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Braking distance on slippery roads

How short you can stop depends on the friction between tyres and road. The friction coefficient is about 0.8 on dry tarmac, but can be 0.1 or less on clear ice. The braking distance then becomes many times longer. Greatly increase your gap, slow down and drive smoothly when it is slippery, especially around zero degrees, on bridges and in shade.

sbrake=v22μgs_{\text{brake}} = \dfrac{v^2}{2 \mu g}the braking distance

Symbols

vvspeedm/s
μ\mufriction coefficient
gggravitational acceleration, 9.81m/s²

Example

You brake from 80 km/h (v≈22v \approx 22 m/s).

Dry tarmac: s=2222⋅0.8⋅9.81≈31s = \dfrac{22^2}{2 \cdot 0.8 \cdot 9.81} \approx 31 m. Clear ice: s=2222⋅0.1⋅9.81≈247s = \dfrac{22^2}{2 \cdot 0.1 \cdot 9.81} \approx 247 m.

Test the grip with a gentle brake somewhere safe when you are unsure of the conditions.
Practise winter, darkness and difficult conditions for free →

← Understeer and oversteer · Reflectors and speed in the dark →

Part of Car licence (class B): Winter, darkness and difficult conditions.