The Theory Test – How To Work Out Stopping Distances
Have you looked at the stopping distance table in the highway code and thought “How am I ever going to remember stopping distances?” If so, don’t worry. Most people feel the same way.
Rather than memorise the table – which is difficult, and can take a long time, we can show you a way to work out stopping distances as you go.
There is no need to remember stopping distances themselves. This is an easier way of dealing with the questions on the theory test.
During the theory test, you might typically be asked a question like ‘what is the overall stopping distance at 40mph?’ You can then calculate the answer. The method of calculation works best using feet rather than metres, but if you prefer you can go on to convert feet to metres if you prefer.
Overall Stopping Distance on dry roads
The overall stopping distance consists of the ‘thinking distance’ – the time the car travels whilst the driver is processing the information to stop, and the ‘braking distance’ – the distance the vehicle travels after the driver has actually applied the brakes to the point the vehicle stops.
Here is a simple table that will help you to calculate overall stopping distances between 20 and 70 mph. The table applies what is known as a calculation ‘factor’, which increases slightly when the speed of travel increases. The factor is multiplies by the speed to work out the overall stopping distance in feet.
The factor of each speed is easy to remember. Here’s how you do it – You need to remember that the factor of 20mph is 2. Then as each speed increases by 10mph, increase the factor by 0.5. So 30mph has a factor of 2.5, and 40mph has a factor of 3 and so-on.
Here is an example:
- Theory test question: What is the overall stopping distance at 70mph?
- Answer: 315 feet. This is worked out by multiplying 70mph by its Factor of 4.5 = 315 feet.
Thinking Distance
In feet the thinking distance is the same as the speed in mph.
Example:
- Theory test question: What is the thinking distance at 40mph?
- Answer: 40 feet. This is worked out by knowing that the thinking distance is the same as the mph in feet.
Braking Distance
The braking distance is the overall stopping distance minus the thinking distance.
Example:
- Theory test question: What is the braking distance at 40mph?
- Answer: 80 feet. This is worked out by working out the Overall stopping distance at 40mph is 120 feet using the Factor.
The thinking distance at 40mph is 40 feet as shown above. So braking distance is 120 – 40 = 80 feet.
If you need to work out the distances in metric rather than imperial measurements, you can do your calculations in feet and convert the final result into metres at the end. Here’s how:
1 foot is 0.3048 metres, or to keep it simple 0.3 metres. To convert feet to metres, divide the number of feet by 10 and multiply by 3.
Here is an example:
- Theory test question: What is the overall stopping distance in metres distance at 70mph?
- Answer: 95m. (It’s actually 96, but in the theory test you’ll have multiple choice answers, so the closest answer to 96 will be correct). This is calculated by working out the overall stopping distance at 70mph using its Factor – 315 feet, then dividing that by 10 and multiplying by 3.
What if the road is not dry?
If the road is wet, double the overall stopping distance, and in icy conditions, the overall stopping conditions can be up to 10 times longer.
Now you know how to work out stopping distances, you will see there is no need to remember the table.
Conclusion
All you really need to remember is this:
- The Factor of 20mph is 2
- The Factors increase by 0.5 every 10mph
- In feet the thinking distance is the same as the speed in mph.
- The braking distance is the overall stopping distance minus the thinking distance.
- To convert to metres from feet divide feet by 10 and multiply by 3