When should you apply the four-second rule? When should you apply the four-second rule?

  • A. If the road is wet or frosty, or you are towing a trailer

  • B. If your brakes are almost worn out

  • C. If you're travelling at more than 100km/h

  • D. At night

    The correct answer is A
    Wet and frosty roads can double the stopping distance, so use the four-second rule. The extra weight of a trailer will also add to your stopping distance. In fact, if it's frosty to icy, it can take you 6-8 seconds to stop.

The four-second rule

You should apply the four-second rule when it's wet, frosty or when you are towing a trailer.

The four-second rule means that you leave four seconds between you and the vehicle in front. It gives you more time to react and more time to stop. In dry conditions, you should leave a minimum of two seconds; some road safety experts say three seconds is more realistic in dry weather because of the reaction time of drivers before they start braking.

Many factors influence stopping distances including the friction of the road surface, how much standing water there is on the road (i.e. puddles), tyre condition and inflation, tyre tread depth, tyre width, braking effectiveness (e.g. vehicles with anti-lock brakes and electronic brakeforce distribution can brake more efficiently), air resistance, speed and whether the road is on a gradient (e.g. braking distances downhill are longer and uphill are shorter).

If you are carrying a heavy load (i.e. your car is full of passengers and/or luggage, or you have a pillion passenger on a motorbike, or your van or truck is laden) then you will take longer to stop because of the additional forces pushing your vehicle forward.

If you are towing a trailer, the extra weight of the trailer will push your car forward. Technically, a trailer should put some downwards force on the towbar which will give better rear-wheel grip leading to more efficient use of all four tyres for braking, but even with this extra braking ability it will still take you longer to stop.

If you are towing a trailer with animals you may want to leave more than four seconds because braking sharply could injure the animal if they hit the front of the trailer. This also applies if you have a pet in the car in a cage, or a dog in your car boot.

Painted road markings can also make a small difference because they are more slippery in the wet, and you should pay particular attention to them if you are riding a motorbike.

Check out our in-depth article on braking distances here.

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