Cross Sections
CE 453 Lecture 22
Iowa DOT Design Manual Chapter 3
1
See also Iowa DOT Standard Plans
2
Objectives
1.
Identify cross section components and design
criteria
See:
http://www.fhwa.dot.gov/environment/flex/ch06.htm
(Chapter 6 from FHWA’s Flexibility in Highway
Design)
3
Cross Section Elements
1.
Roadway
2.
Median
3.
Roadside
Roadway Components
1.
Travel Lanes
2.
Shoulders
4
Source:http://www.fhwa.dot.gov/environment/flex/ch06.htm
Cross Section Elements
1.
Roadway
2.
Median
3.
Border
Roadway Components
1.
Travel Lanes
2.
Auxiliary Lanes
5
Source:http://www.fhwa.dot.gov/environment/flex/ch06.htm
Considerations for Design of
Cross-Section


Volume and
composition (percent
trucks, buses, and
recreational vehicles)
of the vehicular traffic
likelihood of bicyclists
and pedestrians using
the route
Flexibility in Highway Design Chapter 6
6
Considerations for Design of
Cross-Section




Climatic conditions (storage space for
plowed snow, amount of rain)
Presence of natural or human-made
obstructions adjacent to the roadway
(rock cliffs,etc)
Type and intensity of development
along the facility
Safety of the users (speed of traffic)
7
Travel Lanes
Function: guidance to drivers and vehicle support
Pavement types:
 high (modern standards),
 intermediate (surface treatments), and
 low (unpaved)
8
Travel Lanes
Selection Criteria:

Traffic volume and composition

Soil characteristics

Past performance in area

Availability of materials

Energy conservation

Initial cost

Maintenance cost

Overall life-cycle cost
9
Cross Slope



Slope perpendicular to flow of traffic on tangent
section
Rural – normal crown – uniform slope from
center to edge of pavement (cross slope break
typically at centerline)
Urban – parabolic shape (gutter capacity)
10
Cross Slope
•
•
•
Rate of Cross Slope = f(drainage, steering,
and rollover or cross slope break)
Drivers cross the crown line during passing
maneuvers
•
Difficult to negotiate steep slopes
AASHTO Recommends
•
High 1.5 to 2% (0.015 ft/ft m/m)
•
Intermediate 1.5 to 3%
•
Low 2 to 6%
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12
Urban Cross-Sections
13
Urban Cross-Sections
14
HMA Cross-Sections
15
Drainage Considerations
16
Drainage Considerations
17
Roadway Component – Travel
Lane
Lane Width Considerations
•
•
•
•
•
•
What is the impact of weather on
cross slope design?
Safety: Allow steering adjustment
and lateral clearance
Pavement edge crumbling
(deterioration) less with wide lane
Cost/Benefit (depends on traffic)
Bicycle Use
18
Lane Width








Limited by physical
dimension of vehicles
12 ft desirable
11 ft acceptable in urban
areas with restrictions
10 ft okay for low
speed/urban roadways
9 ft – okay low volume rural
and residential roadways
14 ft – shared outside lane
with bike
TWLTL 10 – 16 ft (3.0 –
4.8m)
Auxiliary 10 ft. (3.0 m) or
more
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Shoulders
•
•
•
•
•
•
•
Functions:
Lateral Support
Avoidance Space
Emergency Stop
Ped/Bike Use
Turning/Passing at
Intersections
Mail Delivery, Buses,
etc.
Flexibility in Highway Design Chapter 6
20
Shoulders




Should be flush with roadway surface
Sloped to drain away from traveled
way
Should be stable enough to support
vehicles in all kinds of weather w/out
rutting
Should be visibly different from
traveled way
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Shoulder geometry




Paved/Unpaved (earth, turf, gravel)
Graded and usable width depends on
foreslope and rounding
Consider function, safety, and capacity
impacts
Slope






2 to 6% (paved)
4 to 6% (gravel)
8% (turf)
Min. 2% + lane slope
Max crossover 8%]
Width 0.6 – 3.6 m (2 – 12 ft)
22
Source: A Policy on Geometric
Design of Highways and
Streets (The Green Book).
Washington, DC. American
Association of State23
Highway
and Transportation Officials,
2001 4th Ed.
Curbs



Control access
Control drainage
Type used varies with location and
design speed
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Curbs
25
Curbs
26
Iowa’s Roadway-Related
Fatal Crashes

52% of Iowa’s fatalities are
related to Lane Departure

39% of Iowa’s fatal crashes
are single-vehicle
Run-Off-the-Road (ROR)
crashes
27
Safety Investment Strategy

Candidate Safety Projects









Paved shoulders
Milled-in shoulder rumble strips
2-lane shoulder widening
High severity crash intersections
High severity crash 2-lane roads
High crash curves
Expressway intersections
Centerline rumble strips
Cross-median head-on crashes
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4 Foot Paved Shoulder
US 63
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Milled Shoulder Rumble Strips
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Median Function
►
►
►
►
►
►
►
Separate
opposing traffic
Drainage
Aesthetics
Space for future
lanes
Recovery
Access control
Minimize
headlight glare
Flexibility in Highway Design Chapter 6
31
Median Types/Geometry
Depressed (rural arterials,
6:1 preferred, 4:1 min)
Raised (urban arterials)
Flush (urban/sub. some
rural)





Double yellow to limit
access
TWLTL
Width is determined by




Function
Safety
Need for independent
design
Flexibility in Highway Design Chapter 6
32
Iowa DOT Urban Design Aids
See http://www.dot.state.ia.us/local_systems
New Construction
33
Iowa DOT Urban 3R* Guidelines
See http://www.dot.state.ia.us/local_systems
34
*Resurfacing, Restoration, Rehabilitation
Rural Design - New Construction
35