The stopping distance of a vehicle is the thinking distance plus the braking distance. The thinking distance is how far the vehicle travels during the driver's reaction time. The braking distance is how far it travels under the braking force. For a given braking force, the greater the speed, the greater the stopping distance.
Reaction times vary from person to person, with typical values from 0.2 s to 0.9 s. Tiredness, drugs, alcohol and distractions such as a phone all increase reaction time, which increases the thinking distance. One simple way to measure reaction time is the ruler drop test: a partner drops a ruler without warning, you catch it, and the distance it fell tells you your reaction time. A computer-based test that times a response to a screen signal is another method. Repeating the test with and without a distraction shows its effect.
Braking distance is increased by adverse road conditions, such as wet or icy roads, which reduce the friction between the tyres and the road. It is also increased by poor vehicle condition, which is limited to worn brakes or worn tyres. These factors make it harder to stop in an emergency, so drivers must leave bigger gaps in bad conditions.
When the brakes are applied, the friction force between the brakes and the wheel does work. This reduces the kinetic energy of the vehicle and the brakes get hotter. The greater the speed, the greater the braking force needed to stop in a certain distance. A greater braking force gives a greater deceleration. Very large decelerations can make the brakes overheat or cause a loss of control. Higher tier only: the force can be estimated with F = ma, so a 1000 kg car decelerating at 8 m/s2 needs a braking force of about 8000 N.