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Stopping Distances

Closer than you think: Know your stopping distances

Toy cars failing to observe stopping distances

Being able to travel from A to B at greatly increased speeds has made the world a smaller place, but cars have made it a more dangerous place too.

Like a lot of important technological advances, speed seems to have made us increasingly impatient - the faster we expect something to be, the more impatient we seem to get at anything that slows that process down.

This could be partly why some of us drive too fast for the conditions even though we know it's more dangerous. And it could be partly why we drive too close to the car in front even though we absolutely hate it when someone does that to us - tailgating is drivers' number one pet hate according to our AA-Populus surveys. Despite our best intentions, it seems speeding and tailgating are habits we find it very hard to break. Both habits increase the likelihood of accidents, and this is why awareness of stopping distances is so important.

Car stopping distances

What does stopping distance mean?

When driving, you need to travel at a speed that'll allow you to stop well within the distance you can see to be clear. In traffic, this means leaving enough space between you and the vehicle in front of you to pull up safely if it suddenly stops or slows down. It applies on winding country lanes too, where there could be a stationary tractor just around the corner - will you be able to stop in time if there was?

This stopping distance will obviously increase as you go faster.  The Highway Code includes a table of typical stopping distances which all learner drivers have to remember. It takes time to process what's going on and what you need to do and the Highway Code illustrates this by dividing the stopping distance into 'thinking' and actual 'braking' distances.

What does thinking distance mean?

If the car in front brakes suddenly then, however hard you try, you won't be able apply your own brakes at exactly the same time - it'll take you a bit of time, and therefore distance, to register what's happening, decide that you need to brake, and then move your right foot to apply the brakes.

The thinking distances in the Highway code are based on a thinking time of just under 0.7 seconds - the faster you're going the further you'll travel in that time.

What does braking distance mean?

This is the distance your car will travel once you hit the brakes before it comes to a complete stop. For the same car under the same conditions the braking distance will increase as your speed goes up, so the Highway code gives typical braking distances for a range of speeds. So once you'e applied the brakes, the laws of motion take over.

You should leave at least this distance when driving a well maintained car in good road and weather conditions. Many factors will affect (increase) braking distance:

  • Brakes: The condition of the car's brakes will obviously affect braking distance, so keep them in good working order. If you've got ABS brakes it won't significantly reduce braking distance - it can actually increase it on snow or gravel - but it does allow you to keep control and steer while braking. Suspension also plays a part if worn as weight transfer under braking affects performance.
  • Weather conditions: If the road is wet or icy, this will increase braking distances, and therefore overall stopping distances. For example, the gap should be doubled when it's wet and increased even further - some advice says by ten times - when it's icy.
  • Tyres: Different tyres have different wet and dry grip depending on their tread pattern and the rubber used. A wet grip rating is included on the label on all new tyres. Braking performance is also affected by tyre pressure - both under- and over-inflation will increase braking distance as will tyre wear. Tests have shown that a car on tyres with only 3mm of tread will travel about a third further before coming to a stop than one on new tyres.
  • Road conditions: A poorly maintained or muddy road surface will increase braking distance.
  • Weight: The braking distance will also increase if the car is heavier.

Besides your speed, your reaction time can be affected by other factors too:

  • Drugs and alcohol both slow your reaction time and therefore increase the distance covered before you react to danger ahead.
  • Distractions: If you're not completely focused on the road ahead then it'll take you longer to react.  Passengers and radio/heater controls have always been a potential source of distraction while sat-navs, mobile phones, and other in-car tech just add to the problem.  It's illegal to use a hand-held mobile phone while you're driving but even a hands-free call will take part of your mind away from driving.
  • Tiredness: Lack of sleep severely affects driver attention, awareness and reaction time. The quality of your driving falls off sharply the more tired you are. On longer journeys you should take a break every couple of hours as research has shown that after driving for this long you'll be less able to concentrate and slower to react. All these factors can affect the speed with which you react to a given circumstance and how quickly you can hit the brakes. Once you hit the brakes, how far you travel before stopping will depend on the state of your car as well as the road and weather conditions.

The 2 second rule

A good way to check that you're maintaining a good stopping distance is by observing the 2-second rule. This means you choose a fixed point on the road ahead and watch when the vehicle in front of you passes that point. If you make sure it's 2 seconds or more before you pass that same fixed point, you're probably maintaining a safe distance. This is a good rule of thumb for cars in dry conditions but if it's wet you should double the gap to 4 seconds.

The typical stopping distances taught to learners haven't changed for decades, despite improvements in cars and braking systems over the years. Some have suggested they should be reduced to account for that. However, others say they should be increased, as drivers face more and more potential distractions. But these general guidelines are effective, and there's no sign that they'll be changed anytime soon.