If the brakeefficiency is 100 the vehicle will stop

The efficiency of the brakes depends upon the age of the vehicle, vehicle characteristics etc. If
the brakeefficiency is 100%, the vehicle will stop the moment the brakes are applied. But
practically, it is notpossible to achieve 100% brake efficiency. Therefore the sight distance
required will be more when theefficiency of brakes are less. Also for safe geometric design, we
assume that the vehicles have only 50%brake efficiency.
Frictional resistance between the tyre and the road
The frictional resistance between the tyre and road plays an important role to bring the vehicle to
stop.When the frictional resistance is more, the vehicles stop immediately. Thus sight required
will be less. Noseparate provision for brake efficiency is provided while computing the sight
distance. This is taken intoaccount along with the factor of longitudinal friction. IRC has
specified the value of longitudinal frictionin between 0.35 to 0.4.
Gradient of the road.
Gradient of the road also affects the sight distance. While climbing up a gradient, the vehicle can
stopimmediately. Therefore sight distance required is less. While descending a gradient, gravity
also comesinto action and more time will be required to stop the vehicle. Sight distance required
will be more inthis case.
Stopping sight distance
Stopping sight distance (SSD) is the minimum sight distance available on a highway at any spot
having sufficient length to enable the driver to stop a vehicle traveling at design speed, safely
without collision with any other obstruction.
There is a term called safe stopping distance and is one of the important measures in traffic
engineering. It is the distance a vehicle travels from the point at which a situation is first
perceived to the time the deceleration is complete. Drivers must have adequate time if they are to
suddenly respond to a situation. Thus in highway design, sight distance atleast equal to the safe
stopping distance should be provided. The stopping sight distance is the sum of lag distance and

Dept. Of Civil Engg, SJBIT Page
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