# 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:

• tiredness
• distractions

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.

## 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 by the between the brakes and the wheel. This reduces the 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 . Large decelerations may cause the brakes to overheat, and the driver may also lose control of the vehicle.