Road Safety

Human Reaction Times

Human reaction times are a measure of the time taken for a person to respond to an incoming stimuli. An example would be the time taken for someone to catch a ball once they have seen it.

Road Safety, figure 1

Reaction times can vary greatly from person to person, and can vary for an individual based on many factors, some of these are:



Drug use (legal and illegal)

Presence of alcohol in the bloodstream

Concentration on the task


Practice (learning)


Reaction times can be measured in the lab with a 30 cm ruler and a stopwatch.

Road Safety, figure 2

By measuring either the distance dropped before the ruler is caught or the time taken, it is possible to measure the relative reaction times of individuals.

There are more sophisticated versions of this idea that allow people to respond to visual or audio stimuli and the times can be measured and analysed by computers.

Road Safety, figure 3

Human reaction times are an important factor in road safety. The consumption of alcohol or drugs increases people’s reactions times, as does their level of concentration on the task of driving. This is why the use of mobile phones etc is illegal and subject to heavy fines and points on driver’s licences.

Stopping Distances

The distance a vehicle takes to stop is known as its stopping distance and is made up of two different distances.

A. Thinking distance

B. Braking distance

Thinking distance: The distance travelled by a vehicle from the point that the driver receives to stop; to the point where they activate the braking system. This is in effect the reaction time of the driver and is greatly affected by the factors listed above.

Drugs, (legal or illegal), including alcohol, increase thinking distances significantly. Not only does the drug affect the nervous system and the brain’s ability to react, but they also affect judgement and so a person under the influence of drugs may not react at all and may be unaware of their speed and their surroundings. Distractions, such as mobile phone usage, also increasing thinking distances and can mean a driver fails to stop at all.

[N.B. In more recent years this can also be the reaction time of the sensors and software for automatic braking systems in conventional vehicles or fully automated (self-drive) vehicles, although these are much lower than for human drivers, there is still a ‘thinking’ time for a computer too.]

__Braking distance: __Whether a vehicle is under the control of a human only, a computer or a combination of both, the braking distance is the distance covered by the vehicle once the braking system has been activated. This is affected by:

  1. The condition of the brakes, old worn brakes are much less effective.
  2. The road condition - wet and icy roads have less friction, so have greater braking distances.
  3. The speed of the vehicle
  4. The mass of the vehicle
  5. The condition of the tyres, this affects the grip, worn tyres have less grip and greater braking distances.
  6. Pressing the brakes too hard can make the wheels lock and skip- ABS systems in cars prevent this, reducing braking distance by reducing the risk of a skid.

Road Safety, figure 1

Dangers of Rapid Braking

Rapid braking has several serious consequences. Firstly it increases the likelihood of the wheels locking and the car skidding, resulting in the loss of control of the car. Secondly, the rapid braking will reduce the braking distance, but that reduces the time and increases the forces acting on the car and its passengers.

Example: A car travelling at 30 mph (13.4 m/s) stops in controlled conditions in 4 seconds, (1 second is thinking time and 3 seconds the braking time). The car has a mass of 1500 kg including passengers.

Momentum = mass x velocity = 1500 x 13.4 = 20,100 kgm/s

Force = change in momentum / time = ( 0 - 20,100) / 3 = -6700 N

Road Safety, figure 1

Note that the momentum when stopped is 0 kgm/s. The force is negative as it is in the opposite direction to the direction of movement.

However in the rapid deceleration of a crash that happens in 0.1s

Force = change in momentum / time = (0 - 20100) / 0.1 = - 201,000 N

[To put that into context: If a force of around 4-5000 N is applied to our major bones it is enough to break them.]

The shorter the time for the vehicle to stop the greater the force that must be applied. The very large force also produce large acceleration.

In the controlled braking situation above the acceleration would have been 4.47 m/s/s. In the crash the deceleration experienced by the passengers would rise to 134 m/s/s, that’s over 13 G (force of gravity). It is only for a 1/10 of a second but at 9G most people black out.

Calculating Braking Distances

Road Safety, figure 1

This chart is the standard chart given in the UK highway code for stopping distances. You are expected to know these for your driving theory exam. For the physics exam, you need to know the pattern.

Calculating braking distance in good conditions

A car travelling at 30 mph (13.4 m/s) with a mass of 1500 kg, stops in 2.09s.

Momentum = 20,100 Kgm/s

Force = change in momentum / time =(0 - 20100) / 2.09= -9617.22 N

To calculate the distance we need to know

  1. The Kinetic Energy of the car before it brakes
  2. That the work done to stop the car is equal to its kinetic energy before it brakes.

KE _=1/2 _mv2 =0.5 x 1500 x (13.4)2 = 134,670 J

Kinetic energy = Work done

Work done = force x distance E = F d so d = E / F _ =134670 / 9617.22=14 _m: as stated in the chart.

Road Safety, figure 2

If the car’s brakes are only 91% efficient then the braking time increases to 3 seconds.

F = 20100 / 3 = 6700 N

The work done is the same but over a longer distance:

d = 134670 / 6700 = 20.1m

Although that does not sound a great deal of difference, it is 1 ½ extra car lengths to come to stop. Maybe that is the difference between a safe stop and an accident.