Reaction and braking distance
The only topic in the course where physics applies instead of a norm. The statute contains no table of braking distances at all: art. 19 merely requires you to be able to stop within the clear and visible part. The figures exist so you understand what that sentence means at 100 km/h.
Three distances, not one
Stopping distance is made of two parts:
- reaction distance — how far the car travels while you are thinking;
- braking distance — how far it travels from the moment the brakes bite.
At 50 km/h that is about 14 and 12 metres; at 100 km/h, 28 and 48. The figures are computed from openly stated assumptions (reaction 1 second, deceleration 8 m/s² on dry tarmac); the [reaction, braking and stopping distance](/en/tables/braking-distance) table names them outright, because other tyres and other surfaces give other numbers.
Try it yourself: [speed choice and risk](/en/oefenen?onderwerp=risk-speed-choice) — 76 questions.
The square applies only to braking
The commonest mistake is to extend “twice as fast, four times as far” to the whole stopping distance. Only braking distance goes up fourfold: it depends on the square of the speed. Reaction distance grows linearly, so stopping distance from 50 to 100 grows about 2.9 times, not 4.
The difference is not academic: it explains why a safe gap on the motorway feels “excessive” when it is barely sufficient.
Try it yourself: [tyres and brakes](/en/oefenen?onderwerp=tyres-brakes) — 25 questions.
Wet roads add half again
On a wet surface deceleration drops from about 8 to 5 m/s² — braking distance grows about half as much again. At 100 km/h that is 77 metres instead of 48, nearly thirty metres of difference: the length of a bus with a trailer.
Hence the practical conclusion that gets tested: in rain you double the gap not “to be safe” but because the physics is different.
Try it yourself: [visibility and weather](/en/oefenen?onderwerp=vision-weather) — 30 questions.
The two-second rule
You cannot measure metres from the driver's seat, so keep your gap in time: pick a fixed point, wait for the car ahead to pass it, and count two seconds. That is reaction distance plus margin, and it works at any speed without arithmetic.
In rain, count four.
Try it yourself: [speed choice and risk](/en/oefenen?onderwerp=risk-speed-choice) — 76 questions.
What to remember
Three relationships, and all three are asked: braking distance grows with the square of speed, reaction distance linearly, and a wet road adds about half again. There is no need to memorise the table: the exam tests the relationship, not the figures.