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What is Sight Distance And Stopping Sight Distance? ( SSD )

What is Sight Distance And Stopping Sight Distance? ( SSD )

Sight Distance:

sight distance is the length of road visible to the driver at any instant so he can safely stop his vehicle or overtake another vehicle.

Sight distance requirement is needed in the design of vertical curves and it also governs the set back distances of buildings or any other obstructions adjacent to the carriageway on a horizontal curve.

Sight distances considered by IRC in highway design are:
(i) Stopping Sight Distance (SSD) or absolute minimum sight distance
(ii) Overtaking Sight Distance (OSD) or Passing sight distance
(iii) Safe Sight Distance required for entering in an intersection
(iv) Intermediate Sight Distance (ISD)
(v) Head Light Sight Distance (HSD)

Sight distance on a highway depends upon the following factors:

(1) Speed of Vehicle: The speed of a vehicle is directly related to the sight distance. More will be the speed of the vehicle, more will be the sight distance required.

(II) Driver’s Reaction Time: As we discussed earlier, reaction time is the time taken by the driver from the instant of seeing the object to the instant when brakes are applied. Total reaction time is measured on the basis of PIEV theory which varies from 0.5 seconds for simple situations to 3 – 4 seconds for complex situations.

Remember: IRC recommends a reaction time of 2.5 seconds for stopping sight distance and 20 seconds for
overtaking sight distance calculations.

(iii) Brake’s Efficiency: The efficiency of brakes depends upon the age and characteristics of the vehicle. If we say the brakes are 100% efficient, it means the vehicle will stop at the moment of application of brakes. But practically 100% efficiency of brakes is not achieved, otherwise skidding will occur which is uncontrollable and dangerous for the vehicle and road users.

(iv) Gradient of Pavement: When we are going down on a gradient, the gravitational force also comes into action which causes the vehicle to lake more time to stop the vehicle means more sight distance is required. While climbing up a gradient less Sight distance is required because the time taken to stop the vehicle will be less.

(v) Frictional Resistance: Less distance is required by a vehicle to stop when frictional resistance is more, but more friction will cause more wear and tear in the tire which is not beneficial for the vehicle. That’s why IRC recommends the value of longitudinal friction between 0.35 to 0.40.

Coefficient of longitudinal friction:


Speed (kmph) 20-30 40 50 60 65 80 100
coefficient of friction, f
0.40 0.38 0.37 0.36 0.36 0.35 0.35


Stopping Sight Distance:

The stopping sight distance is defined as the minimum sight distance that should be available and visible in front of the driver so that he can safely stop the vehicle before collusion.

The stopping sight distance depends upon:

(i) the road feather.
(ii) height of the driver above the road.
(iii) height of the object above the road.

Note:- As per IRC the height of the driver’s eye above the road is 1.2 m and the height of the object is 0.15 m above the road surface.

Stopping sight distance is composed of two components:

(i) Lag Distance (d):

It is the distance traveled by the vehicle in total reaction time if v is the design speed in m/s and t is the total reaction time in a sec, then

lag distance (d) = vt metres

If V is design speed in kmph and t in seconds then,
d1 = 0.278 Vt metres

v= design Speed in kmph

t= total reaction time in sec.

(ii) Braking Distance (l):

It is the distance traveled by a vehicle after the application of brakes. It is obtained by equating the work done in stopping the vehicle and kinetic energy stored in the vehicle. Let us consider, Fis the maximum frictional force developed and/is the braking distance, then the
work done against friction in stopping the vehicle will be Fl.

d2 =  (0.278 Vkmph)²/2gυ

g= acceleration due to gravity in m/s²

υ= coefficient of friction

Stopping Sight Distance = d1+d2

= 0.278 Vt + (0.278 Vkmph)²/2gυ


Braking Distance on an Inclined Surface:

braking distance

SSD = 0.278 Vt + (0.278 Vkmph)²/2g(υ±n)

n= gradients

Intermediate Sight Distance: In this case, the minimum sight distance between two vehicles is known as the intermediate sight distance and it is equal to twice the stopping sight distance.

The intermediate sight distance is provided for a single-lane road with 2-way traffic in order to provide limited overtaking opportunities.

Headlight Sight Distance:

It is defined as the minimum sight distance that should be visible ahead to the driver while traveling during right so that he can safely stop his vehicle before the collision.

headlight sight distance equal to the SSD.





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