A.D. Tupper & Associates Limited

PRT

Picture a hazard, or other situation, which suddenly develops as a stimulus that requires an avoidance measure of braking. Once the stimulus presents, it takes the driver a moment or two brake, for he has to:

Perceive that situation as a hazard;

Identify it and decide what he is going to do; and

Then start to implement his decision.

That process of perception and reaction takes time. That period is known as the perception and reaction time or PRT. It has been widely studied and can be taken as 1.5 seconds for average people under average situations.

By definition, the PRT is that period of time that elapses from the instant that a stimulus first becomes available to be seen, until that instant when the brakes start to take effect. The distance traveled during the PRT is the perception and reaction distance, or PRD.

Braking

Once the brakes take effect, they slow the car continuously over a stretch known as the braking distance.

So, the total stopping distance is thus the sum of the PRD and the braking distance. The PRD is a function of the person, and the braking distance is a function of:

The speed at which the brakes are applied;

The vigour with which they are applied; and

The friction that develops between the tire and the road surface.

All other factors being equal, the faster a vehicle is going the further it takes to stop; the harder the brakes are applied (up to a limit) the less distance it takes to stop; the more slippery the road, the further it takes to stop.

There is a mathematical relationship between the initial speed, the rate of retardation, and the travel distance. If two of those elements are known, the third can be calculated. In reconstruction analyses, we usually know the distance traveled, we can get an assessment of the available friction by testing or from the literature, and can thus calculate the speed of the vehicle at the start of braking, and at the first point of possible perception, that is at the start of the PRD.