Understanding vehicle stopping distances

A tree never hits an automobile, except in self defense. – American proverb

Penguin road sign

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If you have to brake in a hurry, how much roadway do you think you need in order to stop? The answer may surprise you! There are a number of factors that determine vehicle stopping distances and here we look at a few of the main ones.

Your reaction time

In an emergency, the quicker you can react, the better the outcome is likely to be. Anything that stops you from reacting quickly will increase the time it takes for your vehicle to come to a stop. Being distracted by a mobile phone will slow you down for example. And it goes without saying that drugs and alcohol definitely don’t mix with driving.

Tiredness can also have a very negative impact on your driving in general, and specifically your reaction speed. In fact, some research has shown it to be equal or worse to driving while under the influence of alcohol. So if you’re feeling drowsy, make sure you pull over and get some rest.

Exactly how long does it take your vehicle to stop? The graph below gives you an idea of how much of a factor your ability to react quickly plays in stopping your car in an emergency.

The condition of your vehicle’s tyres

Your tyres are the only part of your vehicle that is touching the road so they’re a big part of determining how quickly you can stop your vehicle. Bald tires are obviously not going to perform as well as new ones that have a much deeper tread. So make sure your tyres are legal and properly aligned so they’ll do the job they’re designed to.

How heavy your vehicle is

If you have 5 huge rugby players in your car then that’s a fair amount of weight and it’s going to take a lot to stop as opposed to when you’re driving on your own. The same is true if you are towing a trailer or caravan – the extra weight means it will take you longer to slow down. In these cases you need to watch your speed and start braking earlier than normal to compensate for the extra time it will take to stop.

The condition of the road you’re travelling on

Weather can greatly influence stopping distance, most noticeably with ice, snow or rain. These things get between your vehicle’s tyres and the road reducing traction. Chains are essential for ice and snow, but make sure you know how to fit and use them properly. Always be aware of the conditions and adapt your driving to suit them.

Your speed

Perhaps the ultimate determining factor when it comes to stopping distance is your speed. Even going just a little over the speed limit can make a big difference in how your car behaves. Stick to the speed limits – they’re there for a good reason.

And lastly, always follow the two second rule

Always keep a safe distance between yourself and the vehicle in front of you. A good way of measuring a safe space is often described as the two second rule. This means that you should travel at least two seconds behind the vehicle in front of you (four seconds if the weather is bad).

You can do this by watching the vehicle in front of you pass a sign on the road and then counting “one thousand and one, one thousand and two”. You should not pass the same point until you’ve said the whole thing. If you do pass the point before you’ve said this then you’re too close to the vehicle in front of you.

The two second rule is great because it works regardless of the speed you’re travelling – as you increase your speed, the safe stopping distance you need between you and the vehicle in front will increase.