Longitudinal section in roads
Longitudinal section is required for the alignment
of the highway through obligatory points.
Longitudinal section is used for the construction of
horizontal and vertical cuve.
Cross Section
These are lives of levels or short profile perpendicular
to centre line of the road.
Cross section is required for the provision of camber,
drainage pipes and for the design of pavement
sections like subgrade, sub-base course, base
COurse, surface course.
Vertical curve: Due to changes in grade in the vertical
alignment of highway, it is necessary to introduce vertical
Curve at the intersections of different grades to smoothen
out the vertical profile and thus ease off the changes in
gradient for the fast moving vehicles.
Note: Gradient is the rate of rise or fall along the length
of the road with respect to the horizontal.
Horizontal curve: A horizontal highway curve is a curve
in plan to provide change in direction to the central line
of a road.
Transition curve: When a vehicle travelling on a straight
road (ie. infinite radius) enters a curve of finite radius.
it is suddenly subjected to the centrifugal force which
causes shock and sway. In order to avoid this, It is
Customary to provide a transition curve at the beginning
of the circular curve, having a radius equal to infinity at
the end of the straight and gradually reducing the radius
to the radius of circular curve where the curve begins
Stopping sight distance (SSD): It is the minimum
sight distance available on a highway to enable the
driver to stop a vehicle travelling at design speed, safely
without collision with any other obstruction.
The computation of sight distance depends on:
() Reaction time of driver
Reaction time of a driver is the time taken
from the instant the object is visible to the
driver to the instant when the brakes are
applied.
IRC suggest a total reaction time of 2.5 se
(ii) Speed of the vehicle: Higher the speed, more
sight distance will be required to stop the vehicle.
(iii) Efficiency of brakes:
Sight distance required will be more, when the
efficiency of brakes are less which depends
on the age of the vehicle.
For safe geometric design, we assume that
the vehicles have only 50% brake efficiency.
(iv) Frictional resistan ce between the tyre and the
roads:
When the frictional resistance is more, the
vehicle stop immediately.
IRC has specified the value of longitudnal
friction is between 0.35 to 0.4.
(v) Gradient of the road:
. While climbing up a gradient, the vehicle can
stop immediately. Therefore, sight distance
required is less.
. While descending a gradient, gravity also
comes into action and so more sight distance
will be required to stop the vehicle.
There are five objectives for providing transition
curve and are given beloW.
- To introduce gradually the centrifugal force between
the tangent point and the beginning of the circular
Curve, avoiding sudden jerk on the vehicle. This
increases the comfort of passengers. - To enable the driver turn the stearing gradually for
his own comfort and security.
To provide gradual introduction of super elevation. - To provide gradual introduction of extra widening.
Roughometer
Roughometer is a high speed device used for
measuring pavement surface roughness.
The output is in the units of international roughness
index (RI)
Bump Integrator
It is an automatic road unevenness recorder, an
indigenous device developed by (RRI).
Bump integrator measures pavement surface
condition interms of uneveness index.
Uneveness index is a measure of uneveness which
is the cumulative measure of vertical undulation of
the pavement surface recorded per unit horizontal
length of the road.
An uneveness index value less than 1500 mm/km
is considered as good, a value less than
2500 mm/km is satisfactory upto speed of 100kmph
and value greater than 3200mm/km is considered
as uncomfortable even for 55 kmph.
Speedometer: A speedometer ‘or’ speed meter is
a gauge that measures and displays the
instantaneous speed of a vehicle.
At stretch of the road where required overtaking
sight distance can not be provided, then
intermediate sight distance is provided which is
equal to twice of SSD.
Overtaking sight distance
It is the minimum distance open to the vision of
the driver of a vehicle intending to overtake the
slow vehicle ahead safely against the traffic in the
opposite direction.
The overtaking sight distance or passing sight
distance is measured along the centre line of the
road over which a driver with his eye level 1.2 m
above the road surface can see the top of an
object 1.2m above the road surface.
The factors that affect the OSD are:
Velocity of the overtaking vehicle, overtaken
vehicle and of the vehicle coming in the opposite
direction.
Spacing between vehicles, which in-turn
depends on the speed.
Skill and reaction time of the driver.
Rate of acceleration of overtaking vehicle
Gradient of the road
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