Definition and breakdown
- Stopping distance is the total distance a vehicle travels from the instant the driver spots a hazard to the instant the vehicle finally comes to rest
- It splits into two stages:
stopping distance = thinking distance + braking distance
- Thinking distance = the distance the vehicle covers during the driver's reaction time, before the brakes are even pressed
- Braking distance = the distance the vehicle covers while it is decelerating under the braking force, from the moment the brakes are first applied to the moment it stops
Reaction time and thinking distance
- Reaction time is the time gap between a driver noticing a hazard and starting to act on it
- A typical adult driver's reaction time is about 0.7 s (roughly two-thirds of a second) when alert and undistracted. This is the value the UK Highway Code uses for its stopping-distance tables
- Thinking distance grows with reaction time and with vehicle speed:
- longer reaction time → more time before the brakes are pressed
- higher speed → more metres covered during that reaction time
- Reaction time is lengthened by:
- tiredness
- distractions such as using a phone, eating, or fiddling with controls
- alcohol or drugs
Braking distance
- The braking distance counts every metre the vehicle still rolls forward from the moment the brakes first bite until it finally comes to rest
- For a given braking force, braking distance grows with:
- vehicle speed, because a faster vehicle has more kinetic energy that the brakes must remove
- vehicle mass, because heavier vehicles need more work done by the brakes to decelerate
- road conditions, because wet, icy or oily roads reduce the friction between tyre and road, lengthening the braking distance
- brake and tyre condition, because worn brakes or bald tyres also lengthen braking distance
Speed dominates
| Speed / mph | Speed / m/s | Approximate stopping distance / m |
|---|
| 20 | 9 | 12 |
| 30 | 14 | 23 |
| 40 | 18 | 36 |
| 50 | 22 | 53 |
| 60 | 27 | 73 |
- Doubling the speed roughly quadruples the braking distance because braking distance depends on the square of the speed (consistent with v² = u² + 2as)
Example — a vehicle is moving at 22 m/s when the driver spots a child in the road. The stopping distance from that instant is measured as 65 m, of which 18 m is thinking distance. Calculate the braking distance.
- Rearrange stopping distance = thinking + braking → braking = stopping − thinking
- Braking distance = 65 − 18 = 47 m