Stopping Distances & Reaction Times

Stopping Distance

When you stop, it depends on how fast you are going, to how far it takes you to stop.

For the example of a car, let's say you are driving at 30mph through a village, and you need to stop. You apply the brakes, and the car [eventually] comes to a halt. The entire process is called the stopping distance. It takes into account the braking distance (find out more about this on the related page), and also the act of stopping.

The stopping distance for 30mph is generally around 23 metres. But what if you're going faster? Let's say you're driving at 70mph along a motorway. Do you think the whole process will shorter or longer?

Well, in fact, it will definitely be longer. The car will have to travel further in order to come to a full stop, and your reaction time (discussed further down this page) could also be affected.

An equation is formulated for the stopping distance. It goes something like this:

Stopping distance = thinking distance + braking distance

To break it down, see here:

Thinking distance: the distance the car travels in the time it takes you to react and apply the brakes.

Braking distance: the distance the car moves in the time after the reaction to having to brake.

Here is a YouTube video from vt.physics about reaction times:

Reaction Time

As mentioned before, you have a reaction time. This is a natural occurence for anything - dropping a pen and catching it, when someone tells you a joke and you laugh, or driving a car and having to stop.

In the car example, your reaction time needs to be as quick as it can be. If something unexpected happens to you in the car while driving, and you need to process a reaction time (for instance, an animal goes in the road while driving, and you need to emergency stop), this needs to be as quick as possible.

Interesting fact: stopping distances double in wet conditions. In icy conditions, it is as much as ten times greater. At 30mph, that is typically around 230 metres.

There are factors that affect this ability, including:

• tiredness - you shouldn't be driving anywhere while tired
• drugs - unless prescribed by a doctor, which should come with warnings about driving
• alcohol - being inebriated just means there's going to be trouble
• distractions - yes, that sat-nav, your music, something in your glovebox to eat, a drink even

When you have any of these issues, it's best practice to let someone else drive (if possible). If you have some prescription medication that makes you drowsy, you should consult your doctor as soon as possible, and they can prescribe something else. All the other parts in that list you shouldn't be doing anyway while driving.

End Note

We have discussed how a reaction time can be affected by different things, be them in the car around you, or something you have taken. We have also discussed the equation for stopping distances being between the thinking distance and braking distance. And we also know that driving at different speeds means there will be shorter and longer stopping distances.