Forces and Motion: Stopping Distances

Stopping distance refers to the total distance a vehicle travels from the moment the driver notices a need to stop, to the moment the vehicle comes to a complete halt.
It is the sum of two components: thinking distance and braking distance.
Thinking distance is the distance a vehicle travels in the time it takes for the driver to react to a hazard and start applying the brakes; it depends on reaction time and the speed of the vehicle.
Factors affecting thinking distance include driver tiredness, alcohol and drugs, distractions, and age.
Braking distance is the distance a vehicle travels once the brakes have been applied, until it stops completely; it depends on the speed of the vehicle and the condition of the tyres and brakes.
Factors affecting braking distance include speed of the vehicle, condition of the vehicle (tyres and brakes), weather conditions and road surface.
The faster a vehicle is travelling, the greater both the thinking distance and braking distance will be.
On a dry road, at a speed of 30mph, typical total stopping distances are about 23 metres – 9 metres thinking distance plus 14 metres braking distance.
On a wet road, or for vehicles with poor brakes or tyres, or if the driver is inattentive, stopping distances may be much greater.
Stopping distances increase dramatically if the road is icy or if the vehicle is travelling at high speed. In fact, stopping distance is proportional to the square of the speed.
Reaction times can vary greatly from person to person, and even for the same person under different conditions.
Measures to improve safety and reduce stopping distances include keeping the vehicle well-maintained, driving attentively and not under the influence of alcohol or drugs, and fighting driver fatigue.