Driving tests

What is the 4-second rule?

You’ve probably heard of the 2-second rule. It’s a rule that light vehicle drivers can use in dry weather on good road surfaces to maintain a reasonably safe following distance to a vehicle in front. You simply count two seconds from when the vehicle passes a stationary object on the side of the road to when you pass it.

However, the 2-second rule in reality doesn’t give enough time to stop in many scenarios. In this case, you can increase the distance to 3, 4 or even more seconds.

It’s recommended that you use the 4-second rule when:

  • being tailgated (i.e. a person behind you is travelling too close)
  • the driver in front is tailgating another vehicle in front of them
  • towing a trailer or caravan
  • driving a heavy vehicle such as a truck
  • heavily loaded
  • the weather has made the road surface more slippery (e.g. rain)
  • the road surface is poor, e.g. gravel.

In very slippery conditions, such as snow and ice, a driver might want to leave 6-8 seconds.

Electric vehicles, with their additional weight, benefit from a 3-second rule in dry weather.

Typically, a person takes around 1.5 seconds to react, so a 4-second buffer gives much more chance of stopping.

The reason why it’s good to use a rule based on time is that drivers are not very good at judging distances. At 10km/h, a driver should be 5.5m behind the vehicle in front – that’s not too difficult to judge because it’s just a bit longer than a car length. However, at 100km/h, a driver should be 55 metres behind the car in front. This is about 12 average car lengths – not very easy to judge! For every 10km/h, the gap to the vehicle in front increases 5.5 metres for the 2-second rule, or 11 metres for the 4-second rule.

The closer you are to the vehicle in front, the more it blocks your view. Large trucks and buses are particularly bad for this. Dropping back to 4 seconds behind gives much better visibility.

The two trucks on the left are major blockers for the car following.

Of course, leaving more room tends to invite people to fill the gap, especially on multi-lane roads, but in reality, this doesn’t delay your journey as much as you think it would.

Darren has written over 3000 articles about driving and vehicles, plus almost 500 vehicle reviews and numerous driving courses. Connect with him on LinkedIn by clicking the name above

Posted in Advice, Road Code