Stopping distances
It is important to be able to:
- estimate how the stopping distance for a vehicle varies with different speeds
- interpret graphs relating speed to stopping distance
The diagram shows some typical stopping distances for an average car in normal conditions.
Some typical stopping distances
Travelling at 20 mph (32 km/h):
- thinking distance = 6 m
- braking distance = 6 m
- total stopping distance = 12 m
Travelling at 40 mph (64 km/h):
- thinking distance = 12 m
- braking distance = 24 m
- total stopping distance = 36 m
Travelling at 70 mph (112 km/h):
- thinking distance = 21 m
- braking distance = 75 m
- total stopping distance = 96 m
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.