Rule
Braking distance grows with the square of speed
Braking distance scales with the square of speed. Double your speed and braking distance quadruples.
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Lesson5 min
Braking distance quadruples: the maths of speed
Double the speed, four times the braking distance. The quick formula that matters.
You will learn, among other things:
Example
Braking distance by speed
On dry asphalt, using (speed ÷ 10)² × 0.4: • 30 km/h → about 3.6 m • 50 km/h → about 10 m • 70 km/h → about 20 m • 90 km/h → about 32 m • 110 km/h → about 48 m Reaction distance is added on top: at one second that is 9, 15, 21, 27 and 33 m for the same speeds.
Heads up
The quick calculation
Drop the last digit of the speed, multiply the number by itself, then multiply by 0.4. That is your braking distance in metres on dry asphalt. 60 km/h → 6 × 6 × 0.4 means 14 m. For the reaction distance, drop the last digit and multiply by three: 6 × 3 means 18 m. Add them for the stopping distance: 32 m.
Exam trap
On ice it multiplies
On ice the braking distance can be up to ten times the dry-road figure. At 50 km/h that means about 100 m of braking instead of 10, longer than a football pitch, from a speed that feels entirely ordinary. The reaction distance does not change. So the stopping distance at 50 km/h goes from 25 m to roughly 115 m, not to 250.