Stopping Sight Distance- Highway Engineering

It is defined as minimum distance available to driver at any spot of sufficient length on highway to stop the vehicle safely without any collision with any obstruction.
It is also know as absolute sight distance. This is also called absolute minimum sight distance.
The visibility of driver depends upon following factors:

Features of the road ahead
Height of driver's eye above the road surface
Height of the object above the road surface

IRC( Indian Road Congress) recommended the height of driver as 1.2 m and height of object as 0.15m above the road surface for the purpose of measuring SSD.

Factor on which Stopping Sight depends

1.Total reaction time of driver

The total reaction time of an average driver may vary from 0.5 seconds in simple situation to 3-4 seconds in extreme conditions.

2.Speed of vehicle

More the speed, More the stopping distance is required.

3.Efficiency of brakes

The efficiency of brake is 100% means wheel is fully locked. It only result into 100% skidding. To avoid skidding, braking forces should not exceed the Frictional force between tyres and road.

4.Frictional resistance between road and tyres

It depends upon coefficient of friction 'f' between the tyres of vehicle and road. This depends upon type and conditions of thr oad surface and tyres.
IRC recommends specified designed friction coefficient value of 0.35 to 0.40 depends upon design speed.

5.Gradient of Road (if any)

Analysis of SSD

The stopping sight distance is sum of:

Lag Distance

The distance travelled by vehicle at uniform speed during total reaction time 't' (known as lag distance)= v × t where v = design speed (m/s)
t= total reaction time (if not given take t as 2.5 sec)

Braking distance

Braking distance can be obtained by equating the work done in stopping the vehicle and kinetic energy of vehicle moving at design speed

On plain ground