Stopping distances
In an emergency, a driver must bring their vehicle to a stop in the shortest distance possible:
stopping distanceThe sum of thinking distance and braking distance. = thinking distance + braking distance
This is when:
- thinking distanceThe distance a vehicle travels in the time it takes for the driver to apply the brakes after realising they need to stop. 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 distanceThe distance a vehicle travels in the time after the driver has applied the brake. 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 s to 0.9 s. A driverâs reaction time can be affected by:
- tiredness
- drugA substance that can change chemical reactions in the body.
- alcoholA depressant that can be found in wines, spirits and beers. Also known as ethanol.
- 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 someoneâs 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 doneThe amount of energy it takes to do a task. Measured in joules (J). For example, the work done in raising a mass through 10 m would be equal to the gain in potential energy of the mass. by the frictionA force that opposes or prevents movement and converts kinetic energy into heat. between the brakes and the wheel. This reduces the kinetic energyEnergy which an object possesses by being in motion. 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 decelerationSlowing down or negative acceleration, eg the car slowed down with a deceleration of 2 msâ»ÂČ.. Large decelerations may cause the brakes to overheat, and the driver may also lose control of the vehicle.