Wednesday, February 19, 2014

Factors affecting the Stopping sight distance


Factors effecting the Stopping sight distance

As per IRC(Indian Roads Congress), Stopping sight distance is such a distance along the center line of the road that if there is an obstruction of 0.15 m at the other end, it 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, effect the sight distance.

  1. The total reaction time of the driver
  2. Speed of the vehicle
  3. Friction between the Tyre and the pavement surface
  4. Break efficiency
  5. Gradient of the road 
  • The total reaction time of the driver: 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. 
 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 traveled and more will be the stopping sight distance.
Stopping sight distance can be phased into two parts: 1) Perception time 2) 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, friction of the tyre and road surface and the brake efficiency.

PIEVE theory is used to analyse the total reaction time. According to this theory total reaction time of a drive is phased into four classes:

  1. Perception time
  2. Intellect time
  3. Emotion time
  4. Volition time
  • Speed of the vehicle: Speed of the vehicle affects the distance traveled by the driver in the total reaction time, more the speed more will be the distance.  This is known as lag distance.
Similarly,the distance traveled by the driver after the application of the brakes. More the speed more will the braking distance.

  • 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.
  • 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. Efficiency of the brakes are considered by reducing the original value of the friction in a range of 0.35 to 0.40.
  • Gradient of the road: Gradient may be positive or negative and accordingly the required stopping sight distance will be less and more respectively. In case of upward(positive) gradient, a component of the gravity force will help in stopping the vehicle.
In 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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