Stopping distance

Stopping distance has two halves and the test asks about both. Thinking distance is the ground you cover while you react, before the brakes do anything at all — it grows with speed, and it grows with tiredness, alcohol, drugs and distraction. Braking distance is what the vehicle needs once you are actually on the brakes, and it grows much faster than speed does, because it goes up with the square of the speed.

That squaring is the part people get wrong. Doubling your speed does not double the braking distance, it roughly quadruples it. It is why the gap that felt generous at 30 mph is nowhere near enough at 60.

Then conditions multiply the whole thing. On a wet road, allow around double. On ice it can be ten times the dry figure. The numbers in the Highway Code are for a dry road, a good vehicle and an alert driver — treat them as the best case, not the normal one.