In an emergency, a driver must bring their vehicle to a stop in the shortest distance possible:
stopping distance = thinking distance + braking distance
This is when:
Reaction times vary from person to person, but is typically between 0.2 s and 0.9 s. A driver's reaction time can be affected by:
Longer reaction times increase the thinking distance when stopping from a given speed.
There are different ways to measure reaction times. One simple method involves dropping a ruler between someone's open thumb and forefinger. The higher the reaction time needed to grasp the falling ruler, the further the ruler falls before being stopped. The ruler can be adjusted to measure in seconds rather than in millimetres.
The braking distance of a vehicle can be affected by:
The faster a vehicle travels, the greater the braking force needed to stop it in a certain distance. A greater braking force produces a greater deceleration. Large decelerations may cause the brakes to overheat and the driver may also lose control of the vehicle.
It is important to be able to:
The diagram shows some typical stopping distances for an average car in normal conditions.
Travelling at 20 mph (32 km/h):
Travelling at 40 mph (64 km/h):
It is important to note that the thinking distance is proportional to the starting speed. This means that it increases proportionally as speed increases, ie if speed doubles, thinking distance also doubles. However, the braking distance increases by a factor of four each time the starting speed doubles.
For example, if a car doubles its speed from 30 mph to 60 mph, the thinking distance will double from 9 m to 18 m and the braking distance will increase by a factor of four from 14 m to 56 m.