Stopping distances

In an emergency, a driver must bring their vehicle to a stop in the shortest distance possible:

This is when:

  • thinking distance is the distance a vehicle travels in the time it takes for the driver to apply the brakes after realising they need to stop - this time is called their reaction time
  • braking distance is the distance a vehicle travels in the time after the driver has applied the brake

Reaction times

Reaction times vary from person to person but are typically 0.2 to 0.9 seconds (s). A driver's reaction time can be affected by:

Longer reaction times increase the thinking distance when stopping from a given speed.

There are different ways to measure reaction times. One simple method involves dropping a ruler between someone's open thumb and forefinger. The higher the reaction time needed to grasp the falling ruler, the further the ruler falls before being stopped. The ruler can be adjusted to measure in seconds rather than in millimetres.

Braking distance

The braking distance of a vehicle can be increased by:

  • poor road and weather conditions, such as gravel, or wet or icy roads - less friction between tyres and the road
  • poor vehicle conditions, such as worn brakes or worn tyres - less friction between brakes and wheels
  • more mass in the vehicle (extra passengers for example) - the braking friction has to work for a greater distance to remove the larger kinetic energy

When a force is applied to the brakes of a vehicle, there is work done by the friction between the brakes and the wheel. This reduces the kinetic energy of the vehicle, slowing it down and causing the temperature of the brakes to increase.

The faster a vehicle travels, the greater the braking force needed to stop it in a certain distance. A greater braking force produces a greater deceleration. Large decelerations may cause the brakes to overheat, and the driver may also lose control of the vehicle.