impossible
to execute it completely, when homes are constructed close
to the top or bottom of the slope or when the volume of cut
soil would be huge, so it is often combined with another
method (retaining wall, etc.).
One of the most basic countermeasures, it is also one of the
most reliable if it is thoroughly implemented. It is often used
along with drainage works, vegetation works, and structural
protection works using structures.
Because of their durability and environmental properties,
these are not appropriate for steep slope failure
countermeasures, and are rarely used for these purposes.
But they are used for temporary works or partial use.
Application Range and Special Features
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Preventing falling rocks
Principal Goal
Temporary
protective work
Protective work used during
execution of prevention work
7-26
Passive work
Gabion work
Fence work
Temporary
protective fence
work
Passive concrete
retaining wall work
Gabion work
Protecting lives and properties from collapsed
soil and falling rocks during the construction of
failure prevention work.
In cases where it would be difficult to directly
prevent failure of a slope, a gravity retaining wall
is constructed at a distance from the bottom
(toe) of the slope to halt the soil produced by a
failure.
Preventing slope erosion and acting as
counterweight fill work.
Used as supplement to vegetation work in order
to prevent erosion of the surface soil of the slope
by rain and surface water.
Wicker work
Usually provided as a
supplementary measure
with failure prevention work.
The installation of temporary protective fence work is
required when executing steep slope failure prevention
work.
It should be used along with methods executed to improve
slope conditions as much as possible. It is often used on
large slopes. It is effective when it is necessary to preserve
the existing vegetation as much as possible.
As a steep slope failure prevention work method, it should
not be used to completely cover the slope. There are cases
where it is used as a provisional method in a transitional
area with adjoining natural ground.
It is used along with vegetation work and slope grating crib
work on relatively gentle slopes where cutting work has
been done.
It is appropriate for relatively large slopes. It can be
executed while preserving existing vegetation on the slope.
Its foundation is often made
by combining it with
retaining wall work.
Method of protecting dwellings etc. from falling
rocks. It includes preventive network, preventive
fence work and preventive retaining wall work.
Used to prevent failure in cases of relatively
gentle slope with a thin surface soil layer and
prevent such failure from spreading.
Cutting work, drainage work
grating crib work, spraying
work and pitching work are
also used to prevent rocks
from falling.
It is rarely executed by itself, because there is little room for
execution on a steep slope. It is executed along with a
gravity retaining wall.
It is used in special cases of steep slope failure prevention
work. It is used to prevent failure of slopes where landslide
type failure is predicted and of bedrock slopes that act as
dip slopes.
Application Range and Special Features
Work intended to prevent rocks from falling. It
includes rock removal and foot protection works.
Forming an embankment at the bottom of a
place where failure is predicted to stabilize it by
resisting sliding force.
Installing piles in a slope so that the bending
moment and shear resistance of the piles resist
sliding force to improve the stability of the slope.
Purpose or Details of the Work
Earth retaining
fence work
Rock fall
protection work
Counterweight fill
work
Counterweight fill
work
Rock fall
countermeasure
work
Pile work
Work Subcategory
Pile works
Work Category
Work methods that prevent damage
when failure occurs
Methods that combine the functions
of control work and prevention work
Others
Classification
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Figure 7-16
Selection of Natural Slope Failure Countermeasures
Slopes that need Failure
Countermeasures
A. Catch wall
Is it possible to
adopt the measure
in the slope area?
NO
YES
Investigate the Geological Features
Soil
Soft Rock/Rippable Rock
Will it easily collapse?
Will it easily
weather?
NO
YES
YES
Is there spring
water?
NO
NO
YES
E. Drainage
F. Cutting + Drainage +
Retaining wall/Masonry
works/Concrete spraying
+ Vegetation Works
D. Cutting + Drainage +
Retaining wall/Masonry
works/Cast-in place slope
crib works
B. Cutting + Retaining
wall/Masonry works +
Drainage +
Vegetation works
Are there many
cracks?
NO
Is there spring water?
NO
YES
YES
G. Cutting + Drainage +
Retaining wall/Masonry
works/ Cast-in place slope
crib works
C. Cutting + Gabion/
Retaining wall/Masonry
works + Drainage +
Vegetation works
Hard Rock
Are there many cracks?
Is there presence
of rock falls?
NO
NO
H. Drainage
YES
YES
Will it easily
weather?
NO
Is the toe of the slope
at least 20 meters from
the edge of
the road?
YES
K. Catch wall
YES
Is there spring
water?
NO
I. Cutting + Drainage + Retaining wall/
Masonry works/Concrete spraying
YES
NO
L. Rock Fall
Protection System +
Drainage
J. Cutting + Drainage +
Retaining wall/Masonry
works/Concrete pitching
7-27
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Table 7-3
Typical Types of Slope Protection by Vegetation
Kind of Work
Purpose and Feature
Seed Spraying
Seed-mud spraying
Sodding mats
Sodding
Vegetation for the whole surfaces to prevent erosion
due to rain water.
Simple seed matting works
Simple sodding works
For preventing erosion of fill and for partial vegetation
Seed board works
Seed packet works
Coconut mat soil protection
For preventing erosion of slope made of poor soil or
hard soils
The following DPWH Department Orders relate to the use of coconut bioengineering technology solutions:

No. 41 dated August 27, 2010 on the subject of ‘Prescribing the Use of
Coconut Bio-Engineering Technology Solutions in DPWH Projects’.

No. 68 dated September 20, 2012 on the subject of “Prescribing Guidelines on
the Design of Slope Protection Works’.

No. 23 dated February 22, 2013 on the subject of ‘Clarifications on the
Specification of Coconut Coir Fiber Materials’.
Structural Method
Slope protection works with structures are used for slopes not suited to
vegetation, slopes whose stability cannot be assured with vegetation alone for a
long time, or slopes requiring protection against failures and rockfalls.
Retaining walls, pile works, and slope anchor works are mainly used for covering
slopes required to resist earth pressure. Other types of slope protection by
structures that are not required to resist earth pressure, or have limited capacity
as shown in Table 7-4.
Table 7- 4
Types of Structural Protection
Kind of Work
Purpose and Feature
Mortar spraying
Concrete spraying
Stone pitching
Block pitching
Concrete block crib-works
For preventing weathering and erosion
Concrete pitching
Cast-in-place concrete crib-works
Slope anchor works
For preventing collapse of surface layer of slope,
preventing separation of bedrock, and retaining earth
where there is a light earth pressure
Wicker works
Slope gabion works
For controlling erosion of surface layer of slope and
outflow of surface layer due to spring water.
Active Rock fall Protection System
Passive/Rock fall barrier
Rock fall shed
For preventing rock falls
The following DPWH Department Orders relate to the use of shotcrete and
rockfall protection systems:

7-28
No. 26 dated 15 May 2007 on the subject of ‘DPWH Standard Specification for
Shotcrete (Concrete Spray), Item 514’.
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design

No. 33 dated 19 March 2013 on the subject of ‘DPWH Generic Specification
for Rockfall Protection Systems, Item 522’.

No. 63 dated 6 June 2014 on the subject of ‘DPWH Standard Specification for
Item 522A – Protection Systems for Unstable Slopes.
Combined Vegetation and Structure Method
A hybrid solution may sometimes be employed for slope protection works which
may be applicable for high slopes to provide a more cost-efficient design. It is
however suggested to consult a Geotechnical Engineer for such cases.
7.7.2
Precautions for Applying Protection Works According to Soils and Geology
Colluvial Deposit, Strongly Weathered Zone and Clayey Soil
Slopes made of colluvial deposits, strongly weathered materials, volcanic
mudflow, loam, or other cohesive soils have low degrees of solidification with
high water contents and, thus, are frequently collapsed. Slopes made of these
kinds of soils generally have good conditions for vegetation.
Proper protection works in this case other than vegetation are described below.
Where There Is Much Spring Water. If the slope ratio is steeper than 1:1, mat
gabions or crib retaining walls capable of slightly resisting the earth pressure are
suitable. Ditches must be provided in each berm. If the slope ratio is gentler than
1:1, the gabions or crib-works filled up with gravel are suited since they are able
to prevent the surface sediment from being run off by spring water. In addition to
the above, water drainage on berms or groundwater drainage by means of
borings made in a horizontal direction are also performed to remove
groundwater.
Where There Is Some Spring Water. If the slope ratio is steeper than 1:1, stone
masonry or block masonry, which is able to resist to a certain degree against
earth pressure, is frequently used.
If the slope ratio is gentler than 1:1, vegetation alone is generally considered to
be sufficient but other works such as crib-works filled up with gravel or sediment
and block pitching may be used if necessary to prevent the sediment from
running due to surface water.
Sandy Soils (easily erodible sediment)
Slopes made of weathered granite, lahar, and sandstone with a low degree of
solidification, or sand of diluvialepockare easily eroded by spring water or
surface water, or surface layers of these slopes are often run off by seepage
water.
Considerable damage will result if heavy rain occurs during the execution of road
construction work. To prevent this, it is necessary to install temporary drainage
facilities using a soil cement mixture, or vinyl sheets on top of slopes or berms.
Works described below are suited to the slope protection work.
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Cut Slope. Where considerable spring water is present, gabions, crib-works filled
up with cobblestones, and wicker works are chosen according to the degree of
spring water. However, these methods are permeable to surface water at the
same time and thus the rear portions are sometimes scoured. In this case,
groundwater drainage facilities may be installed in the form of tree branches and
then protected by concrete blocks.
Vegetation works are generally employed where little spring water is present, or
crib-works filled up with sediment and wicker works are suited, both of which
are used together with vegetation as auxiliary methods.
Where vegetation alone is used, sodding works and sodding mats which are able
to cover the whole slope from the beginning are suitable but, in the case of seedmud spraying, it is required to protect the slopes with emulsion, nets or straws
until the grass can grow thickly.
Regardless of the amount of spring water, it is desirable to provide drainage
facilities on the top of slope and berms.
Fill Slope. It is desirable to protect the slope with blanket soil (soil suited to
vegetation) of 30 to 50 cm thick where sandy soils are used as filling materials.
It is required to apply sodding mats or sodding works which are able to cover the
whole slope, or to protect the slope with emulsion, nets or straw in the case of
seed-mud spraying where blanket soil is not used.
In the case of a high-fill slope, the portion near the toe of the slope may
sometimes scour and collapse in the form of a mudflow due to seepage water. If
this is the case, it is desirable to cope with the problem by means of not only
sodding but also filters or groundwater drainage works, or by means of wicker
work or concrete block cribwork used in conjunction with sodding.
Hard Soil
It is necessary to perform grooving and soil dressing or digging and soil dressing
at some portions of a slope so as to allow roots to grow, and sodding should then
be performed on the dressed soil of hard slopes made of dense sandy soils, hard
clayey soils (both exceeding the soil hardness of 27 mm), soft rocks or hard clays
(exceeding the soil hardness of 23 mm).
Sediment Creating Chemical Problems
Measurement of Chemical Components of Soils. A weak acidity of pH 5 to 7 is
generally the best suited to the growth of plants for slope. Sodding can be
performed without special treatment for acidity of this degree. However, if an old
stratum formed by the upheaval of mud at the bottom of a lake is suddenly
exposed to air after excavation, the excavated earth is sometimes turned into the
soil with an extremely strong acidity in a short time. If this is the case, or pH is
lower than 4 from the beginning, the soil must be neutralized with lime.
Plants often die because of salt when the soil is near the seashore. Chemical
components of soil can be simply checked by a soil analyzer. According to this
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tester, a reagent is added to a sample solution for coloring and then the color is
compared to the color sample in order to find the approximate value.
Countermeasures. When performing vegetation work on excavated earth with a
strong acidity, or earth containing a large amount of salt, the work should be
performed after a dressing of good quality soil to a thickness greater than 20 cm.
Where filling materials contain sulfur, salt, or other components harmful to
plants, or the acidity is very strong (i.e. less than pH 4), the soil must be improved
with lime or dressed soil suited to vegetation must be used. The same treatment
is necessary where the soil has a strong basicity (greater than pH 8.5).
7.7.3
Vegetation
Features of Vegetation
The purpose of vegetation is to prevent the erosion of a slope immediately after
the completion of work, and one of the features of sodding works is that the face
of the slope can be restored naturally, unlike other slope protection works.
Survey Required for the Execution of Vegetation
Success or failure of vegetation is governed by the growth of plants, and so the
weather and soil on the site should be surveyed, the species suited to the weather
and soil should be selected, and the conditions capable of assuring the complete
growth of the selected species should be provided. Thus, prior to the execution of
vegetation, the items for survey are:
Area, Gradient, Height of Slope. Execution of work by machinery is suitable
when the area of slope is large and places of work are located closely to each
other, but handwork is sometimes more economical if the area is small and the
places of work are scattered. Also, the gradient of slope compared to the standard
gradient, and the maximum height of slope, should be considered when selecting
the type of work and determining the degree of difficulty in executing the work.
Condition of Adjacent Land. Spray materials may sometimes scatter and pollute
crops, houses or structures, and therefore allowances for scattering and pollution
should be made in advance.
Soil Conditions. Physical and chemical composition, water content and hardness
of soil, unevenness, presence of spring water:
Checks should be carried out to see whether the soil is easily eroded as sandy
soil; whether the rooting of plants is difficult as in the case of solidified soil, clay
or mudstone; whether the soil is very dry because several days have passed after
the formation of the slope; whether the growth of plants is difficult because of
strong acidic soil or other harmful components are involved; and whether there
is much spring water present. In addition, the finishing requirements for the
slopes will vary depending upon the kind of work. For example, a certain
irregularity is desirable for seed spraying while a smooth surface is needed for
sodding mats; and thus, the degree of finish of slope surface should be
determined before the execution of work.
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Weather Conditions (air temperature, rainfall, and slope direction). Yearly
mean air temperature should be found to determine what types of plants will
grow, and daily mean air temperature to determine what season is suited to the
execution of work. The slope direction and degree of light should be found since
they are helpful for selecting shade-tolerance grasses. Also, the weather during
scheduled terms of work and the possible occurrence of heavy rain during work
should be examined, and then the work and curing methods should be planned.
Other Considerations. The degree of difficulty in securing local materials (such
as earth and water), their qualities, and conditions of the access road for bringing
in machinery and materials should also be surveyed.
Sodding Materials
Materials used for sodding are seed, fertilizer, and curing materials.
Seed
Kinds and characterizations of seed, amount of seed, and optimum season for
growth should be well understood. It is fundamental to select species suited to
the weather and to determine the proper amount of seed and the seeding season
using upon the results of a germination test.
Fertilizer
A good fertilizer should contain the three elements of nitrogen, phosphorus, and
potassium almost equally mixed with a ratio of 1:1:1. There are many kinds of
soils which fix and make phosphorus inactive, and it is desirable to use a fertilizer
containing a large amount of phosphorus in order to grow grasses with strong
stems and leaves.
A large amount of fertilizer is desirable as an original fertilizer during execution
of work but the amount of fertilizer should then be restricted since an excessive
amount of fertilizer may check the germination. Damage is caused when the
nitrogen content exceeds 10 g/m2 and thus the amount of fertilizer not exceeding
this limit should be determined.
Curing Materials
The functions of curing materials are to protect seed from being washed away by
rain water until the germination is completed, and also to prevent the erosion of
the slope until the vegetation will cover the whole area of slope and the effects of
erosion prevention will be realized.
Chemical Curing Agents. There are many kinds of chemical curing agents used
in seed spraying. Some of them form films over the surface layer of a slope, while
others permeate into the layer to a certain depth and harden this layer. The most
common agent of the film type is asphalt emulsion (cationic), which also has the
advantage of erosion prevention.
Also, polyvinyl acetate is frequently used, and many synthetic resins as permeate
and hardening types are available.
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Fibers. More recently, fibers are being frequently used for seed spraying.
Ligneous fibers have been developed in the USA, and as well bark fibers and slag
fibers are also available. In order to fully realize the effects of erosion prevention
by fibers, the amount of fiber should be greater than 150 g/m2. If the amount of
spraying is less than this, no effects of seed protection and erosion resistance can
be obtained.
Covering Materials. Covering materials available for slopes are synthetic fiber
nets, straw products, fiber mats and paper products. Mats and nets made of straw
or fiber products have high erosion resistance. However, in order to realize the
full effects, a smooth face of slope and careful work are necessary, so that work
by machinery is not appropriate, thereby decreasing the efficiency in some cases.
Some covering materials are pre-mixed with seed and fertilizer.
Individual Sodding Works
Seed Spraying
In seed spraying, the seed, fertilizer and fibers are scattered in water and sprayed
through a pump to the face of either cut or fill slopes. Seed spraying is suited to
relatively low land or to slopes with gentle gradient. A tank with an agitator is
used for the execution of seed spraying, in which (1) water, (2) fibers, (3)
cohesive agent, (4) fertilizer, and (5) seed are placed in tank in the order listed
and fully agitated in order to obtain uniform slurry. Green-colored ligneous fibers
are often used. Where the covering and curing are required after the execution of
work in the typhoon season or heavy rain season, the curing should be
performed by using asphalt emulsion or the like.
Seed-Mud Spraying
Seed, soil, fertilizer and water are mixed together to form a mud-like mixture and
then sprayed to the face of the slope in seed-mud spraying. This spraying is
suited to cut slopes and can be applied to places where the gradients of slopes
are steep, as well as high places. A spray gun is employed in this method in
combination with an air compressor, in which seed, dressing soil and others are
sprayed by means of compressed air, and then asphalt emulsion is sprayed to
perform the film curing. Wet type guns for mortar spraying are used.
The thickness of soil to be covered over the seed should be 2 to 3 times the size of
seed to assure good germination; an normal amount of soil to be used is 0.01
m3/m2. Also, if the amount of water is maintained to about 30 to 40% of the
amount of soil, the soil to be sprayed will have a relatively stiff consistency,
thereby preventing the sprayed soil from flowing down. When mixing materials
(1) soil, (2) water, (3) fertilizer, (4) seed, and (5) others should be placed in the
chamber in the order listed and well mixed.
Different from cement mortar spraying, seed spraying should be performed
without holding the tip of spray normal to the face of slope, and instead the
spraying distance and angle of nozzle should be adjusted in response to the
hardness of the ground so as not to roughen the face of the slope. The thickness
of the sprayed mixture should be as uniform as possible.
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Curing by asphalt film provides a high erosion resistance, and this film is able to
withstand heavy rains during typhoon or rainy seasons. A doubled solution of a
cationic type is normally used at a rate of 1 l/m2 as asphalt emulsion.
Sodding Mats
For sodding mats, the face of the slope is covered with mats containing seed and
fertilizer. This method is also called ‘artificial sodding’. This method offers the
inherent protection of the mats until the completion of vegetation and, thus, can
be executed in any season.
Mat materials used for artificial sodding are non-woven cloth, rough cloth, paper,
straw blind, straw mats and cut-straw felt. Some mats are also reinforced with
nets.
The slope should be smoothly finished without irregularity, and the mats should
be firmly fixed with pegs or rope to the ground so as to fit the mats with the
ground without floating, and so as to prevent the mats flying away with the wind.
Mats should be extended at least 20 cm from the edge of the top for coverage, and
the edge of mats should be embedded in the ground so as to prevent water from
entering underneath the mats from the top of the slope. If the mats float
germination may be delayed and water may flow underneath the mats resulting
in the occurrence of scour.
If long mats are to be used, they should be laid longitudinally where the face of
the slope has been finished, and be laid transversely where they are to be
installed while tamping the slope. Mats should be overlapped by approximately
5cm at each joint in all cases.
Sodding
This is a conventional method in which sods are directly laid on the face of the
slope, and is suited to easily erodible soils since the protection effects can be
realized immediately after the placement of sods.
Normally, wild sods are used for sodding. The standard size of a field sod is 36 x
28 cm, and each bundle contains ten units of sods which are good for 1 square
meter.
When laying sods, they are laid flat directly over the face of the slope with the
long side of each sod directed in the horizontal direction without providing
longitudinal joints. If joints are provided, scouring may start from the joints. Each
sod should tightly contact the ground by hitting it with a tamping board. More
than two pegs should be used per sod unit. For sods laid on a fill slope, drying of
sods can be effectively prevented by thinly sprinkling good quality soil on the
sods. But this is usually not so effective for a cut slope since the gradient is steep
and covering soil may easily flow down.
Simple Seed Matting Works
This method is also called ‘artificial simple sodding works’. Strip-shaped cloth or
paper containing seed and fertilizer is horizontally inserted into the fill slope at
the time of tamping.
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Artificial sods are basic strip of cloth, paper, cut-straw or synthetic net, with seed
and fertilizer attached to the strip.
The artificial sods are inserted horizontally into the slope at 30cm on centers
along the face of slope while tamping the slope in the same way as simple
sodding work.
Simple Sodding Works
This is a conventional method in which sods are inserted horizontally in the form
of streaks into the slope when tamping the face of the slope. Since the field sod
grows slowly, many years are required until the whole surface is covered and
considerable scouring may occur in the case of sandy soil during this time.
Therefore, the growth of grasses should be accelerated by fertilization during the
work and soil should be fully compacted. When using a metric supply of sods (36
x 28 cm), each sod should be cut in half so as to have a width of 14 cm, and then
sods should be horizontally laid in such a manner that the long side of each sod is
flush with the face of the slope. The spacing between the streaks should be
normally 30 cm along the face of slope.
A row of edge sods should be placed at the edge of the top of the slope to prevent
edge collapse.
Seed Board Works
Earth mixed with seed and fertilizer is molded in the form of boards, and the
molded boards are laid in the form of strips in horizontal grooves made in the
face of the slope according to seed board work. These boards have a soil dressing
effect because of the thickness of the board. Seed boards may be molded either
on the site or in a factory. The spacing of the grooves in which the boards are to
be laid is normally 50 cm.
Seed Packet Works
For seed packet works, seed and fertilized earth are filled in net packets and laid
in the form of strips in horizontal grooves made in the face of a slope. Seed and
fertilized earth do not run off since they are enclosed with the net packet. The
packet is flexible and can thus be firmly bonded to the ground. Seed packets can
be filled up with seed and fertilizer earth either on the site or in a factory.
Synthetic resin net is used for the packet net, and the depth of the groove should
be determined in such a manner that the top of the packed will become flush with
the face of the slope, or be slightly projected out from the face of slope. The
normal spacing of grooves is 50 cm.
Precautions for Design and Execution of Sodding Works
Hardness of soil: If soil is too hard, the roots are unable to enter the soil even
though germination has begun. It is necessary to check whether the roots are
able to enter into the ground or not by measuring the hardness of the soil. A soil
improvement method must be selected if the soil is considered to be too hard.
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Soil improvement: Soil improvement becomes necessary when the soil is
physically or chemically not suited to the plants.
One of the physically obstructive factors can be the hardness of the soil. For soils
whose hardness exceeds the limit, it is required to select a method which
includes partial cultivation or soil dressing.
One of the chemically obstructive factors can be strong acidity of soil.
Neutralization by lime is desirable for improving acidic soil, but soil
improvement by lime is limited for cut slopes and thus a method consisting of
both slope cribwork and soil dressing is recommended in this case.
Sodding Works on Cut Slopes: Roots of lawn grasses are able to penetrate
sandy soils, clayey soils, and clay (of hardness lower than 23 mm) so that sodding
can be performed directly in these soils. Seed spraying should be applied if
sodding works can be performed in a suitable season on slopes of these soils.
Sodding or sodding mats are suitable if the area is small. If the soil hardness is
greater than 23 mm but less than 27 mm, seed-mud spraying should be used. If
the soil hardness of a slope exceeds 27 mm, grooving and soil dressing or digging
and soil dressing should be performed to certain portions of the slope to allow
the penetration of roots, and then sodding work should be carried out.
Sodding Works on Fill Slopes: Seed spraying is normally used on fill slopes.
However, if the area is small, simple seed matting works or simple sodding works
should be used. If it is required to complete the vegetation cover as quickly as
possible and if the easily scoured borrowed sandy soil is to be tamped and used
as soil for vegetation, sodding mats or sodding should be performed. As long as
these works are executed in stages in response to the progress of slope
construction, the faces of the slope will remain exposed for only a short time.
Coconut Mat Soil Protection
Coir mats are best for controlling soil erosion and conditioning soil. Made from
coir fiber, they are naturally resistant to rot and they hold soil in place and
prevent erosion, dissipating the force of heavy rains and run-off water. It
provides good soil support for years, allowing natural vegetation to become
established. Coir mats promote the growth of new vegetation by absorbing water
and preventing the topsoil from drying out. As it has strength and durability, it
protects slopes and helps natural vegetation to take root.
7.8
Retaining Walls
7.8.1
Definition and Applications of Retaining Walls
Definition: Retaining walls are structures to support earth for preventing
sediment failures and are constructed for filling or cutting earthworks in places
where stability cannot be maintained by earth slope alone because of site and
topographic conditions. They are also built for protection and consolidation of
slope foundations when roads are to be constructed along rivers or lakes.
Depending upon their shape and mechanical characteristics, retaining walls can
be classified into: stone masonry or block masonry type, gravity type, semi-
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gravity type, supported type, cantilever beam type, counterfort type and the
buttressed type. There are also other special types of retaining walls.
Applications: Requirements described herein should be applied to the design
and construction of standard retaining walls in conjunction with the road works.
Design concepts for earthquake are important. However, design calculations
against earthquakes are normally not required for the ordinary retaining walls
considered in this guide since the load increase by a seismic force can
alternatively be compensated by a slightly increased factor of safety for the
normal design calculations, and by resisting forces which are not taken into
account in the calculations.
However, it is required to design taking into account the effects of earthquake for
retaining walls higher than 8m, or retaining walls which may cause serious
damage and which cannot be easily restored after failure.
Also, where deep slips or consolidation settlement below the bottom of a
foundation are expected, where scouring of the foundation due to running water
or waves along rivers or coasts is expected, or where some problems are
expected because of a retaining wall curved outwardly, rational engineering
judgment must be made during design based upon experience.
For structures along or near the coastline, which is a highly corrosive
environment, the use of corrosion-resistant materials is evidently warranted.
This may involve the utilization of geosynthetic facing or zinc-coated wires for
gabions.
Structures along or near coastlines are subjected to highly corrosion
environment. The use of corrosion-resistant materials is evidently warranted.
This may involve the utilization of geosynthetic facing or zinc-coated wires for
gablons.
Conventionally, the earth pressure method proposed by Coulomb, Rankine, or
Terzaghi-Peck may be used. In these design guidelines, Terzaghi-Peck’s earth
pressure diagram and table has been adopted for standard retaining walls.
7.8.2
Classifications of Retaining Walls
The various types of retaining wall and their features are described below.
Gravity Type Retaining Walls
These walls support the earth pressure by means of dead-weight, and can be
built more easily than other types of concrete retaining walls. The gravity type is
often utilized when the height is relatively low (less than about 4 m) and the
foundation ground has good bearing stratum. Refer Figure 7-17.
Stone, masonry, brick or plain concrete retaining walls are frequently used at the
tail portions of cut slopes or fill slopes. Their advantages are that the gradient,
length, and horizontal alignment of slope can be freely changed, so that they have
been the most widely utilized at the joining portions with other structures.
Though stone masonry has conventionally been the most often utilized, concrete
block masonry is now widely utilized because of the shortage of stone materials
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and the time-consuming work of stone masonry. The height of these retaining
walls is normally less than 7 m.
Figure 7-17
Gravity Walls of Brick, Stone Masonry or Plain Concrete
Semi-Gravity Type Retaining Walls
These are made of concrete but contain steel reinforcing bars for resisting tensile
force, so that the amount of concrete is less than in a gravity type wall. Refer
Figure 7-18.
Figure 7-18
Semi-Gravity Retaining Wall
Crib Type Retaining Walls
These are used to stabilize cut slopes, but are not able to stand by themselves.
Calculations are very difficult to perform for this type of wall, although they are
often used as countermeasures for slopes in mountainous areas. Refer Figure 719.
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Figure 7-19
Crib Type Retaining Wall
Cantilever Beam Type Retaining Walls
This type consists of a vertical wall and bottom slab, with the stability of the
vertical wall maintained by utilizing the weight of earth placed over the bottom
slab. The amount of concrete of this wall can be smaller than that of a gravity
type or semi-gravity type retaining wall. Inverted T-types, L-types and inverted
L-types may beused, depending upon the position of the vertical wall relative to
the bottom slab. These types are selected depending upon the terrain, and the
height of this type of retaining wall is normally about 3 to 8 m. Refer Figure 7-20.
Figure 7-20
Cantilevered Retaining Wall
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Counterfort or Buttressed Type Retaining Walls
The rigidity of both the vertical wall and bottom slab is maintained by the
counterforts which are placed either earth-pressure-acting side or earthpressure-receiving side. The latter case is normally called the buttressed type.
The amount of concrete in this wall is less than those of other types of concrete
retaining walls, and their height is normally larger than 6 m. However,
construction work is more difficult to perform compared to other types of walls
because it needs the placement of reinforcing bars, forms, and other operations.
A counterfort retaining wall is shown in Figure 7-21.
Figure 7-21
Counterfort Retaining Wall
MSE Wall or Reinforced Soil System
Department Order no. 50, dated 04 September 2007 on Mechanically-Stabilized
Earth (MSE) retaining walls was already published by the DPWH. The concept of
reinforced soil systems essentially focuses on the increase of tensile resistance of
the soil system due to the reinforcements. It is known that soil, like concrete, is
very poor in tension. Placing reinforcements on the same direction as the
principal strain direction will increase the soil system’s tensile resistance.
Reinforced soil systems are basically composed of three components; backfill,
reinforcements, and facing.
Backfill. Backfill Reinforced soil systems mostly depend on the friction between
the soil and reinforcements. As such, good soil-reinforcement interaction should
be achieved. Well graded granular materials are preferred as backfill material for
RSS because these materials have relatively high angle of friction hence resulting
in better interaction. Moreover, granular backfill has better drainage
characteristics.
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Backfill Grading Requirements should comply with DPWH Item 515 (DPWH
Standard Specifications for Highways, Bridges and Airports 2013 Edition) .
Reinforcements. Can either be inextensible (steel) or extensible (geosynthetics).
Thses are usually fastened to the facing components and extend on the backfill at
a certain distance, depending on the designed length.
Steel strips and steel grids are among the first reinforcements used for RSS which
are composed of galvanized or epoxy coated steel. Geosynthetic reinforcements
are now widely used for RSS. There are three general types of geosynthetics
depending on the material; high density polyenthylene (HDPE) geogrid, polvester
(PET) geogrid and geotextiles which are made of polyester and polypropylene.
Facing. Facing units or systems are used not just for aesthetics purposes but
more importantly to avoid shallow erosion on gap between reinforcements.
Certain facing units provide drainage paths which are critical for earth retaining
structures. Types of facing units are concrete panels, modular blocks, metallic
facings, welded wire grids, gabion facing and geosynthetics facing.
Some types of facings such as gabions and geosynthetic bags can blend with
green areas where certain vegetation may grow on it. Geotextile filters are also
used and usually laid between the interfaces of the facing or gravel drains and
soil mass to prevent migration of fines from the backfill.
In rural areas, the selection of materials used for facing units will depend on
availability and labor constraints of the project at the particular site.
A MSE wall is shown in Figure 7-22.
Figure 7-22
7.8.3
Mechanically Stabilized Earth Retaining Wall
Design of Retaining Walls
Refer to the structural design guidelines contained in Volume 5.
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7.8.4
Precautions for the Design of Retaining Walls
Precautions to be taken for the design of retaining walls are described
hereinafter for each type of wall.
Gravity Type Retaining Walls
Height of wall and gradient of slope should be determined by referring to the
limits in Table 7-5.
Table 7-5
Height and Gradient Limits for Stone and Concrete Block Masonry Walls
Height (m)
Gradient
of slope
H:V
0 to 1.5
1.5 to 3.0
3.0 to 5.0
5.0 to 7.0
Fill
0.3:1
0.4:1
0.5:1
0.6:1
Cut
0.3:1
0.3:1
0.4:1
0.5:1
Semi-Gravity Type Retaining Walls
This type of wall will support earth pressure by means of a dead-weight. The wall
should be designed in such a manner that the resultant of earth pressure and
deadweight will not create tensile stresses in the horizontal section of the body of
the wall.
In determining dimensions of the wall, it is desirable for the width of the bottom
slab to be about 0.5 to 0.7 times the height of the retaining wall, and that the
thickness of the member at the top will become greater than 35 cm by taking
account of the workability and the installation of a protection fence at the top.
Semi-gravity type retaining walls are reinforced with steel bars in order to resist
the tensile force created in the horizontal section of the wall due to the resultant
of the earth pressure and dead-weight.
Crib Type Retaining Walls
This type of retaining wall is frequently used for mountain roads for the purpose
of widening the existing road, and is able to support the earth pressure by its
own deadweight while being supported by the earth at the rear or by the backfill.
Generally, the thickness of wall at the top is greater than 40 cm, the slope ratio of
the front wall is 1:0.3 to 1:0.6, and the height of the wall is about 5 to 15 m.
Cantilever Beam Type Retaining Walls
This wall is comprised of a vertical wall and bottom slab, and each member
resists the external force as a cantilever beam.
Each member of the inverted T-type is so designed as to resist the earth pressure
acting on the wall body by means of the weight of the body and the weight of the
backfilled material over the heel portion of the bottom slab. In this case, the
width B of the bottom slab is often 0.5 to 0.8B, and the thickness of end member
is greater than 30cm.
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Counterfort or Buttressed Type Retaining Walls
It is desirable to design the vertical wall or bottom slab as a slab supported at
three sides. Alternatively, the wall or slab may be designed as a continuous slab
supported by counterforts or buttresses without considering the influence of
fixing. The width of the bottom slab is usually 0.5 to 0.7 the height, the thickness
of the wall at the top is greater than 30 cm, and the interval of the counterforts is
about one third to two thirds the wall height.
Retaining Walls in Landslide Areas
Flexible structures should be provided as a rule for retaining walls in a landslide
area, by taking account of the displacement of ground and spring water. Another
landslide may be triggered if the amount of cutting or the excavation of the
foundation is large, so that safety must be fully taken into account when
designing the retaining wall.
A retaining wall can be used if the toe of a slope at the tail portion of a landslide
area is likely to collapse, thereby invoking subsequent failures up to the highest
portion. Thus, retaining walls alone are not so effective for preventing a largescale landslide. Generally, retaining walls are used for sheathing in combination
with counterweights fills.
The ground generally deforms considerably and the amount of spring water is
large in a landslide area, and thus crib retaining walls which are flexible and very
permeable are frequently used.
Also, small retaining walls are sometimes built on piles or shafts as a foundation.
Typical types of retaining walls for this situation are:
7.8.5

Reinforced concrete crib retaining wall

Mat gabion retaining wall

Steel crib retaining wall

Large-type concrete block retaining wall

Concrete retaining wall.
Execution of Retaining Wall Works
Foundation Works
If the bearing ground is bedrock, the bearing ground should be cut to a depth
required for placing the footing, the new surface of bedrock should be cleaned,
then the spread footing should be placed. Refer Figure 7-23.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Figure 7-23
Retaining Wall on Bedrock
If the bearing ground is earth or gravel, rubble-stones should be laid over the
excavated surface and rolled fully and uniformly, leveling concrete should be
poured over the rubble-stones, and then the spread foundation should be placed
over it. Refer Figure 7-24.
Figure 7-24
Retaining Wall on Earth Stratum
If the bearing ground surface is slanted, the portion at the valley side should be
cut in the form of steps and the rock should be replaced with concrete to the
bedrock line to form a horizontal, uniform foundation. The body of the retaining
wall should then be directly built over the foundation. Refer Figure 7-25.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Figure 7-25
Partially Replaced Stratum
A pile foundation is normally used when building a retaining wall above poor
ground. If the poor stratum is thin or if replacement material is easily available,
the poor stratum should be replaced with good quality gravelly soil or the like to
provide a uniform bearing ability so that the retaining wall may be built directly
over the replaced material. Refer Figure 7-26.
Figure 7-26
Replaced Foundation in Poor Ground
7-45
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Main Body of Retaining Wall
It is desirable to pour concrete monolithically for both the footing and wall
portions. However, if this is not possible, it becomes necessary to provide groove,
tenon or half-lap construction joints or to insert steel dowels at the joints.
Expansion joints should be provided at 10m intervals for gravity type retaining
walls, and every 15 to 20 m for the cantilever beam and counterfort type
retaining walls.
Good quality material should be used as backfill material for counterfort type
retaining walls since compaction of fill at the rear is not easy. Also a small
spreading depth should be used and the backfill material should be fully
compacted with rammers.
7.9
Erosion Control and Landscaping
Erosion is the removal of solids (sediment, soil, rock, and other particles) in the
natural environment. It usually occurs due to transport by wind or water; by
down-slope creep of soil and other material under the force of gravity. Refer
Figure 7-27.
Figure 7-27
Erosion Control: (left) the problem; and (right) the solution
Two general reasons for erosion control are:
1. During the erosion process of soil particles are transported, resulting in the
gradual deformation and destruction of slopes, banks, shores, river bottoms.
2. Elsewhere, erosion causes undesirable sedimentation on agricultural land
and roads, in rivers and drainage systems.
Several ways to prevent erosion control for slopes are:
7-46

Prevent removal of the vegetation

In case of new slopes, protect the fertile surface layer by means of:
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
-
Coarse, less erodible surface
-
Flatter slopes/terraces
-
Surface cover with concrete, stones, plastic sheets
-
Temporary plastic sheets over the surface in rainy season
-
Geosynthetic mats
-
Biodegradable mats
DPWH Department Orders issued relevant to erosion control are:

No. 06 dated 17 February 2012 on the subject ‘DPWH Standard Generic
Specification for Coconut Bio-Engineering Solutions, Item 622’ provides an
updated specification for use of coconut coir fiber materials, superseding
earlier DPWH Order No.28, s.2008; plus DPWH Order No.29, s.2008.

No. 68, dated 20 September 2012 on the subject ‘Prescribing Guidelines on
the Design of Slope Protection Works’ proposes the use of:

-
Flexible slope protection for flood flow velocities of not more than 3
m/sec that can adapt to foundation movement and allow growth of
vegetation.
-
Modular concrete materials in rivers and streams with more than 3
m/sec flow velocities.
-
Riprap, concrete revetments, mattresses and geosynthetic materials, and
other types of flexible slope protection subject to evaluation by the
Bureau of design and approval by the Secretary.
No. 23 dated 22 February 2013 on the subject ‘Clarifications on the
Specification of Coconut Coir Fiber Materials’ amends DPWH Order No. 68,
s.2012 to include DPWH Order No. 06 dated 17 February 2012.
Landscaping should be provided for urban collector roads in keeping with the
character of the street and its environment. The landscape design should permit
a sufficiently wide, clear, and safe pedestrian walkway that allows for the needs
of individuals with disabilities and bicyclists as well as pedestrians.
However care should be taken to provide sight distances, lateral offsets, and clear
zones.
Landscaping should also consider maintenance operations and costs, plus future
road improvements.
DPWH Orders relevant to landscaping are No. 15 dated 24 January 2000 and No.
38 dated 10 August 2006 on the subject of ‘Tree Planting Along National Roads’,
which directs the planting of trees:

In a neat row at 10-20 m spacing.

Within the right-of-way (ROW) and as close as possible to the ROW limit
without encroaching on adjoining property or obstructing overhead utility
lines.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
7.10

For roads without curbs, a minimum clear distance of 0.60 m beyond the edge
of shoulder, not less than 3.0 m from the edge of pavement, and with a
minimum clear distance of 1.0 m from the ROW limit.

For roads with curbs, a minimum setback of 1.00 m beyond the face of the
curb.

Except for roads with narrow ROW where no tree planting shall be
undertaken to preclude accidents and allow space for vehicles to pull over
during emergency.
References
Highway Earthwork Series. Manual for Slope Protection. 1984. Japan Road
Association. Tokyo, Japan.
Landscaping and Erosion Control. www.geosyntheticsworld.com
7-48
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
8
Road Facilities
8.1
Road Safety and Clear Zone Requirements
With the development of higher standard highways and expressways, the nature
and characteristic of crashes changed from head-on crashes and tree collisions to
cases of drivers running off the road and colliding with man-made objects. The
man-made objects include bridge piers, sign supports, culverts, ditches and other
design features of the roadside. In response the clear zone concept has been
developed – an unobstructed, traversable area provided beyond the edge of the
through traveled way for the recovery of errant vehicles. The clear zone includes
auxiliary lanes other than those that function like through lanes, bike lanes, and
shoulders, plus varying widths of foreslope and backslope beyond the shoulder,
depending upon the design speed and traffic volume.
Figure 8-1
Clear Zone Distance
Source: DPWH Highway Safety Design Standard, May 2012
8-1
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Table 8-1 provides AASHTO recommended clear-zone foreslope and backslope
distances for different slopes, highway capacity, and design speeds.
Table 8-1
Design
Speed
N(kph)
o
t≤ 60
e
s
t
o70 – 80
T
a
b
l 90
e
8
1
100
:
S
o
u
r
c
e
110d
d
f
r
o
m
AASHTO Suggested Clear Zone Distances in Meters from Edge of Through Travel
Lane
Design ADT
Foreslope
1V:6H or
flatter
Backslope
1V:5H to
1V:4H
1V:3H
1V:3H
1V:5V to
1V:4H
1V:6H or
flatter
Under 750c
2.0 – 3.0
2.0 – 3.0
b
2.0 – 3.0
2.0 – 3.0
2.0 – 3.0
750 – 1500
3.0 – 3.5
3.5 – 4.5
b
3.0 – 3.5
3.0 – 3.5
3.0 – 3.5
1500 – 6000
3.5 – 4.5
4.5 – 5.0
b
3.5 – 4.5
3.5 – 4.5
3.5 – 4.5
Over 6000
4.5 – 5.0
5.0 – 5.5
b
4.5 – 5.0
4.5 – 5.0
4.5 – 5.0
Under 750c
3.0 – 3.5
3.5 – 4.5
b
2.5 – 3.0
2.5 – 3.0
3.0 – 3.5
750 – 1500
4.5 – 5.0
5.0 – 6.0
b
3.0 – 3.5
3.5 – 4.5
4.5 – 5.0
1500 – 6000
5.0 – 5.5
6.0 – 8.0
b
3.5 – 4.5
4.5 – 5.0
5.0 – 5.5
Over 6000
6.0 – 6.5
7.5 – 8.5
b
4.5 – 5.0
5.5 – 6.0
6.0 – 6.5
Under 750c
3.5 – 4.5
4.5 – 5.5
b
2.5 – 3.0
3.0 – 3.5
3.0 – 3.5
750 – 1500
5.0 – 5.5
6.0 – 7.5
b
3.0 – 3.5
4.5 – 5.0
5.0 – 5.5
1500 – 6000
6.0 – 6.5
7.5 – 9.0
b
4.5 – 5.0
5.0 – 5.5
6.0 – 6.5
Over 6000
6.5 – 7.5
8.0 – 10.0a
b
5.0 – 5.5
6.0 – 6.5
6.5 – 7.5
Under 750c
5.0 – 5.5
6.0 – 7.5
b
3.0 – 3.5
3.5 – 4.5
4.5 – 5.0
750 – 1500
6.0 – 7.5
8.0 – 10.0a
b
3.5 – 4.5
5.0 – 5.5
6.0 – 6.5
1500 – 6000
8.0 – 9.0
10.0
12.0a
–
b
4.5 – 5.5
5.5 – 6.5
7.5 – 8.0
Over 6000
9.0 – 10.0a
11.0
13.5a
–
b
6.0 – 6.5
7.5 – 8.0
8.0 – 8.5
Under 750c
5.5 – 6.0
6.0 – 8.0
b
3.0 – 3.5
4.5 – 5.0
4.5 – 5.0
750 – 1500
7.5 – 8.0
a
8.5 – 11.0
b
3.5 – 5.0
5.5 – 6.0
6.0 – 6.5
1500 – 6000
8.5 – 10.0a
10.5
13.0a
–
b
5.0 – 6.0
6.5 – 7.5
8.0 – 8.5
Over 6000
9.0 – 10.5a
11.5
14.0a
–
b
6.5 – 7.5
8.0 – 9.0
8.5 – 9.0
T
Source: Table 3.1 in AASHTO, 2011, Roadside Design Guide4th Edition, American Association of State
Highway and Transportation Officials, Washington DC. Used by Permission
Also refer to DPWH Order No. 217 dated 17 November 2000 on the subject of ‘Prohibiting the Installation of
Advertisements, Billboards and Signs within the Road Right-of-Way of National Roads’.
8-2
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
8.2
Road Safety Facilities
Although a traversable and unobstructed roadside is highly desirable, some
appurtenances should be placed near the traveled way, such as protective
guardrails and barriers, highway signs, roadway lighting, traffic signals, railroad
warning devices, etc. There are also some structures that must be located near
the traveled way for the structural functioning of the roadway pavement, such as
drainage works.
Safety facilities related to these provisions are discussed below.
8.2.1
Safety Barrier
A roadside barrier is a longitudinal barrier used to shield motorists from natural
or man-made hazards located along either side of a traveled way. It may also be
used to protect bystanders, pedestrians, and cyclists from vehicular traffic.
Barrier recommendations are based on the premise that a traffic barrier should
generally be installed if it reduces the severity of potential crashes. It is
important to note that the probability or frequency of run-off-the-road crashes is
not directly related to the severity of the potential crash. The installation of
barriers could lead to higher incident rates due to the proximity of the barriers to
the traveled way.
Barrier installation has often been the result of a subjective assessment of
whether the consequences of a vehicle striking a fixed object or running off the
road are believed to more serious than hitting a safety barrier. Safety barrier is
expensive, so it is also necessary to assess how likely it is that vehicles will run off
the road. This requires consideration of traffic volumes, traffic speeds and road
alignment. Costs associated with installing, maintaining and repairing a barrier
also need to be considered.
General Warrants for Use of Highway Safety Barrier
Highway conditions that are shielded by a roadside barrier are generally one of
two basic categories: embankments, or roadside obstacles.
Height and side slope are the basic factors in determining barrier needs for
embankments. AASHTO, 2011, Roadside Design Guide 4thEdition Figures 5.1 and
5.2 provide the results of American studies on these and other factors for the
consideration of barrier use. DPWH should consider preparing similar costeffectiveness evaluations to guide the use of barriers on embankments in the
Philippines.
Barrier guidelines recommended by AASHTO for roadside obstacles are
summarized in Table 8-2.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Table 8-2
Barrier Guidelines Recommended by AASHTO for Roadside Obstacles
Obstacle
Guidelines
Bridge piers, abutments, and
railing ends
Shielding generally needed
Boulders
Judgment decision based on nature of fixed object and likelihood of
impact
Culverts, pipes, headwalls
Judgment decision based on size, shape and location of obstacle
Foreslopes and backslopes
(smooth)
Shielding generally not needed other than for embankment
Foreslopes and backslopes
(rough)
Judgment decision based on likelihood of impact
Ditches (parallel)
Judgment decision depending on geometry of ditch
Ditches (transverse)
Shielding generally needed if likelihood of head-on impact is high
Retaining walls
Judgment decision based on relative smoothness of wall and
anticipated maximum angle of impact
Sign / luminaire supports
Shielding generally needed unless break-away supports used
Traffic signal supports
Isolated traffic signals within clear zone on high-speed rural facilities
may need shielding
Trees
Judgment decision based on site-specific circumstances
Utility poles
Shielding may be needed on case-by-case basis
Permanent bodies of water
Judgment decision based on location, depth of water and likelihood of
encroachment
There may also be situations where barriers may be required to separate
pedestrians and cyclists from vehicular traffic, but such requirements depend on
the specific situation.
Curbs should not be placed in front of guardrail because they may cause a vehicle
to vault over the guardrail.
Safety barrier terminals are potentially hazardous. The risk of collision can be
reduced by flaring the end section of the barrier away from the road. For
guardrail it is also necessary to either fit a crashworthy terminal piece, or, if
speed is no more than 60 kph, ramp the end beam down into the ground.
Types of Safety Barrier
AASHTO, 2011, Roadside Design Guide 4th Edition, Table 5-3 lists five types of
flexible barrier system, twelve types of semi-rigid system, and ten types of rigid
roadside barrier system, some of which are specific to the USA.
DPWH Highway Safety Design Standards, Part 1, Road Safety Design Manual
Section 20.6.1 lists three types of flexible wire rope safety barrier system, three
types of semi-rigid system, five types of rigid system, and three types of
temporary roadwork barrier system for use in the Philippines.
Refer to these publications for information on the applications and merits of each
type.
Factors to be considered in the selection of a specific type of barrier include the
highway classification, road design speed, traffic volume and composition,
roadway alignment, intersection sight distance, expected impact frequency. A list
of general selection criteria is provided in Table 8-3. Test levels for US Safety
Barriers are provided in Table 8-4.
8-4
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Table 8-3
Factors to be Considered in the Selection of Specific Types of Safety Barriers
Criteria
Comments
Performance
capability
Barrier should be structurally able to contain and redirect the design vehicle
for the appropriate test level – refer to Table 8-2 below
Deflection
Expected deflection of barrier should not exceed available deflection distance
Site conditions
Slope approaching the barrier and distance from traveled way may preclude
use of some barrier types
Compatibility
Barrier should be compatible with planned terminal or anchorage and
capable of transitioning to other barrier systems (such as bridge railing)
Cost
Standard barrier systems may be relatively consistent in cost, but highperformance systems can cost significantly more
Maintenance
Routine
Few systems require a significant amount of routine maintenance
Collision
Generally flexible or semi-rigid systems require significantly
maintenance after a collision than rigid or high performance systems
Material storage
The fewer the number of systems used, the fewer inventory items/storage
space required
Simplicity
Simpler designs are easier to maintain and more likely to be reconstructed
properly by field personnel
Aesthetics
Barrier aesthetics may be a consideration in selection
Field experience
The performance and maintenance requirements of existing systems should
be monitored to identify problems that could be reduced with correct barrier
type selection
more
Source: AASHTO, 2011, Roadside Design Guide. Used by Permission.
Table 8-4
US Safety Barriers Test Levels
Test level
Acceptance Test
Vehicle type
Speed (kph)
Impact angle
(degrees)
TL-1
1,100kg car and 2,270kg pickup
50
50
25
25
TL-2
1,100kg car and 2,270kg pickup
70
70
25
25
TL-3
1,100kg car and 2,270kg pickup
100
100
25
25
TL-4
1,100kg car 2,270kg pickup 10,000kg
single unit truck
100
100
90
25
25
15
TL-5
1,100kg car 2,270kg pickup 36,000kg
single unit truck
100
100
80
25
25
15
TL-6
1,100kg car 2,270kg pickup 36,000kg
tractor tank trailer
100
100
80
25
25
15
Source: AASHTO, 2009, Manual for Assessing Safety Hardware (MASH), First Edition. Used by
Permission.
8.2.2
Median Barriers
Median barriers are longitudinal barriers most commonly used to separate
opposing traffic on a divided highway. They may also be used along heavily
traveled roadways to separate through traffic from local traffic or to separate
high occupancy lanes from general purpose lanes.
8-5
Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Median barriers can reduce the incidence of cross-median crashes, and the
overall severity of such crashes. Their disadvantages are initial cost, an increase
in the number of crashes due to the reduction of recovery area, maintenance
costs associated with increased crashes, and a reduction in median access
opportunities for maintenance and emergency service vehicles.
Standard practice in Europe is to use barriers on all expressway medians less
than 15 m wide. AASHTO guidelines for the use of median barriers on high-speed,
fully controlled-access roadways are:

A barrier is required where the median width is less than 9.1 m and the ADT
is greater than 20,000 vehicles per day.

A barrier is optional for locations with a median width less than 15.2 m and
ADT less than 20,000 vehicles per day.

Where the median with is between 9.1 m and 15.2 m, and the ADT is greater
than 20,000 vehicles per day, a cost/benefit analysis should be conducted to
determine the need for a barrier.
Alternative crashworthy median barrier systems are listed in AASHTO, 2011,
Roadside Design Guide 4th Edition, Table 6-1, and shown in DPWH Highway Safety
Design Standards, Part 1, Road Safety Design Manual Figure 20.12.
Design safety strategies for channelized islands and medians in urban areas are
provided in Table 8-5.
Table 8-5
Design Safety Strategies for Channelized Islands and Medians in Urban Areas
Purpose
8.2.3
Strategy
Reduce likelihood of run-off-the-road collision
Widen median
Reduce crash severity
Place only frangible items in channelized island or
median
Shield rigid objects in median
Drainage
In urban areas, where drainage ditches are a potential hazard for motorcyclists,
pedal cyclists and pedestrians, only shallow gutters or covered drains should be
provided. A closed drainage system with curbs and drop inlets should be
considered for higher speed roads.
Drainage inlets, grates, and similar devices should be placed flush with the
pavement ground surface and must be able to support wheel loads. Traversable
drains should be used within clear zones.
Curbs and curbs with gutters are commonly used in urban settings to separate
pedestrians from the traffic flow. However curbs have limited re-directional
capability, particularly where vehicle speed is above 40 kph.
Design safety strategies for curbs in urban areas are provided in Table 8-6.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
Table 8-6
8.2.4
Design Safety Strategies for Curbs in Urban Areas
Purpose
Strategy
Design curb to minimize
potential for vaulting vehicles
Use appropriate curb height compatible with expected vehicle
trajectories
Orient barriers with respect to curbs so as to improve curb-barrier
interaction
Grade adjacent terrain flush with the top of the curb
Gateways/Traffic Calming
Gateways are localized features in urban environments where slower and more
cautious vehicle operation is desirable.
Design safety strategies for gateways/traffic calming in urban areas are provided
in Table 8-7.
Table 8-7
Design Safety Strategies for Gateways/Traffic Calming in Urban Areas
Purpose
Strategy
Reduce likelihood of run-ofthe-road crashes
Apply speed reduction signs, pavement markings, narrowed
crossings with raised pavement, and other traffic calming treatments
Reduce severity of run-offthe-road crash
Construct roundabouts with traversable island centers in initial islands
Inappropriate speed is a major contributory factor in road crashes in the
Philippines. Very often this problem arises because the road has to meet two
conflicting functions: providing for through traffic, which wants to go fast, and
local traffic (pedestrian and vehicular) that is moving slowly and is vulnerable.
The best solution is to keep all through roads out of towns and villages, but this is
not going to be possible in the foreseeable future. The inescapable conclusion is
that the speed of the through traffic in towns and villages must be reduced to a
level which is safe for a mix of pedestrians and vehicles.
This level is ideally 30 kph, because this is the speed at which collisions with
vulnerable road users become mostly survivable. However, most drivers would
not accept such a low speed limit unless there were many pedestrians in the
road, so 50 kph has become the norm. Roads with a 50 kph design speed must be
designed so as to make it difficult to exceed 50 kph. This implies narrow
carriageways, and sidewalks instead of shoulders. This is unlikely to always be
enough however, and more forceful speed management measures may be
needed. The application of speed management measures is sometimes called
‘traffic calming’ and Figure 8-2 to Figure 8-4 shows some examples that have
been found to be effective in other countries. They may not suit every situation,
and they must be designed so that they are not in themselves a hazard.
Gateways help to alert drivers to the need to slow down because they are
entering a town or village where there may be pedestrians and others in the
road. However, they are not very effective on their own.
Well-designed humps like these are highly effective in reducing speeds and
accidents. They must be very well signed.
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
Figure 8-2
Gateway/Traffic Calming Devices
5
5
POYOPOY
POYOPOY
Figure 8-3
Well Designed Hump
Figure 8-4
Center Island to Reduce Speed
PLAN
A
Ramp
12m
32m
24m
Ramp
Ramp
Ramp
A
CROSS SECTION A - A
3.0 - 3.5 m
3m
Flexible marker post
3.0 - 3.5 m
Narrowing the road by building a center island can be effective in reducing
speeds. They also provide a safer crossing point for pedestrians. They must be
very well signed.
8-8
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
8.2.5
Noise Barriers
Refer to Section 3.6.5.7.
8.2.6
Motorcycle Facilities
Motorcyclists have a higher risk of being seriously injured in a crash compared to
occupants in automobiles. They are also at risk in crashes with some types of
open-faced traffic barriers. Consideration should be given to modifying barriers
with a lower rubrail and post padding, or replacing them with smooth faced
concrete barriers at locations where motorcycle crashes are occurring.
8.2.7
Bicycle Facilities
Bicycle facilities may include standard road lanes, wide outside lanes, dedicated
bicycle lanes, and off-road bicycle paths. They also include roadside bicycle racks.
Design safety strategies for bicycles are provided in Table 8-8.
Table 8-8
Design Safety Strategies for Bicycles
Purpose
8.2.8
Strategy
Reduce likelihood of crash
Use wider curbside lanes
Increase operational offsets
Reduce severity of crashes
Locate bicycle racks as far away from the road as possible
Pedestrian Facilities
On low-speed streets, pedestrians are separated from vehicular traffic by a
sidewalk which is separated from the roadway by a raised curb. However above a
speed of 40 kph for relatively flat approach angles, a vehicle may mount the curb.
For roadways with design speeds over 40 kph, separating the sidewalk from the
edge of the roadway with a buffer space is recommended.
When sidewalks or multi-use paths are adjacent to the traveled way of highspeed facilities, provision should be made to shield the sidewalk in consideration
of vehicle and pedestrian traffic volumes, roadway geometry, sidewalk/path
offset, and cross-section features.
Design safety strategies for pedestrians are provided in Table 8-9.
Table 8-9
Design Safety Strategies for Pedestrians
Purpose
Strategy
Reduce motor vehiclepedestrian crash likelihood at
roadside locations
Provide continuous pedestrian facilities
Install pedestrian refuge medians or channelized islands
Offset pedestrian locations away from traveled way with pedestrian
buffers
Physically separate pedestrians from traveled way at high-risk
locations
Improve sight distances by removing objects that obscure driver or
pedestrian visibility
Reduce severity of motor
vehicle-pedestrian crashes
at roadside locations
Reduce roadway design speed, operating speed, or both in high
pedestrian volume locations
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
8.2.9
Parking
On street parking can narrow the effective width of a roadway, resulting in speed
reduction and reduced crash severity. However it also may increase collisions
associated with vehicles attempting to pull in or out of parking spaces.
Design safety strategies for on-street parking are provided in Table 8-10..
Table 8-10
Design Safety Strategies for On-Street Parking
Purpose
8.2.10
Strategy
Reduce likelihood of crash
Restrict on-street parking to low-speed roads
Reduce crash severity
Where parking is appropriate, use parallel parking rather than angular
parking
Sign Posts and Roadside Hardware
Design safety strategies for vertical roadside utility poles, light poles, and street
signs in urban areas are provided in Table 8-11.
Table 8-11
8.2.11
Design Safety Strategies for Roadside Utility Poles, Light Poles and Street signs
in Urban Areas
Purpose
Strategy
Treat individual poles or
posts in high risk locations
Remove or relocate poles
Place poles on inside of horizontal curves and avoid placement on
outside of roundabouts or too close to intersection corner
Use breakaway or yielding poles
Shield poles
Improve pole visibility
Treat multiple poles or
posts in high-risk locations
Establish urban-enhanced lateral offset guidance for pole setback
distance from curb
Place utilities underground while maintaining appropriate night-time
visibility
Combine utilities and signs onto shared poles
Replace poles with building-mounted suspended lighting (where
suitable)
Minimize level of severity
Reduce travel speed on adjacent road
Disabled Person Facilities
Refer to DPWH Order No. 37 dated 26 August 2009 on the subject ‘Enforcement
of the Accessibility Law (BP 344) Along National Roads’.
8.3
Traffic Control Facilities / Devices
Text in this section refers to the DPWH, Highway Safety Design Standards Part 2:
Road Signs and Pavement Markings Manual, May 2012 (referred to below as ‘the
Manual’).
8.3.1
Functional Classification of Traffic Control Devices
Traffic Control devices have been classified under the categories shown below.

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Road Signs
-
Regulatory Signs (Type R)
-
Warning Signs (Type W)
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design

8.3.2
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Guide or Information Signs (Type G)
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Expressway Signs (Type GE)
-
Special Purpose Signs for Traffic Instruction (Type S)
-
Hazard Markers (Type HM)
Pavement Markings
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B1. Longitudinal Lines
-
B2. Transverse Lines
-
B3. Other Lines
-
B4. Other Markings
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B5. Messages and Symbols
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B6. Object Markings
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B7. Raised Pavement Markers
Basic Principles in the Design, Installation and Maintenance of Traffic Signs
Uniformity in the design of signs facilitates identification by the road user.
Standardization of shape, color, dimensions, legends and illumination or
reflectorization is important.
8.3.3
Uniformity of Traffic Control Devices
Uniformity of locations is also important for road user identification. Refer to
Sections 1.8 to 1.13 of the Manual covering the topics of placement, shape, size,
color, letter series, letter size, overhead signs, reflectorization and illumination,
installation, excessive use, and maintenance.
8.3.4
Types of Traffic Control Devices
Classes of Traffic Signs
Road signs are as listed in Section 8.3.1, plus Part A also includes Guide Posts and
Delineators.
Regulatory Signs (Type R)
Refer to Section 2 of the Manual.
Warning Signs (Type W)
Refer to Section 3 of the Manual.
Guide or Information Signs (Type G)
Refer to Section 4 of the Manual.
Expressway Signs (Type GE)
Refer to Section 5 of the Manual.
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Special Purpose Traffic Instruction Signs (Type S)
Refer to Section 6 of the Manual.
Hazard Markers (Type HM)
Refer to Section 7 of the Manual.
Guide Posts and Delineators
Refer to Section 8 of the Manual.
8.3.5
Markings
Section 9.1 to 9.3 of the Manual confirms the functions and limitations, legal
authority and standardization of line marking.
Types of Marking
Section 9.4 of the Manual discusses types of markings
Fundamental Requirements of Marking
Sections 9.5 to 9.8 of the Manual discuss line marking materials, color, types of
line, plus widths and tolerances.
Longitudinal Lines
Section 11 of the Manual details center, lane, barrier, edge, continuity, and
transition lines.
Transverse Lines
Section 12 of the Manual details Stop, Give Way (or Yield or Holding),
Roundabout Holding lines, and Pedestrian Crossing markings.
Other Lines
Section 13 of the Manual details Turn, Parking Bay, Median, Bus and PUJ Lane,
Loading and Unloading Bay, and ‘Do Not Block Intersection’ lines.
Other Markings
Section 14 of the Manual details:
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
Approach Markings to Islands and Obstructions

Chevron Markings

Diagonal Markings

Rumble Strips

Markings on Expressway Exit and Entrance Ramps

Curb Markings for Parking and Loading/Unloading Restrictions

Markings for the approach to Railroad Crossings
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design

Refer to DPWH Orders

No. 31 dated 15 June 2010 on the subject ‘DPWH Standard Specification for
Reflectorized Thermoplastic Rumble Strips, Item 618’

No. 10 dated 21 February 2011 on the subject ‘DPWH Standard Specification
for Chevron Signs, Item 620’
Messages and Symbols
Section 15 of the Manual details the marking of messages and symbols on the
pavement.
Object Marking
Section 16 of the Manual details the marking of objects off the roadway.
Raised Pavement Markers
Section 17 of the Manual details the use of raised pavement markers.
8.3.6
Speed Humps
A speed hump is a raised area in the roadway pavement surface extending
transversely across the travel way. Speed humps are sometimes referred to as
“pavement undulations” or “sleeping policemen”. Most agencies implement speed
humps with a height of 3 to 3.5 inches (76 to 90 mm) and a travel length of 12 to
14 feet (3.7 to 4.3 m). Speed humps are generally used on residential local
streets. See Figure 8-1 and Section of Speed Hump.
A speed bump is also a raised pavement area across a roadway. Speed bumps are
typically found on private roadways and parking lots and do not tend to exhibit
consistent design parameters from one installation to another. Speed bumps
generally have a height of 3 to 6 inches (76 to 152 mm) with a travel length of 1
to 3 feet (0.3 to 1 m).
From an operational standpoint, speed humps and bumps have critically
different impacts on vehicles. Within typical residential operational speed ranges,
vehicles slow to about 20 mph (32 km/h) on streets with properly spaced speed
humps. A speed bump, on the other hand, causes significant driver discomfort at
typical residential operational speed ranges and generally results in vehicles
slowing to 5 mph or less at each bump.
Speed bumps of varying design have been routinely installed on private
roadways and parking lots without the benefit of proper engineering study
regarding their design and placement. Speed humps, on the other hand, have
evolved from extensive research and testing and have been design to achieve a
specific result on vehicle operations without imposing unreasonable or
unacceptable safety risks.
Speed humps are generally installed on roadway functionally classified as local
roads and residential collector streets as defined in AASHTO’s A Policy on
Geometric Design of Highway and Streets (2011 Edition).
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Many agencies install speed humps on roads with an urban cross section (i.e.
curb and gutter). Street where speed humps and applied may or may not have
side walks as bicycle facilities. The surrounding land use for street where speed
humps are applied is generally residential in nature and may include schools,
parks or community center.
Figure 8-5
Plan and Section of Speed Hump
Source: Guidelines for the Design and Application of Speed Humps. Report RP-023 A, ITE Traffic
Engineering Council Speed Hump Task Force 1997
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
9
Roadway & Street Lighting Design (Road, Bridge,
Vehicular Tunnel, Underpass, Walkway and Bicycle Way)
9.1
Introduction
Good visibility under day or night conditions is one of the fundamental needs for
motorists to travel on roadways in a safe and coordinated manner. Properly
designed and maintained street lighting will produce comfortable and accurate
visibility at night, which will facilitate and encourage both vehicular and
pedestrian traffic. Determination of the need for lighting should be coordinated
with crime prevention programs and other community needs.
Warrants for the justification of street lighting involve not only identifying the
functional classification of the roadway, but also pedestrian and vehicular
volume, night-to-day crash ratios, roadway geometry, merging lanes, curves, and
intersections to establish appropriate illumination levels.
Because glare also indicates the quality of lighting, the type of fixtures and the
height at which the light sources are mounted are also factors in designing street
lighting systems. The objectives of the designer should be to minimize visual
discomfort and impairment of driver and pedestrian due to glare.
Where only intersections are lighted, a gradual transition from dark to light to
dark should be provided so that drivers may have time to adapt their vision.
9.2
Factors in Lightning Design
The expertise required for lighting designs includes:

Lamp types and characteristics, including depreciation factors

Ballast types and characteristics

Fixture mechanical characteristics

Lens types
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Photometric performance of luminaries and factors impacting such
performance

Fixture mounting types

Pole mechanical and electrical characteristics

Breakaway device options and when appropriate to use

Clear zone criteria

Pole types, mounting options, and loading considerations

Foundation and support details

Pavement reflection factors

Mounting height and spacing options

Light trespass and sky glow issues including laws and ordinances
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9.3

Lighting quality requirements, such as illuminance, luminance, veiling
luminance, and visibility

Maintenance considerations for individual components and the lighting
system as a whole

Energy and life-cycle costs

Coordination with master lighting plans
Master Lighting Plan
A master lighting plan is a formal arrangement between relevant government
agencies and other entities within a regional area to coordinate and standardize
the design, operation, and maintenance of public lighting.
The basic benefits of lighting include safety, beautification, and security for
people and property. Additional benefits derived from a master lighting plan
include:

Improved safety through maximizing resources

A consistent image, reflecting the local culture and tastes

Night-time linking of various sections of the region

Systems that better identify the nature of the site

Better management of energy use

Tighter control of sky glow and light trespass

Aid in implementing lighting curfews

Increased public security

Coordinated maintenance

Easier coordination of maintenance specifications, such as poles, breakaway
devices, and luminaires
The master lighting plan development process involves the following steps:

Coordinate with other participants to set goals

Consult with and consider the concerns of various groups having a stake in
public lighting

Conduct a study to justify and determine the feasibility of the planning
strategies
Master lighting plans allow for benefits to be derived from modern electronic
monitoring and control systems, known as electrical and lighting management
systems (ELMS).
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
9.4
Techniques of Lighting Design
9.4.1
Introduction
Accepted methods exist for achieving a given lighting condition known as either
level of illuminance or level of luminance. These methods permit ready analysis
of alternative lamps, luminaires, mounting height, luminaire spacing, energy
consumption, etc., to determine a preferred design.
The design of a roadway lighting installation is a process of applying known or
specified photometric characteristics of selected lamp-luminaire combinations. A
trial-and-adjust process of assumed luminaire locations is used in making
calculations of either the average amount of illuminance or the average
luminance over the roadway.
The level and uniformity of illuminance or luminance along a highway depends
on the lumen output of the light source, luminaire distribution, mounting height,
luminaire position, pavement reflectance, and spacing and arrangement. All
appropriate light sources should be considered, and the size or sizes that will
give the most effective and economical lighting system should be used.
9.4.2
Illuminance and Luminance Considerations
Illuminance in roadway lighting is a measure of the light incident on the
pavement surface measured in foot-candles (Lux). The illuminance at any certain
point will be the sum of illuminance from one or several contributing sources.
Luminance in roadway lighting is a measure of the reflected light from the
pavement surface that is visible to the motorist’s eye. Different road surface
materials, such as Portland cement concrete or asphalt, have different luminance
coefficients. For a section of roadway, luminance uniformity is calculated both as
the ratio of average level to minimum point, and maximum point to minimum
point. The evaluation of glare from the fixed lighting system is also relevant and
included with the luminance criteria.
9.4.3
Warranting Conditions
Lighting benefits motorists by improving their ability to see roadway geometry
and other vehicles at extended distance ahead. This results in greater driver
confidence and improved safety, which in turn improves highway capacity,
pedestrian safety, public safety, security and convenience.
Warrants for continuous expressway lighting, complete interchange lighting, and
partial interchange lighting are provided in Table 9-1.
A continuous lighting system provides relatively uniform lighting on all main
lanes and direct connections, and complete interchange lighting of all
interchanges within the section, using conventional luminaires or high mast
assemblies or both. Frontage roads are not normally included.
Complete interchange lighting is defined as a lighting system that provides
relatively uniform lighting within the limits of the interchange, including main
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
lanes, direct connections, ramp terminals, and frontage road or crossroad
intersections. Refer Table 9-2.
Partial interchange lighting is defined as a lighting system that provides
illumination only of decision making areas of roadways, including acceleration
and deceleration lanes, ramp terminals, crossroads at frontage road or ramp
intersections, and other areas of nighttime hazard. Refer Table 9-3.
Table 9-1
Warranting Conditions for Continuous Expressway Lighting (CEL)
Case
Warranting Conditions
CEL-1
Sections in and near cities where the current average daily traffic (ADT) is 30,000 or
greater.
CEL-2
Sections where three or more successive interchanges are located with an average spacing
of 2.3 km or less, and adjacent areas outside the right-of-way are substantially urban in
character.
CEL-3
Sections of 3 km or more passing through a substantially developed suburban or urban
areas in which one or more of the following conditions exist:
Local traffic operates on a complete street grid having some form of street lighting, parts of
which are visible from the expressway,
The expressway passes through a series of developments – such as residential,
commercial, industrial, and civic areas, colleges, parks, terminals, etc., that include lighted
roads, streets, parking areas, yards, etc., that are lighted,
Separate cross streets, both with and without connecting ramps, occur with an average
spacing of 0.75 km or less, some of which are lighted as part of the local street system,
The expressway cross section elements, such as median and borders, are substantially
reduced in width below desirable sections used in relatively open country.
CEL-4
Sections where the ratio of night to day crash rate is at least 2.0 times the region average
for all unlighted similar sections, and a study indicates that lighting may be expected to
result in a significant reduction in the night crash rate. Where crash rate data is not
available, rate comparison may be used as a general guideline for crash severity.
Source: AASHTO, 2005, Roadway Lighting Design Guide. Used by Permission.
Table 9-2
Warranting Conditions for Complete Interchange Lighting (CIL)
Case
Warranting Conditions
CIL-1
Where the total current ADT ramp traffic entering and leaving the expressway within the
interchange areas exceeds 10,000 for urban conditions, 8,000 for suburban conditions, or
5,000 for rural conditions.
CIL-2
Where the current ADT on the crossroad exceeds 10,000 for urban conditions, 8,000 for
suburban conditions, or 5,000 for rural conditions.
CIL-3
Where existing substantial commercial or industrial development that is lighted during hours
of darkness is located in the immediate vicinity of the interchange, or where the crossroad
approach legs are lighted for 0.75 km or more on each side of the interchange.
CIL-4
Where the ratio of night to day crash rate within the interchange area is at least 1.5 times the
region average for all unlighted similar sections, and a study indicates that lighting may be
expected to result in a significant reduction in the night crash rate. Where crash data is not
available, rate comparison may be used as a general guideline for crash severity.
Source: AASHTO, 2005, Roadway Lighting Design Guide. Used by Permission.
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Table 9-3
Warranting Conditions for Partial Interchange Lighting (PIL)
Case
Warranting Conditions
PIL-1
Where the total current ADTramp traffic entering and leaving the expressway within the
interchange area exceeds 5,000 for urban conditions, 3,000 for suburban conditions, or 1,000
for rural conditions.
PIL-2
Where the current ADT on the expressway through traffic lanes exceeds 25,000 for urban
conditions, 20,000 for suburban conditions, or 10,000 for rural conditions.
PIL-3
Where the ratio of night to day crash rate within the interchange area is at least 1.25 times
the region average for all unlighted similar sections, and a study indicates that lighting may
be expected to result in a significant reduction in the night crash rate. Where crash severity
data is not available, rate comparison may be used as a general guideline for crash severity.
Source: AASHTO, 2005, Roadway Lighting Design Guide. Used by Permission.
9.4.4
Design Values for Expressways
Suggested lighting design values for Principal Arterial, Minor Arterial, Collector,
and Local roads, plus Alleys, Sidewalks, Pedestrian Ways and Bicycle Ways are
provided in Table 3-5 of AASHTO, October 2005, Roadway Lighting Design Guide,
American Association of State Highway and Transportation Officials, Washington
DC.
The selection of light source, luminaire distribution, mounting height, and
luminaire overhang is an engineering decision based on geometry, character of
the road, environment, proposed maintenance, economics, aesthetics, and overall
lighting objectives.
Lighting levels on crossroad approaches should not be reduced through the
interchange area.
Where partial interchange lighting is provided, luminaires should be located to
best light the through lanes and speed change lanes at diverging and merging
locations.
On continuously lighted expressways and lighted interchanges, the lighting of
bridges and overpasses should be at the same level as the roadway. Lighting
poles on bridges should be located within the protection of railings or parapets,
away from the inside of superelevated curves.
The installed lighting system should have a pleasant daytime appearance.
Provisions for present and future lighting should be included with roadway and
structural work.
9.4.5
Streets and Highways other than Expressways
Considerations for the lighting of urban streets and highways are traffic volumes
(both vehicle and pedestrian), at-grade intersections, turning movements,
signalization, varying geometrics, weather conditions, crime deterrence, and
general safety.
Lighting may be provided for all major arterials in urbanized areas, and for
locations or sections of streets and highways where the ratio of night to day crash
rates is higher that the region average for similar locations, and a study indicates
that lighting would significantly reduce the nighttime crash rate.
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Lighting of spot locations in rural areas should be considered whenever a driver
is required to pass through a section of road with complex geometry or raised
channelization. The lighting design treatment is typically similar to that for
expressway ramp terminals.
Lighting design valves using the illuminance or luminance technique are
provided in AASHTO 2005.
9.4.6
Pole Placement Guidelines
Structural supports for lighting units should be designed and located so that they
do not distract the attention of the motorist or interfere with their view of the
roadway, or obstruct the view of signs.
Height restrictions may apply for lighting poles adjacent to airports and landing
zones.
Locating structural supports for lighting units within a median area may be
appropriate if the width of the median is sufficient or if concrete median barriers
are used. Locations within the clear zone of a main lane, and a ramp at a gore
area is not desirable unless positioned behind or atop a longitudinal traffic
barrier or crash cushion. They should not be located on the traffic side of guide
rail or deflecting barrier. In locating lighting poles behind rail, consideration
should also be given to the distance necessary for rail deflection.
The maintenance and servicing of roadway and sign lighting units should be
considered when designing the lighting system, including consideration of the
potential hazards to maintenance personnel.
Options for upgrading existing lighting system structural supports include:
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
Reducing the number of poles by combining lighting, traffic control, and
electrical power functions where possible,

Redesigning lighting systems so that supports are located outside the clear
zone using higher mounting heights or off-set luminaires,

Installing Non-breakaway lighting poles within the clear zone along the edge
of the traveled way must be protected from impact by longitudinal guardrail
or barrier,

When lighting poles cannot be placed outside the roadside clear zone or
behind longitudinal guardrail, barrier or crash cushion, the design should
include breakaway supports,

Only when the use of breakaway supports is not practical should a traffic
barrier or crash cushion be used exclusively the shield light poles, and

Where the above alternatives are not appropriate, the designer should
investigate delineating the obstacle to provide quicker recognition and
response by motorists.
Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
9.5
High-Mast Lighting
High-mast lighting is comprised of groups of luminaires mounted on freestanding poles at mounting heights that can vary from 18 to 55 m. High mast
poles are usually provided with luminaire lowering devices for maintenance.
They are used principally at interchanges, toll plazas, rest areas, parking areas,
and for continuous lighting on highways.
The benefits of high-mast lighting include improved uniformity, lower glare, and
fewer pole locations.
9.6
Tunnels and Underpasses
9.6.1
Underpasses
An underpass is defined as a portion of a roadway that extends through and
beneath some natural or man-made structure, which requires no supplementary
daytime lighting.
Underpass lighting is warranted in areas that have frequent night-time
pedestrian traffic, or where unusual or critical roadway geometry occurs
adjacent to or in the underpass area. Continuous lighting on the associated
expressway lanes also warrants the installation of underpass lighting.
Night-time lighting levels and uniformity should be aimed at the lighting levels on
the adjacent roadways. Higher levels of lighting may result because of luminaire
mounting height and spacing limitations, and lighting from other nearby sources,
but increased levels should not exceed approximately twice that of the adjacent
roadway.
Luminaires attached to the structure along the roadside in full or partial view of
the motorist may necessitate glare control or the use of lower wattages. It is
generally better to minimize source glare by using several lower output
luminaires than one or two high output luminaires.
9.6.2
Vehicular Tunnels
A structure of any type that surrounds a vehicular roadway and is longer than an
underpass is considered as a tunnel. Tunnels normally require supplementary
day lighting to provide adequate roadway visibility for safe and efficient traffic
operation.
A tunnel zone is a length of tunnel roadway equal to the wet pavement minimum
stopping sight distance for the vehicle operating speed of the tunnel roadway and
its approaches. A tunnel is considered ‘short’ if its length from entry portal to exit
portal is equal to or less than one zone, and ‘long’ if it has more than one zone.
The physical features of a tunnel can have a significant effect on reducing the day
lighting needs, through the appropriate lighting of tunnel portals, adjacent walls,
and approach pavement, use of high brightness internal ceiling and wall surfaces,
and consideration of pavement surface type.
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9.6.3
Lighting of Tunnel Interiors
Short vehicular tunnels that have relatively straight and level approach
alignments may offer adequate visibility to entering motorists by silhouette
viewing of other vehicles and objects on the roadway against the far end exit
portal. Such tunnels may be treated as underpasses. In multi-lane one-way
tunnels, or un-separated two-way tunnels, lighting should be provided to the
extent that motorists can distinguish lane markings or other delineation
important to safe travel.
The most critical portion of a tunnel that affects visibility is at the entrance
portal. Visibility of this first entrance zone is essential for the motorist to identify
and safely react to the presence of vehicles and objects that may be present on
the tunnel roadways. This is accomplished by lighting the entrance zone in
proper proportion to the outside ambient luminance to which the motorists’ eyes
are adapted. Beyond the entrance zone, if the tunnel is classified as a short
tunnel, the entrance zone lighting level applies throughout its entire length. In
long tunnels, lighting beyond zone one should be reduced progressively in
successive zones until a minimum level is reached.
Night-time lighting levels in a tunnel should be somewhat higher, but not
exceeding three times that of the lighting requirements for roadways adjacent to
the tunnel. Uniformity of lighting should closely match that of requirements for
adjacent roadways.
The choice of type of tunnel luminaire should consider such items as luminous
efficacy, source glare, light distribution characteristics, physical placement
limitations, frequency of maintenance, and resistance to damage. Entrance zone
lighting levels may need to be adjusted to match ambient conditions at different
seasons and during cloudy or inclement weather, and ideally lighting levels in
subsequent tunnel zones should vary proportionately. Tunnel lighting systems
should be designed to be as fail-safe as practical.
9.7
Work Zone Lighting and Temporary Roadway Lighting
Safety in work zones is important, where motorists may need to negotiate
detours, sections with reduced shoulders, reduced lane widths, limited ‘pull-off’
areas, unusual maneuvering, temporary pavement markings, rough pavement,
and many other conditions. Roadway lighting can be an effective tool in work
zones, providing additional visual information.
An increasing amount of highway construction and maintenance is being
performed at night to avoid daytime congestion. Temporary lighting for work
zones must consider the glare effect on motorists.
Types of temporary lighting may include the early installation of permanent
lighting, installing temporary fixtures on permanent poles, or installing
permanent poles in temporary locations.
Temporary lighting should meet all the protection or breakaway requirements
that permanent lighting must meet.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
9.8
Roundabouts
Roundabouts require special attention with respect to lighting. Motorists
unfamiliar with the roundabout need the navigation assistance of good lighting to
deal with limited sight distances, weaving traffic, direction and other signs, and
the need for quick decisions.
Roundabouts should be lit to a level that is 1.3 to 2 times the value used on the
best lit approach. The illuminance method should be used, with light levels
extended 2 to 3 m outside the outer curb of the sidewalk or other roadside
features. The lighting should be extended a minimum of 125 m along each road
connecting to the roundabout.
Provision of god pedestrian recognition is important at roundabouts. Crosswalks
should be typically lit with the pedestrians in positive contract by installing light
poles 3 to 10 m before crosswalks. Roundabouts should be lit from the outer edge
of the roadway.
9.9
Electrical System Requirements
Breakaway rated fuse holders should be used anytime breakaway pole devices
are used. Breakaway poles should not be wired from overhead without an
adequate means of electrical disconnection.
All equipment must be grounded, including metal ground box lids, exposed metal
conduit, metal poles, and supplemental ground rods at pole foundations.
The voltage available at each light pole affects the light output of the fixture.
Voltage drop values should be considered in the determination of lumen
maintenance factors.
9.10
Safety Rest Areas
Rest areas may be available for use at night as well as by day, and their general
appearance should generate a feeling of safety and security. Properly designed
lighting will enhance such facilities.
Lighting design may consider the rest area entrance and exit, interior roadways,
parking areas, activity areas, and main lanes.
9.11
Roadway Sign Lighting
9.11.1
Introduction
Traffic signs are placed along the roadway in strategic locations and are used to
convey specific, consistent messages to the motorist. Sign legibility at night can
be achieved in one of two ways:

The retro-reflection of the letters and background of the sign by vehicle
headlights.

The illumination of the sign face by an internal or external fixed-source sign
lighting system.
The visibility and legibility of a sign during the hours of darkness depend on:
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Design Guidelines, Criteria and Standards: Volume 4 - Highway Design
9.11.2

Ambient Luminance – the amount of ambient luminance adjacent to the sign.

Sign Luminance Above Ambient – the sign luminance in excess of the ambient
luminance determines how well the sign can be viewed against its
background.

Uniformity Ratio of Light Levels – the level of uniformity of light over the
entire face of the sign.

Reflectivity of Legend and Background – the reflectivity of the letters that
make up the legend as well as the background that they are installed on
should be optimized without delivering excessive glare to the motorist.

Contract between Legend and Background – the contracts should be
optimized in order for a passing motorist to quickly read and process the sign
message.
Sign Lighting Recommendations
Where required, signs can be illuminated externally, internally, or using a
luminous source message. Recommended illuminous and luminous lighting levels
for illuminated signs are provided in Table 9-4.
Table 9-4
Recommended Illuminous and Luminous Lighting Levels for Illuminated Signs
Ambient Luminance
Sign Illuminance
Sign Luminance
Footcandles
Lux
Candelas per m2
Low
10 – 20
100 – 200
22 – 44
Medium
20 – 40
200 – 400
44 – 89
High
40 – 80
400 – 800
89 – 78
Source: AASHTO, 2005, Roadway Lighting Design Guide. Used by Permission.
Lighting units that illuminate the face of a sign may be located either on top of the
sign, on the bottom of the sign, or remotely located on an adjacent support in
consideration of:

The luminaire housing should not obstruct the view of the sign message.

The reflected light should not reduce the visual performance of the sign
message.

Contribution to sky-glow should be limited as much as is practicable.

The spill light should not be directed into the eyes of motorists.

The luminaire mounting arrangement should not create maintenance
problems.
Solar street lights are raised light sources powered by photovoltaic panels
typically mounted on the street light support structure. Photovoltaic panels
charge a rechargeable battery to provide power to a fluorescent or LED lamp
during the night. LED lamps are usually used as they provide much higher
Lumens with lower energy consumption. Most solar panels turn on and turn off
automatically by sensing outdoor light.
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Design Guidelines, Criteria and Standards: Volume 4 – Highway Design
9.12
Maintenance Considerations in Roadway Lighting Design
All lighting systems depreciate with time and need continuing surveillance and
maintenance to provide the service for which they were designed and installed.
The design of lighting should consider the extent and frequency of maintenance.
Lighting maintenance can be categorized into several basic areas including
luminaires, support structures, electrical distribution and control, and external
factors. Each of these areas is important to the overall utility and efficiency of a
lighting installation and should be included in any good maintenance program.
9.13
Sky Glow and Light Trespass
9.13.1
Overview
Lighting systems affect the area surrounding the roadway.
Sky glow is defined as the added sky brightness caused by the scattering of light
into the atmosphere. That portion of scattered light that is redirected back
towards the ground is, in essence, light that is emitted by the sky. At high enough
levels, the sky will appear as a self-luminous body, and will glow. Sky glow is of
concern to astronomers and others who like to see the moon and starts, or just
wish to enjoy the natural night-time environment.
The term light trespass describes light that strays from its intended target and
illuminates adjacent properties. Most complaints about this impact are from the
public, when lighting from roadway luminaires shines onto their property.
9.13.2
Mitigating Sky Glow and Light Trespass
The least expensive and most successful approach to objectionable light
problems is prevention. For prevention efforts to work the designer should:
9.14

Perform a review or walk-through of the site during the pre-design stage, and
consider adjacent property and nearby developments as well as investigate
community desires for lighting systems.

Select a luminaire whose candela distribution pattern matches the need.

Consider internal and external shields if necessary to limit the candela in
certain directions.

Consider pole location, mounting height, spacing, finished terrain, and
landscaping as design variables that can be used to mitigate light trespass.

Choose luminaires and placements with care as glare or visual clutter can be
produced by almost any luminaire when observed against a dark
background.
Reference
AASHTO, October 2005, Roadway Lighting Design Guide, American Association of
State Highway and Transportation Officials, Washington DC
Department of Energy, Philippines, December 2008, Roadway Lighting Guidelines.
9-11