Everything You Wanted To Know About Stopping Distances But Were Afraid To Ask

The distance it takes to stop a vehicle depends on a number of factors. AA Roadwatch’s Esther O’Moore Donohoe explains…

You’ve probably heard us say ‘make sure you increase your braking distance’ in our Roadwatch traffic reports whenever there are slippery road conditions. If you’re the type of person who has said to yourself ‘by how much should I increase my braking distance? How long does it take to stop? What’s a tracker mortgage?’ Well you are in luck, because we’re going to break/brake it all down in this blog post.

BE COOL AND FOLLOW THE TWO SECOND RULE

In the Rules Of The Road handbook, it is advised that drivers make sure they are at least two seconds behind the vehicle in front of them. They even have a little test you can use to gauge this. If you’re driving on a dry road, choose a fixed point ahead, such as a street light. When the vehicle in front passes that point, say out loud, ‘only a fool breaks the two-second rule’. Not only will you wow any passengers in the vehicle with this catchy rhyme, but it will also help you work out if you’ve enough distance between you and the vehicle ahead. If you have already passed your chosen point by the time you get to the end of the sentence, you are driving too close to the vehicle in front and need to pull back.

If the road is wet, you have to double this distance, i.e. repeat ’only a fool breaks the two-second rule’ twice. When driving in snowy, foggy or icy conditions, say ‘only a fool breaks the two-second rule’ four or five times. That’s because it takes longer to come to a complete stop when roads are wet or icy. You should always adjust your driving style to match the conditions; bear in mind that hailstones in particular can change driving conditions very suddenly.

The distance it takes you to fully stop might surprise you: if you are driving on a dry road surface at 50kmph, it will take you 25m to stop, which is just over the length of two double decker buses. At 120kmph in wet conditions, you’ll have travelled 169m by the time you come to a complete stop: that’s about the length of 17 buses.

OTHER FACTORS THAT AFFECT STOPPING DISTANCE

It’s not as simple as seeing a hazard ahead and applying your brakes, however. There are a number of factors that affect stopping distance.

Perception and reaction time

According to the RSA, the length of time it takes us to perceive a hazard and then realise it’s a hazard that we need to do something about, can take anywhere between 0.5 and 0.25 of a second. You then need to consider your reaction time which is how quickly we can take action in relation to the hazard ahead. The amount of time it takes from taking your foot off the accelerator to press the brake pedal can range from 0.25 to 1.5 seconds.

Keep in mind too, that both your reaction and perception times can be affected by things like tiredness, poor concentration and drugs and alcohol. To dazzle you with more facts and figures: ‘A perception and reaction time of four seconds at 100kmh means a car travels 110 metres before the brakes are applied’. This is longer than the height of Big Ben in London.

Your vehicle’s capacity to react and brake

But wait, there’s more. Aside from the human element, you also have to consider the condition of the vehicle you are driving. Every vehicle takes time to react once the brake pedal has been tapped. This will depend on its condition, especially the condition of the braking system.

The final thing to bear in mind is the braking capability of your vehicle which relies on things like brakes, the tread and grip of your tyres, the road surface, brakes, the weight of the vehicle and the vehicle’s suspension. You can check the tread of your tyres by placing a €1 coin in the grooves – if you can see the gold rim they might need replacing!

So, next time you hear a Roadwatcher advise you to ‘increase your braking distance’ you’ll know exactly what to do. Another mystery solved!

Credit for figures and source information: The RSA

This blog about stopping distances is one of a series on safe driving and road use. You can check out our previous post about how to correctly use your car lights here.