Gradient of the road also affects the sight distance. While climbing up a gradient, the vehicle can stop immediately. Therefore the sight distance required is less. While descending a gradient, gravity also comes into action and more time will be required to stop the vehicle. Sight distance required will be more in this 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 at least 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/sec2 .

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 Fl = fWl where W is the total weight of the vehicle. The kinetic energy at the design speed is

Factors governing stopping sight distance

•  Total reaction time of driver

• It is the time taken by instant by the driver to identify the visible object

• The further time is divided into different components

• ‘Perception time’ is the time sensations received by the eye or ears of the driver to transfer to the brain through the spinal cord.

• ‘Intellection time’ is the time required for the driver to understand the situation. It is also the time required for comparing the different thoughts, regrouping, and registering new sensations.

• ‘Emotion time’ is the time elapsed during emotional sensations and other mental disturbances such as fear, anger, or any other emotional feelings like superstition, etc. with reference to the situation. The emotion time varies for different drivers, but even for a particular driver, the emotion time is likely to vary considerably depending upon the situation or the actual problem involved.

• ‘Volition time’ is the time taken by the driver for the final action, such as brake application.

• The 'PIEV' process has been illustrated in

It is also possible that the driver may apply brakes or take any other avoiding action like turning, by the 'reflex action', without the normal thinking process, which is probably the minimum time for taking a preventive action like brake application.

The PIEV time of a driver also depends on several factors such as the physical and psychological characteristics of the driver, type of problem involved. environmental conditions and temporary factors (eg. motive of the trip, travel speed, fatigue, consumption of alcohol, etc.).

The total reaction time of an average driver may vary from 0.5 seconds for simple situations to as much as 3 to 4 seconds or even more in complex problems.

 India (IRC) 2.5 Sec Australia National Association of Australian State Road Authorities (NAASRA) 2.5 sec Speeds more than 100 km/h 2.0 sec Speeds less than 100 km/h 1.5 sec Restricted situations and difficult terrain Austria 2.0 sec Britain 2.0 sec Canada Similar to the U.S. policy France 2.0 sec Germany 2.0 sec For rural roads 1.5 sec For urban streets Greece 2.0 sec For rural roads 1.5 sec For urban streets South Africa 2.5 sec Sweden Swedish national road administration (SNRA) 2.0 sec Switzerland 2.0 sec United States 2.5 sec t
•  It is also possible that the driver may apply brakes or take any other avoiding action,like turning, by the reflex action, without normal thinking process that is observed to be min time for avoiding the collison.

• This reaction time depends on several factors. The driver still, type of obstruction involved, environmental condition, mental health of the driver shift, type of obstruction involved, environmental conditions, age , mental health.