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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.

**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 tra c engineering. It is the distance a vehicle
travels from the point at which a situation is rst 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 the braking distance. Lag distance is
the distance the vehicle traveled during the reaction time t and is given by

vt, where
v is the velocity in m=sec^{2}. Braking distance is the distance
traveled by the vehicle during

braking operation. For a level road this is obtained by
equating the work done in stopping the vehicle and the kinetic energy of the
vehicle. If F is the maximum frictional force developed and the braking
distance is l, then work done against friction in stopping the vehicle is F l =
f W l where W is the total weight of the vehicle. The kinetic energy at the
design speed is

Therefore, the
SSD = lag distance
+ braking distance
and given by:

where v is the design speed in m=sec^{2}, t is the
reaction time in sec, g is the acceleration due to gravity and f is the
coefficient of friction. The coefficient of friction f is given below for
various design speed.

When
there is an ascending gradient of say +n%, the component of gravity adds to
braking action and hence braking distance is decreased. The component of
gravity acting parallel to the surface which adds to the braking force is equal
to W sin W tan = W n=100. Equating kinetic energy and work done:

Similarly
the braking distance can be derived for a descending gradient.

Tags : Civil - Highway Engineering - Geometric Design Of Highways

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