5.1 Introduction to stopping distances and the speed of road vehicles

What is the formula for stopping distance?

When driving a car you obviously need to be alert to any sudden change in your situation, particularly if you have to do an emergency brake to come to a halt.

In this situation you want to stop the car (or any other road vehicle) in the shortest time as possible to effect an appropriate emergency stop!

This means applying the maximum force on the brake pedal.

The longer it takes for you to react and longer it takes to come to a halt, the greater the risk of crashing into an object in your path. Everybody's 'thinking' reaction time to a situation requiring a quick physical response is different, though typically in the range 0.2 to 0.8 seconds. In biology you may have studied the nervous system including the reflex arc (GCSE level biology notes).

The distance it takes to stop a road vehicle in an emergency situation is given by the following formula:

STOPPING DISTANCE = THINKING DISTANCE + BRAKING DISTANCE

The thinking distance is how far you travel during your reaction time, which is the time interval from you perceiving a hazard and actually starting to take action e.g. apply the brakes.

The braking distance is the actual distance you travel from when you first apply the brakes until you come to a halt.

The stopping distance is the total time required from initial visual stimulus to actually stopping moving.

The chart above gives typical or average values for thinking distances, braking distances and stopping distance and are quoted from the UK Highway Code guidance booklet.

You can see the thinking distance is quite an appreciable portion of the total stopping distance, particularly at lower speeds, BUT, look how dramatically the total stopping distance increases with increase in speed.

These values should be doubled for wet roads and multiplied by 10 for ice coated roads. Snow would be somewhere in between, but where?, so just take great care in driving in any of these adverse driving conditions.

Later in this page I've used this data to do some graphs and calculations relating stopping distance to speed and the kinetic energy of a car.

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INDEX of physics notes on reaction times, stopping distances of road vehicles, Newton's 2nd Law, braking friction force, KE calculations