Calculating stopping distances for your Theory Test

I’ve been asked a number of times by students how they can go about calculating stopping distances, if they can’t remember the numbers, for the Theory Test. Below is a method you can use to work out the answer but first of all we’re going to look at the different elements that make up the stopping distance and why these numbers matter in the first place.

The Thinking Distance

The stopping distance is not simply how long it takes the car to stop from when you press the brake pedal. It also includes what’s know as the Thinking Distance. This thinking distance is the time it takes for you to recognise the situation and press the brake pedal. It also includes the distance you will have travelled before the car begins to slow down.

The faster you’re going the more distance you will cover before you’re able to react to the situation. The thinking distance can be remembered quite easily as it’s approximately 1 foot for every mph that you’re doing. So if you were driving at 40mph your thinking distance is going to be 40 feet.

The Braking Distance

As well as the Thinking Distance you also need to take into account the Braking Distance when you’re calculating stopping distances. The Braking Distance is the distance the vehicle will travel from the moment the brakes are applied to when it comes to a complete stop. The faster you’re going the longer it takes for the vehicle to come to a stop.

To calculate the Braking Distance you need to know the overall Stopping Distance and subtract the Thinking Distance.

The Stopping Distance

When calculating Stopping Distances it is quite simply the total distance a vehicle will travel before coming to a stop.

Stopping Distance = Thinking Distance + Braking Distance

Why it’s important to understand the Stopping Distances

It’s important to know not just how we’re calculating stopping distances but why they are a part of the Theory Test. It’s because having an idea of these stopping distances is going to help keep us safe on the roads. I’ll often tell my pupils that they need to be able to stop within the distance they can see, knowing how far a vehicle will travel will help you judge what is an appropriate speed for the road conditions. I’ll also talk about the two second rule, which is a safe following distance from the vehicle in front, and again allows you to stop the car safely for the speed you’re driving. If you’re driving closer then your stopping distance to the car ahead then you’re increasing the risk of being in an accident.

What can affect your Stopping Distance?

The stopping distances quoted in the Highway Code and shown in the picture above are approximate, there are a lot of things that can change the stopping distance.

  • Attention – Your attention (Thinking Distance) can have a significant affect on your stopping distance. Have you been drinking or are you on medication that makes you drowsy? It’s one of the reasons people shouldn’t be using a mobile phone when driving.
  • The Weather – Are the roads dry, wet or icy? Remember on a wet road your stopping distance can be twice as much, on an icy road it can be as much as 10 times your normal stopping distance.
  • Brakes – Do your brakes work well or are they old and worn? Are they stopping you in a straight line or do they pull your car to one side? It should go without saying but the quality of your brakes will alter your stopping distance.
  • Tyres – Are your tyres worn or badly inflated? It’s why you should check your tread depth and correctly inflate your tyres. Maintaining your tyres could make the difference in stopping safely. Don’t ignore them.

Calculating Stopping Distances

Now we know why they want us to know and understand these numbers for the Theory Test lets have a look at calculating stopping distances that makes remembering them easy. If you look at the picture above you’ll see that the stopping distances are given in both metres and feet. The easiest way of calculating stopping distances is to do it in feet. All we need to is take the speed and multiply it using intervals of 0.5. We start at 20mph and multiply that by 2 to give us 40 feet. So our stopping distance for 20mph is 40 feet. Then we move to 30mph, add 0.5 to 2 and then multiply 30 by 2.5 to give us 75 feet. We just keep adding 0.5 each time we increase the speed until we get to 70mph. The table below shows how this done.

Speed (mph)FactorStopping Distance
in feet
(Speed x Factor)
20240
302.575
403120
503.5175
604240
704.5315

This should then allow you to work out the stopping distance if you can’t remember the number when you’re taking your Theory Test.