What rule should you use to judge a safe following distance in wet or frosty conditions if driving a car? What rule should you use to judge a safe following distance in wet or frosty conditions if driving a car?

  • A. Two-second rule

  • B. Four-second rule

  • C. 100 metre rule

  •  
    The correct answer is B
     
    When it's wet and frosty it can take up to twice as long to stop, so you need to observe the four-second rule. This also applies any time if you're towing a trailer.
     
 
 
 

Stopping distances explained

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
Dry concrete 0.75
Wet concrete 0.60
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).