= the distance the vehicle covers during the driver's reaction time, before the brakes are even pressed
= 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 , 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
13
23
40
18
36
50
22
53
Doubling the speed roughly quadruples the braking distance because braking distance depends on the square of the speed (consistent with v² = u² + 2as)
Exam tip
Effect of a factor on thinking distance vs braking distance
What comes up: "Explain the effect, if any, of [factor] on the thinking distance and on the braking distance."
Write (two marks each component, four marks total): For thinking distance — state whether it increases or decreases, then link it to a change in the driver's reaction time. For braking distance — state there is no effect, and explain that braking distance depends on the braking force and road conditions, not on the driver's reaction time.
Watch out: tiredness lengthens reaction time and therefore increases thinking distance, but has no effect on braking distance. Confusing the two components in an answer (saying tiredness increases braking distance) scores zero for the braking distance part.