When a driver sees a hazard, the vehicle does not stop at once. The stopping distance is the total distance travelled from the moment the driver sees the hazard to the moment the vehicle stops. It is made up of two parts added together: stopping distance = thinking distance + braking distance.
The thinking distance is the distance the vehicle travels during the driver's reaction time, before the brakes are applied. During this time the vehicle keeps moving at a constant speed, so thinking distance = speed × reaction time. A car at 20 m/s with a reaction time of 0.6 s travels 20 × 0.6 = 12 m before the driver starts braking.
The braking distance is the distance the vehicle travels after the brakes are applied, while the braking force slows it to a stop. If the thinking distance is 12 m and the braking distance is 24 m, the stopping distance is 12 + 24 = 36 m.
