The primary
consideration in the design of geometric cross sections for highways are ,
run-ways,and taxiways is drainage. Details
vary depending on the type of facility Highway cross sections consist of traveled way,
shoulders (or parking lanes),
and drainage channels. Shoulders are intended primarily as a safety feature.They provide for accommodation of
stopped vehicles, emergency use, and lateral support of the pavement.Shoulders may be either paved or unpaved.
Drainage channels may consist of ditches (usually grassed swales) or of paved shoulders with berms or curbs and gut-ters. Cross section of
various roads are given bellow.
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| Two-lane highway cross section, with ditches.
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Divided highway cross section, depressed median, with ditches
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Pavement surface
characteristics
For a safe and comfortable
driving four aspects of the pavement surface are important; the friction between the wheels and
the pavement surface, smoothness of the road surface, the light re action characteristics of the top of pavement
surface, and drainage to water.
Friction
Friction between the wheel and
the pavement surface is a crucial factor in the design of horizontal curves and thus the
safe operating speed. Further, it also act as the acceleration and deceleration ability of vehicles.
Lack of adequate friction can cause skidding or slipping of vehicles.Skidding happens when the path
traveled along the road surface is more than the circumferential movement of the
wheels due to friction Slip occurs when the wheel
revolves more than the corresponding longitudinal movement along the road. Various factors
that act on friction are:
The frictional force that
develops between the wheel and the pavement is the load
acting multiplied by a factor called the coefficient of friction and denoted as of The choice of the value of friction is a very complicated issue
since it depends on many variables. IRC suggests the coefficient of longitudinal friction as 0.35-0.4
depending on the speed and coefficient of later
friction as 0.15. The former is useful in sight distance calculation and the latter in horizontal
curve design.
Unevenness
It is always desirable to have an
even surface, but it is seldom possible to have such one. Even if a road is constructed with high
quality pavers, it is possible to develop unevenness due to pavement failures. Unevenness a
ect the vehicle operating cost, speed, riding comfort, safety,fuel consumption and wear and
tear of tyres. Unevenness index is a measure of
unevenness which is the cumulative measure of vertical undulation of the pavement
surface recorded per unit horizontal length of the road. An unevenness index value less than
1500 mm/km is considered as good, a value less than 2500 mm.km is satisfactory up to speed
of 100 kmph and values greater than 3200 mm/km is considered as uncomfortable even
for 55 kmph.
Light reflection
Drainage
The pavement surface should be
absolutely impermeable to prevent seepage of water into the pavement layers. Further, both
the geometry and texture of pavement surface should help in draining out the water from the surface in less
time.
Camber
Camber or cant is the cross slope
provided to raise middle of the road surface in the transverse direction to drain o rain water
from road surface.Too steep slope is undesirable
for it will erode the surface. Camber is measured in 1 in n or n% (Eg. 1 in 50 or 2%) and the value
depends on the type of pavement surface.
Width of
carriage way
Width of the carriage way or the
width of the pavement depends on the width of the traffic lane and number of lanes. Width of a
traffic lane depends on the width of the vehicle and the clearance. Side clearance
improves operating speed and safety.
Kerbs
Kerbs indicate the boundary
between the carriage way and the shoulder or islands of footpaths.Different types of kerbs are
Low or mountable kerbs :
Semi-barrier type kerbs :
Barrier type kerbs :
Road margins
The portion of the road beyond
the carriageway and on the roadway can be generally called road margin. Various elements that
form the road margins are given below.
Shoulders
Parking lanes
Bus-bays
Service roads
Cycle track
Footpath
Guard rails
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