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.

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.