A. Two-second rule
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A. Two-second rule
B. Four-second rule
C. 100 metre rule
Use the two-second rule in dry conditions.
Use the four-second rule in wet conditions or when towing a trailer in dry conditions.
On ice it can take up to 10 times the distance to stop.
The stopping distance is the distance it takes your vehicle to come to a complete stop. Many factors influence it, such as
Stopping distances will be longer with worn tyres, worn suspension, no ABS, a heavier vehicle, narrower tyres and a slippery road surface.
The table below from Noon (1994) is a selection of reasonable average values between tyre and road.
|Surface Type||Coefficient of Friction (µ)|
|Gravel and dirt road||0.35|
|Wet, grassy field||0.20|
|Dry asphaltic concrete||0.65|
|Wet asphaltic concrete||0.50|
|Snow||0.20 – 0.25|
|Ice||0.10 – 0.15|
|Loose moist dirt that allows tyre to sink about 5cm||0.60 – 0.65|
The more water present on the road, the lower the coefficient of friction. As little as 0.4mm of water should be considered “wet” (Tulloch, Stocker 2011).