Cross Section Elements (1)
CE562
Text: A Policy on Geometric Design, pp.305 - 375
Pavement
Surface Types
High-type pavement has smooth riding qualities, good skid-
resistance properties in all weather, long life span without
fatigue failure.
Low-type surface ranges from surface-treated earth roads to
loose surface such as earth, crashed stone, gravel. It requires
greater steering effort.
Intermediate-type surface ranges from surface treatments to
pavements only slightly lower quality than high-type
pavements.
High traffic volume and high design speed warrant the use of
the high-type surface type. Low traffic volumes and low design
speed warrant the use of the low-type surface.
Pavement
Cross Slopes
Undivided traveled ways (two- and multilane) on tangents and
flat curves have a crown in the middle and slope downward
toward both edges. 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 (almost
always downward to the outer edge).
Exhibit 4-2
Exhibit 4-3
Crowns vs. Unidirectional Slopes
Divided Roadways
Type of Pros Cons
Roadway
Crowned •rapid drainage during •inlets and underground drainage
separately rainstorms (drainage towards the median)
•difference between low and •difficult design of at-grade
high points is minimal intersection elevation
•use of such sections should be
limited to regions with high rainfall
Unidirectional •more comfortable for drivers •drainage is slower
cross slopes changing lanes •difference between low and high
•drainage away from the median points of the cross section is larger
saves inlets and drains
•simplifies treatment of
intersections
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. Constructing transition cross-sections
from tangent to superelevated pavement may be difficult.
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.
Lane Widths
• Lane width strongly influences traffic safety and comfort
• Lane width range is 2.7-3.6 m with the 3.6 m lane predominant
on high-type highways
• Two-lane two-way highways with the 3.6 lane provide safe
clearance between large commercial vehicles
Circumstances that justify narrower lanes
• Urban areas with land restrictions -- 3.3 m
• Low speed facilities -- 3.0 m
• Auxiliary lanes at intersections -- 3.0 m
• Low-volume roads in rural and residential areas -- 2.7 m
• Continuous two-way left-turn lanes -- 3.0-4.8 m
Anything wrong
with this cross section?
Shoulders
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
Graded shoulder = between the edge of the traveled way and the
intersection of the shoulder slope and the foreslope plane
Usable shoulder = part of the graded shoulder that can be used by the
stopped vehicle
Exhibit 4-3
Shoulders
Shoulder can be surfaced by using:
turf, gravel, crushed rock, mineral or chemical additives, bituminous
treatment, asphalt or concrete pavements.
Functions of shoulders on low-type vs. high-type roads
Width of shoulders
Low-type roads -- minimum 0.6 m, recommended 1.8-2.4 m,
Shoulder usable by bicycles -- minimum 1.2 m,
High-type roads -- minimum 3.0 m, recommended 3.6 m.
Clearance to roadside elements (barriers, walls, signs, etc.)
High-type roads -- 0.6 m to the edge of the usable shoulder,
Low-type roads -- minimum 1.2 m to the edge of the traveled way.
Shoulders
Other rules
• Shoulders should be continuous.
• Shoulders on bridges should have the same width as on the
approach sections.
Cross slopes
• Bituminous and concrete shoulders -- 2-6 %,
• Gravel and crushed rock shoulders -- 4-6 %,
• Turf shoulder -- about 8 %.
Clear zones
Continuous and clear of exposed obstructions – uncurbed high-speed
roads governed by AASHTO Roadside Design Guide, - uncurbed
low-speed roads – 3.0 m, curbed roads – 0.5 m.
Shoulders
Shoulder stability
• All types of shoulders must be flush with the traveled way. Unstabilized
shoulders need regular maintenance.
• Paved or stabilized shoulders reduce maintenance and improve lateral
support for the pavement of the traveled way.
• In the most firm solution, the whole roadway is surfaced including
shoulders.
Shoulder contrast
• Different color and surface of the shoulder discourage drivers from
using it as a traveled way. Bituminous, crushed stone, gravel and turf
shoulders offer excellent contrast with concrete pavements. Stone
aggregates and turf offer good contrast with bituminous pavements.
• Edge lines should be used when the whole roadway is paved.
• Depressed rumble strips alert drivers that cross over onto the shoulder
but they make the shoulder useless for bicycles.
Curbs
Functions of curbs
• drainage control
• roadway edge delineation
• right-of-way reduction
• delineation of pedestrian walkways
• reduction in maintenance operation
Types of curbs:
Vertical curbs -- discourage vehicles from leaving the roadway,
not desirable on high-speed highways, desirable on urban
roads, and along long walls, tunnels to protect safety walks.
Sloping curbs -- vehicle can cross them readily when required,
used at median edges, to outline channelizing islands, at the
outer edge of the shoulder.
Vertical Curb
Sloping Curbs
Exhibit 4-6
Sidewalks
Sidewalks are an integral part of city roads, sometimes provided in
rural areas.
A border area (minimum 2.4 m) separates the roadway from the
homes and businesses. The border provides space for sidewalks. In
residential areas the sidewalk width varies between 1.2 to 2.4 m.
The side walk should be separated from the traveled way with a curb
and planted strip of minimum 0.6 m width. Sidewalks covering the
full border are justified in commercial areas, near schools, and other
pedestrian generators. Sidewalks built along a rural highway must
be well removed from the traveled way.