As per IRC(Indian Roads Congress), Stopping sight distance is such a distance, along the centreline of the road, that any obstruction of 0.15 m, at the other end, must become visible to the eye of a driver at a height of 1.2 m from the road surface.
In order to analyse the stopping sight distance which is required to design a given highway, we have to consider the following factors which, affect the sight distance.
- The total reaction time of the driver
- Speed of the vehicle
- Friction between the Tyre and the pavement surface
- Break efficiency
- The gradient of the roadÂ
(1) The total reaction time of the driver
The total reaction time of a driver is the time from the instance the obstruction is visible to the driver to the instance when he effectively applies the break.Â
 In the total reaction, time vehicle moves at the speed at which the driver is moving or taken as the design speed. So, if the total reaction time of the driver is more, more will be the distance travelled and more will be the stopping sight distance.
Total time required for stopping within the stopping sight distance can be phased into two parts – a) Perception time b) Brake reaction time
Perception time is the time taken by the driver to realize that there is an obstruction on the road.
Brake reaction time is the time taken by the driver to effectively apply the brakes. This depends upon the skills of the driver, the friction of the tyre and road surface and the brake efficiency.
Now, there is this PIEVE theory, according to which total reaction time of a drive is phased into four classes: i) Perception time ii) intellect time iii)Emotion time iv)Volition time
(2) Speed of the vehicle
Speed of the vehicle affects the distance travelled by the driver within the total reaction time. The more the speed, the more will be the distance. Â This is known as the lag distance. Similarly, the distance travelled by the driver after the application of the brakes depends on the initial velocity, braking efficiency and friction. More the initial speed more will be the braking distance.
(3) The Friction between the Tyre and the road surface
The friction between the Tyre and the road surface depends upon the type of road surface and the condition of the tyre. Also, it depends upon the speed of the vehicle. More the friction, less will the stopping sight distance required but, if less is the friction, more it will be.
(4) Brake Efficiency
100% brake efficiency means the rotation of the Tyre is completely locked, but it will surely result in the skidding of the vehicle. The efficiency of the brakes is considered by reducing the original value of the friction in a range of 0.35 to 0.40.
(5) The Gradient of the road
The gradient may be positive or negative and accordingly, the required stopping sight distance will be less and more respectively. In the case of the upward(positive) gradient, a component of the gravity force will help in stopping the vehicle.
In the case of the downward gradient, the component of the gravity will be in opposite direction to the direction of the friction force so this will increase the stopping sight distance.
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good simple good explanation