Braking and reaction times

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

Stopping distance = thinking distance + braking distance

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
  • 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 seconds (s) to 0.9 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 further the ruler falls before being stopped, the higher the reaction time of the person grabbing the ruler. The ruler can be marked in s rather than in mm.

On the left-hand side two hands hold both end of a ruler from top to bottom. On the right hand side one hand has just let go and the second hand at the bottom has caught ruler.

Braking distance

The braking distance of a vehicle can be affected by:

  • poor road and weather conditions, such as wet or icy roads
  • poor vehicle conditions, such as worn brakes or worn tyres
  • a greater speed
  • the car's mass – more mass means a greater braking distance

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.