Elements of a
Typical Cross-section of
Road and Highway drainage
Transportation Engineering – I
Dr. Attaullah Shah
Road
• A way or path over which cyclists,
vehicles and pedestrians can pass
lawfully.
• Roads are normally used for
transportation within a country.
Advantages of Roads
• Nearest to the man, as for going to airport,
harbor or railway station.
• Can be used by all types of vehicles from
cycles to trailers.
• Can lead to any remote area and road
users have freedom of movement.
• Vehicle movements are not time bound,
roads are open to traffic for 24 hours.
Typical Road Cross-Section
Typical Road Cross-Section
Cross-Section Elements
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The cross section of a road includes some
or all of the following elements:
Traveled way
Roadway
Median
Shoulder
Kerb
Traffic Barriers
Bicycle and pedestrian facilities
Drainage channels and side slopes
Two Lane Rural Highway
Cross-Section
Urban Highway Cross-Section
Selection of appropriate
cross-section elements
In selecting the appropriate cross-section
elements and dimensions, designers need
to consider a number of factors:
• Volume and composition (percent trucks,
buses, and recreational vehicles) of the
vehicular traffic expected to use the facility
• The likelihood that cyclists and pedestrians
will use the route
• Climatic conditions
Selection of appropriate
cross-section elements
• The presence of natural or human made
obstructions adjacent to the roadway (e.g.,
rock cliffs, large trees, wetlands, buildings,
power lines)
• Type and intensity of development along the
section of the highway facility that is being
designed
• Safety of the users
The most appropriate design is the one that
balances the mobility needs of the people
using the facility (motorists, pedestrians, or
cyclists) with the physical constraints of the
corridor within which the facility is located.
Right of Way
• The right of way can be described generally
as the publicly owned area of land that
encompasses all the various cross-section
elements.
• The right of way is the land set aside for use
as a highway corridor.
• Rights of way are purchased prior to the
construction of a new road, and usually
enough extra land is purchased.
• Sometimes, rights of way are left vacant after
the initial roadway facility is constructed to
allow for future highway expansion.
Right of Way
Requirements of area for right of way
are as follows:
• For 2 lane road = 150 ft width of area
• For 4 lane road = 250 ft width of area
• For 8 lane road = 300 ft width of area
Traveled Way or Carriage Way
• The portion of the roadway provided for
the movement of vehicles, exclusive of
shoulders.
• Number of lanes on a traveled way are
decided on the basis of expected traffic
volumes and appropriate level of
service required for the facility.
Traveled Way or Carriage Way
• Lane width strongly influences traffic
safety and comfort
• Lane width ranges from 2.7-3.6 m
with 3.6 m lane predominant on hightype highways
• Two-lane two-way highways with the
3.6 m lane provide safe clearance
between large commercial vehicles
Dual Carriage Way
When traffic volumes are quite heavy,
carriageway may be divided into two
parts by providing a median strip and
each portion of the carriage way is
reserved for traffic moving in opposite
direction.
Road Way
• The portion of a highway provided for
vehicular use.
• It includes both carriageway and
shoulders.
Formation Width
• It is the sum of widths of carriage way,
shoulders and median strips if provided.
• In case of embankments, it is measured
as the top width.
• In case of cutting, it is the bottom width
of the cutting from which side drains are
excluded.
Shoulders
• Shoulders are the strips provided on both
sides of the carriage way.
Functions of shoulders
• accommodation of stopped vehicles
(disabled vehicles, bus stops)
• emergency use
• lateral support for the pavement
• space for roadside facilities
• space for bicycles and pedestrians
• driving comfort (freedom from strain)
• improvement in sight distance
• improvement in capacity
Width of shoulders
• Low-type roads -- minimum 0.6 m,
recommended 1.8-2.4 m
• Shoulder provided for bicycles -- minimum
1.2 m wide
• High-type roads -- minimum 3.0 m,
recommended 3.6 m
• Shoulders should be continuous.
• Shoulders on bridges should have the
same width as on the approach sections.
Median or Traffic Separators
• It is the physical or painted separation
provided on divided highways between
two adjacent roadways.
• Medians can also be used to isolate
slow and fast moving traffic in the same
direction.
• Width of medians ranges from
1.2 to 24 m.
Median
Function of medians
• separate opposing traffic
• recovery area for out-of-control
vehicles
• stopping area
• storage of right-turning and U-turning
vehicles
• minimize headlight glare
• provision for future lanes
Kerb
• It is the dividing line between carriageway
and footpath.
Functions of Kerbs
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drainage control
roadway edge delineation
right-of-way reduction
delineation of pedestrian walkways
reduction in maintenance operation
Assistance in roadside development
Types of Kerbs
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Class I Kerb: Height 7-9 cms
Class II Kerb: Height 15-20 cms
Barrier: 23-45 cms
Submerged: provide lateral stability
Foot Path or side Walk
• Foot paths are provided in Urban
roads
• These are raised strips constructed
along both the edges of roads.
• Their minimum recommended width
is 1.3 m.
Foot Path or side Walk
Foot Path or side Walk
Bicycle and Parking lane
• Bicycle lane is a portion of the
roadway designated by striping,
signing, and/or pavement markings
for preferential or exclusive use by
bicycles and/or other non-motorized
vehicles.
• Parking lane is an additional lane
provided on Urban roads and streets
for on-street parking.
Bicycle and Parking lane
Minimum Width Requirements
Bicycle and Parking lane
Traffic Barriers
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A longitudinal barrier, including bridge rail,
or an impact attenuator used to
Redirect vehicles from hazards located
within an established Design Clear Zone
To prevent median crossovers
To prevent errant vehicles from going over
the side of a bridge structure
To protect workers (occasionally)
To protect pedestrians, or bicyclists from
vehicular traffic
Traffic Barriers
Traffic Barriers
Drainage Channels and
Side slopes
• Drainage channels and side slopes are
provided along the length of road for storm
water drainage etc.
Drainage Channels and
Side slopes
Drainage channels should:
• have adequate capacity for the design
runoff,
• minimize damage to the highway caused
by unusual storm water,
• minimize risk for motorists,
• be resistant to the high speed water flows
where expected,
• prevent sedimentation of the particles
carried by water.
Drainage Channels and
Side slopes
Side slopes should
• insure the stability of the roadway
• provide opportunity for recovery of an outof-control vehicle
Part 2
Highway Drainage
Transportation Engineering - I
Highway Drainage
• A means by which surface water is
removed from pavement and ROW
• Redirects water into appropriately
designed channels
• Eventually discharges into natural
water systems
Inadequate Drainage
• Damage to highway structures
• Loss of capacity
• Visibility problems with spray and
retro-reflectivity
• Safety problems, reduced friction and
hydroplaning
Highway Drainage
• Transverse slopes
– Removes water from pavement surface
– Facilitated by cross-section elements
(cross-slope, shoulder slope)
• Longitudinal slopes
– Minimum gradient to maintain adequate
slope in longitudinal channels
• Longitudinal channels
– Ditches along side of road to collect
surface water after run-off
Transverse slope
Longitudinal slope
Longitudinal channel
Drainage System
Three phases
1. To Estimate the quantity of water to
reach the system
2. Hydraulic design of system elements
3. Comparison of different materials to
serve the purpose
Steep slopes provide good hydraulic
capacity and lower ROW costs, but
reduces safety and increases
erosion and maintenance costs
Hydrologic Analysis
Q = CIA (english) or Q = 0.0028CIA (metric)
Q = runoff (ft3/sec) or (m3/sec)
C = coefficient representing ratio or runoff to
rainfall
I = intensity of rainfall (in/hour or mm/hour)
A = drainage area (acres or hectares)
Transverse Slope
Undivided traveled ways (two- and multilane)
on tangents and flat curves have a crown in the
middle and slope downward toward both edges
(camber). The downward cross slope may be a
plane or rounded section (parabolic), or a
combination of the two.
One-way traveled ways on divided highways
may be crowned separately or may have a
unidirectional cross slope/cross fall.
Transverse Slope
Transverse Slope
Transverse Slope
Crowns vs. Unidirectional Slopes
Type of
Roadway
Pros
Cons
Crowned
separately
•rapid drainage during
rainstorms
•difference between low
and high points is
minimal
•inlets and underground
drainage (drainage towards
the median)
•difficult design of at-grade
intersection elevation
•use of such sections
should be limited to regions
with high rainfall
Unidirectional
cross slopes
•more comfortable for
drivers changing lanes
•drainage away from the
median saves inlets and
drains
•simplifies treatment of
intersections
•drainage is slower
•difference between low
and high points of the cross
section is larger
Cross Slopes on Tangents
Contradictory design controls
A steep lateral slope reduces water ponding and the width of the
water flow along the curb.
A flat lateral slope reduces vehicles' drift towards the low edge.
Recommended design controls
Lateral drift of vehicles at high speed is barely perceptible on
cross slopes up to 2%. The slope of 1.5-2.0% is acceptable on
high-speed highways. In the areas of intense rainfall a
maximum cross slope is 2.5%.
Crown section
Change in the cross slope of 3-4% causes swaying of high body
vehicles. Rounded crowns reduce discomfort.
Curbed Highways
The minimum slop values of 1.5-2 % in areas with
intense rainfalls will cause wide sheet of water on
the curbed traveled way.
Possible improvements:
• parabolic cross section with increasing cross slope
towards the outer edges,
• gutter along the curb with the cross slope larger
than on the traveled way,
• on multilane traveled way, cross slope broken
along traffic lane edges, increasing from the
minimum value on the innermost lane up to the
maximum value on the outermost lane. This
solution is used on uncurbed sections as well.
Drainage Channels and
Sideslopes
Design considerations of highway drainage
includes
• safety
• good appearance
• control of pollutants
• economy in maintenance
This can be achieved by applying
• flat side slopes
• wide drainage channels
• rounding
Drainage Channels
Types of Drainage Channels
Intercepting
channel
Flume
Toe-of-slope
channel
Roadside
channel
Drainage Channels
Drainage channels should:
• have adequate capacity for the design
runoff,
• minimize damage to the highway caused
by unusual storm water,
• minimize risk for motorists,
• be resistant to the high speed water flows
where expected,
• prevent sedimentation of the particles
carried by water.
Side slopes
Side slopes should:
• insure the stability of the roadway
• provide opportunity for recovery of an out-of-control
vehicles
Roadside Channels
• Steep sides improve hydraulic efficiency and reduce right of
way costs
• Flatter sides improve slope stability and traffic safety, reduce
maintenance costs
• Side slopes 1:4 or flatter provides a good chance of recovery
for errant vehicles and relax drivers' tension (roadside channel
is visible to drivers)
• Side slopes of 1:5 or 1:6 are recommended in the flat areas
• Intercepting channels have a flat cross section form by a dike
made with borrow material
• Median drainage channels are shallow depressed areas with
inlets
• Flumes are open channels or pipes used to connect
intercepting channels or shoulder curbs with roadside channels
• Channel lining prevents channels erosion caused by fast
stream of water.
Examples: grass (where possible), concrete, stone etc.
Side slopes
Safety consideration
• Rounded hinge point reduces the chance of an errant vehicle
becoming airborne
• Fore slopes 1:6 or flatter can be negotiated by errant vehicles
• Fore slopes 1:3 with liberal rounding provide a good chance
for recovery
• Slopes steeper than 1:3 can be used only where justified by
local conditions. The use of roadside barriers should be
considered
Maintenance consideration
• Flat and well-rounded side slopes simplify establishment of
turf and its maintenance
• Slopes 1:3 or flatter enable the use of motorized equipment
Side slopes
Other rules
Flat, well-rounded side slopes create a streamlined
cross section. Advantages for the streamlined
cross sections are:
• natural, pleasant appearance,
• improved traffic safety,
• snow drift prevented,
• easy maintenance.
Retaining walls should be considered where slopes
would be steeper than 1:2.
Standard slope for rock cuts is 2:1. In good-quality
rock, slopes ranges from 6:1.
Sideslopes