Stopping distances for a typical car on a dry road are given in the Highway Code. They show how the distance changes over a range of normal speeds. Each pair below is thinking distance plus braking distance.
- 20 mph: 6 m + 6 m = 12 m
- 30 mph: 9 m + 14 m = 23 m
- 40 mph: 12 m + 24 m = 36 m
- 50 mph: 15 m + 38 m = 53 m
- 60 mph: 18 m + 55 m = 73 m
- 70 mph: 21 m + 75 m = 96 m
The thinking distance is directly proportional to speed: when the speed doubles from 20 mph to 40 mph, it doubles from 6 m to 12 m. The braking distance rises much faster. From 20 mph to 40 mph it goes from 6 m to 24 m, which is 4 times as far.
This means the stopping distance grows faster than the speed does. These values are for a dry road and an alert driver, so in wet weather or when tired the real distances are greater. You can use the figures to estimate the distance at a speed between the ones listed.