Stopping Sight Distance

Stopping Sight Distance or Non-Passing Sight Distance

Stopping Sight Distance is the minimum distance available on a highway at any spot having sufficient length enable the driver to stop a vehicle without collision with any obstruction. Stopping sight distance should be provided throughout the length of all roads.  

Assumption:

• Height of driver eyes = 1.2 m
• Minimum height of obstruction = 0.15 m
• Brake efficiency = 100 %

1. Time when driver seen the obstruction

2. Break applied by driver 

Lag distance is the distance travelled by the vehicle in, total reaction time, if V is the design speed in km/hr and tr is the total reaction time in sec, then 

Lag Distance = 0.278 V tr (km/hr Sec)  

3. Vehicle completely stopped

Braking distance is the distance travelled by 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. Lets consider, F is the maximum frictional force developed and L is the braking distance, then the workdone against friction in stopping the vehicle will be FL.

Friction force, F = f W

therefore, 

Workdone in stopping the vehicle, = f W L

Kinetic energy stored in vehicle, = ( mv² ) / 2

but, W = mg

therefore, 

Kinetic Energy stored in vehicle = ( Wv² ) / 2 g

hence, 

f W L = ( Wv² ) / 2 g

L =  v² / 2 g f

therefore, 

Stopping Sight Distance = Lag Distance  + Braking Distance 

Stopping Sight Distance=  V tr + v² /2gf (v is in m/s, g is acceleration in m/s²)

Stopping Sight Distance   = 0.278 V tr +  V ² / 254 f  (V is in km/hr)

Reaction Time (tr):

Reaction time is calculated by using PIEV theory, where P is Perception which is time required for transmission of the sensations received through eyes, ears and body to the brain and nervous system, I is Intellection is the formation of new thoughts and ideas and time required for understanding the situation, E is Emotion based on the situations like fear or anger and V is for Volition is the final decision sent by the brain to the muscle and this actual act of taking a decision to produce action is done. 

As per IRC recommendation, 

Sr. No	Conditions	tr (sec)
1	Stopping Sight Distance	2.5
2	Overtaking Sight Distance	2.0
3	Minimum Space Headway	0.7

Stopping sight distance as per Indian practice,

Design Speed (kmph)	20	30	40	50	60	65	80	100
Stopping Sight Distance (m)	20	30	45	60	80	90	120	180