Stopping distances, and why they are not what you think

What you need to know

Stopping is two separate things happening one after the other, and only one of them is about the car.

Thinking distance is how far you travel between seeing a problem and the brakes actually starting to work. It is your reaction time, converted into metres. A rough rule is one foot for every mile per hour: at 30 mph you cover about 9 metres before the brakes do anything at all.

Braking distance is how far the car travels once the brakes are working. It depends on speed, the road surface, your tyres and your brakes.

Add them together and you get the overall stopping distance.

Speed	Thinking	Braking	Overall
20 mph	6 m	6 m	12 m
30 mph	9 m	14 m	23 m
40 mph	12 m	24 m	36 m
50 mph	15 m	38 m	53 m
60 mph	18 m	55 m	73 m
70 mph	21 m	75 m	96 m

These are figures for a dry road, good tyres, good brakes and an alert driver. Every one of those can be worse than assumed.

The bit that catches people out

Look at what happens to the two columns as speed rises.

Thinking distance grows in a straight line — double the speed and you double it. That makes sense: your reaction time has not changed, you are just covering ground faster.

Braking distance does something far less comfortable. It rises with the square of speed. Double your speed and you need roughly four times the distance to stop.

From 30 to 60 mph, your speed doubles. Your braking distance goes from 14 metres to 55 metres — almost exactly four times.

This is why the difference between 20 and 30 mph outside a school is not a tenth of anything. It is the difference between stopping short of a child and arriving at them still doing a substantial speed.

Making the numbers real

Metres are hard to judge from the driver’s seat. Convert them:

• A typical car is about 4 metres long.
• 23 metres at 30 mph is about six car lengths.
• 96 metres at 70 mph is about 24 car lengths, or most of a football pitch.

If those figures feel absurdly long compared with the gaps you see on a motorway, that is the point. Most of the traffic around you is not leaving enough room.

What makes it worse

Thinking distance gets longer if you are tired, distracted, ill, or have been drinking. A hands-free phone call is legal, but it still occupies your attention, and attention is the whole of thinking distance.

Braking distance gets longer on a wet road — at least double — and on ice, where it can be up to ten times the dry figure. Worn tyres, worn brakes and a heavily loaded car all add to it too.

Because the two halves are affected by different things, a tired driver in the rain has lengthened both at once.

The rule that replaces the table

You will not do arithmetic at the wheel. What you will do is leave a gap, and the gap that matches these figures is the two-second rule:

Pick a fixed point — a sign, a bridge, a lamp post. When the car in front passes it, say “only a fool breaks the two-second rule”. If you reach the point before you finish saying it, you are too close.

It works at any speed, because it measures time rather than distance — and time is what you actually need. In the wet, make it at least four seconds. On ice, leave a gap that feels ridiculous.