Driving: The Essential Skills (TES) is the official DVSA guide to driving. Under the heading “Separation Distance”, it says the following:

The two-second rule

In good dry conditions an alert driver, who is driving a vehicle with first class tyres and brakes, needs to be at least two seconds behind the vehicle in front.

In bad conditions, double the safety gap to at least four seconds or even more.

The Highway Code (HC) says the following:

126

Stopping Distances. Drive at a speed that will allow you to stop well within the distance you can see to be clear. You should

• leave enough space between you and the vehicle in front so that you can pull up safely if it suddenly slows down or stops. The safe rule is never to get closer than the overall stopping distance (see Typical Stopping Distances diagram, shown below)
• allow at least a two-second gap between you and the vehicle in front on roads carrying faster-moving traffic and in tunnels where visibility is reduced. The gap should be at least doubled on wet roads and increased still further on icy roads

You will note the use of the words “at least” and “or even more”. Neither publication (nor the DSA) is advising people to drive at precisely two seconds behind the driver in front all of the time.

On lessons, it is common for pupils to get too close to the vehicle in front at some stage of their training. It is also common for them – when asked how big a gap they should leave – to answer “two car lengths”. They do not mean that: it is just an answer with the number “two” in it, and if they actually thought about it (which some do) they would realise that two car lengths is a ridiculously short distance. The bottom line is that they have some vague recollection of the two-second rule, but not enough to recall it correctly.

As an aside, they also often give a nonsense answer when you ask them how far ahead they should be able to see before using their fog lights. Answers of “two metres” or “ten metres” are common (at which point I usually comment that I wouldn’t even walk outside, let alone drive, if visibility was that bad). They also commonly give nonsense answers to the tyre tread depth show-me-tell-me question based on a vague memory of having read it while studying for their Theory Test.

The simple fact is that the Theory Test is complementary to their lessons. It is the ADI’s job to help them apply the theory to the practicalities of being out on the road.

Instructors should be careful about making up their own rules about separation distances, especially if they don’t understand the topic as well as they think they do. Trying to leave too large a gap could easily lead to a candidate driving too slowly, and apart from the obvious problems this can cause in terms of the outcome of the test, it is guaranteed to encourage other drivers to overtake in frustration. TES and HC say you should leave “at least” two seconds in good weather. This does not automatically mean that ten or twenty seconds would be OK in free moving traffic (unless it is icy), because people would overtake and drop into the gap, and the learner would then have to slow down even more to open up the artificially large gap again. A “two-second rule” sized gap discourages all but the biggest arseholes (i.e. Audi drivers) from cutting in because the driver applying it is driving sensibly.

A confident and well-trained driver should be able to maintain a safe distance using the two-second rule without resorting to complicated additional rules. I stress again that when we say “two-second rule” it encompasses the principles of “at least” or “even more” mentioned in TES and the HC.

Measuring the gap is easy. As the car in front passes a sign, a lamp post, or some other feature, just say in a normal voice “only a fool breaks the two-second rule”. If you get to the sign or whatever before you finish saying it then you’re too close. And with a little driving experience under your belt you will know automatically if you’re too close without having to keep doing it.

Why is it measured in seconds and not car lengths?

Referring to overall stopping distances for a moment, if you have to brake suddenly the stopping distance varies depending on how fast you were travelling. At 20mph you can stop in about 12 metres, whereas at 70mph it will take 96 metres – or eight times further. In other words, it is a different number of car lengths for every single speed. Learners have enough trouble learning stopping distances as it is.

Leaving a gap of at least two seconds applies at any speed. It is much easier to apply than individual numbers of car lengths.

Why two seconds?

It’s only a rough rule. At 1mph, a car will be travelling 0.447 metres per second. Therefore, at 20mph it will cover about 18m in two seconds. At 30mph it’ll travel 27m, at 40mph nearly 36m, at 50mph about 45m, at 60mph about 54m, and at 70mph it’s about 62m.

These distances are not the same as stopping distances, and they aren’t supposed to be. Stopping distances are about stopping dead. The two-second rule is really aimed at giving the driver time to react to vehicles in front slowing down. It’s two separate – but related – things.

Does it have to be exactly two seconds?

No. Less than two seconds is dangerous and is almost guaranteed to get you a fail if you do it on your test. If you are closer than two seconds away from the car in front then you are what people refer to as a “tailgater”. But within reason, more than two seconds is fine.

Is it always two seconds?

No. TES and the HC both refer to doubling the gap in wet weather – so it becomes four seconds. Likewise, they mention that in icy weather it can take ten times the normal distance to stop (which seems vague, but skidding on ice is vague. Believe me).

There’s nothing wrong with a gap of, say, three seconds in good conditions. Four seconds is perhaps beginning to raise the question of adequate progress if traffic is flowing freely. Any more than that in free-moving traffic and other problems – such as people overtaking – becomes an issue. Just use common sense.