Thinking, braking and stopping distances
Stopping vehicles as quick as possible in an emergency is important but many factors affect this.
The driver's reactions and the road and vehicle conditions play a part, as well as mass and speed.
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 - this time is called their reaction time
- 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
Factors affecting thinking distance
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:
- tiredness;
- alcoholA depressant that can be found in wines, spirits and beers. Also known as ethanol.
- medication prescribed by a doctor;
- illegal drugA substance that can change chemical reactions in the body.;
- distractions – such as texting, reading text, adjusting the radio, background noise.
The greater the reaction time the longer the thinking distance.
During the reaction time, a car travels at constant speed because the driver has stopped pressing the accelerator but has not yet pressed the brake.
This means that if the speed of the car is known and the driver’s reaction is known, thinking distance can be calculated using:
Distance = speed x time.
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.
A conversion table can be used to determine the reaction time in seconds (see below).
Factors affecting braking distance
The braking distance of a vehicle can be increased by:
- poor road conditions, such as oil on the road - less friction between tyres and the road;
- poor weather conditions, such as wet or icy roads or snow - less friction between tyres and the road;
- poor vehicle conditions, such as worn brakes or worn tyres - less friction between brakes and wheels and wheels and road;
- 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.
The kinetic energy of the car is converted to heat energy, causing the temperature of the brakes to increase.
Some of the kinetic energy is also converted to sound energy.
The heat and sound energy are spread out to the surroundings and “lost”
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 ms⁻²..
Large decelerations may cause the brakes to overheat, and the driver may also lose control of the vehicle.