Stopping distances and following distances explained

Depending on where you are in the world you will hear about either the two-second rule or the three-second rule. In the UK and New Zealand, the two-second rule is promoted, whereas here we use the three-second rule. The distance is checked by timing three seconds from when the back of the vehicle in front clears a particular place and then the front of your vehicle passes that place.

Maintaining a gap of three seconds between you and the vehicle in front is definitely safer than two seconds, but there comes a limit of pragmatic motoring, i.e. keeping 3 seconds behind adds 50% more distance between vehicles which actually contributes to traffic jams. Although, so do nose-to-tail accidents which would be virtually eliminated if we all followed at least 3 seconds behind the vehicle in front.

At night it is more difficult to judge distances so increasing the gap to four seconds can help.

Stopping distances

We covered braking distances on motorbikes in this article. Your stopping distance is your thinking (or reaction) distance plus your braking distance.

Reaction distance

Your reaction distance will be longer if you are tired, distracted (e.g. by loud music, children, or your own thoughts), have been drinking (check drink driving limits here), have taken drugs (read about driving and drugs here), have taken some kinds of medication (see our article here), or are older.

For example, if you take one second to react you will travel 14m at 50kph. At two seconds this is 28m. Two seconds is considered to be fairly common if you are not expecting something to happen; one second would mean you are very alert and half a second would mean you expected the event to happen (e.g. in drag racing a 0.5s reaction time is considered a 'perfect' reaction time when the race starts).

Braking distance

At the end of your reaction time is when you press the brake, and then a number of things happen. Your vehicle has a brake balance set which will divert more of the braking power to the front tyres than the rear tyres. Very few road cars have any ability to change this in the cabin.

No ABS or other electronics

Your braking power will be diverted in proportion to the front wheels and back wheels. Assuming the brakes are of equal performance each side), each front wheel gets the same braking power and each back wheel gets the same braking power. If one front wheel has less grip than the other it will start locking (skidding first). As there is no ABS (anti-lock brakes) you will need to cadence brake (pump the brakes as you feel them start to lock) to get the best braking power.

If there is a difference in the strength of the brakes on either side, one side will consistently lock before the other and could cause you to spin.

This is why you should avoid heavy braking on wet roads.

ABS, but no other electronics

If you brake enough to lock one or more wheels ABS will reduce the braking power in a fraction of a second to stop it locking, but also reduces it on all wheels.

ABS + electronic brakeforce distribution

EBD is an electronic system that knows when the braking friction has been overcome on one wheel and then diverts more braking power to wheels that are still turning until they start to lock. You get better braking performance without having to push the brake pedal as hard.

ABS + EBD is the optimal combination for rapid braking.

Mechanical and tyres

In a vehicle with four wheels you have four contact patches on the road that provide friction to help slow down the tyre. You also have four wheels that will have brake discs or drums that provide friction to slow down the rotation.

Your braking performance is ultimately down to how much friction your tyres have on the road. Worn tyres won't have as much grip. Softer tyres will have more grip (but will wear easier). Heavily cambered tyres (tyres leaning in at the top) don't provide as much braking performance in a straight line as tyres that are vertical. 

Vehicles will worn suspension will 'dive' under brakes putting more weight over the front wheels. Vehicles that maintain a good weight distribution between front and back wheels get more effective braking.

Slippery surfaces

If it starts to rain the roads can become very slippery as the layer of grime and oil on the surface will mix with water and create a slick film that makes corners more difficult to negotiate. If it's wet or you are towing a trailer then 4-6 seconds is recommended. If it's icy it can take up to ten times as long to stop. Sand and gravel act like marbles on the road. Wet leaves and wet road markings are slippery, too.