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Stopping distance

Stopping distance is the distance from spotting a hazard until the car stands still. It is the sum of the reaction distance (while you react) and the braking distance (while the car brakes). Reaction distance grows in step with speed, while braking distance grows with the square of speed. So a little more speed gives a much longer stopping distance.

sstop=sreaction+sbrakes_{\text{stop}} = s_{\text{reaction}} + s_{\text{brake}}stopping distance
sreaction=v⋅trs_{\text{reaction}} = v \cdot t_rreaction distance with speed in m/s
sbrake=v22as_{\text{brake}} = \dfrac{v^2}{2a}braking distance with deceleration a

Symbols

vvspeedm/s
trt_rreaction time, often about 1 ss
aadeceleration, about 7 on dry tarmacm/s²

Example

At 50 km/h, v≈13.9v \approx 13.9 m/s.

Reaction distance with 1 s: 13.913.9 m. Braking distance: 13.92/(2⋅7)≈13.813.9^2/(2\cdot 7) \approx 13.8 m.

The stopping distance is about 28 m.

Double the speed gives four times the braking distance. Divide km/h by 3.6 to get m/s.
Practise speed, distance and stopping distance for free →

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Part of Car licence (class B): Speed, distance and stopping distance.