What is stopping distance and how can I work it out

What is stopping distance and how can I work it out

Too close to call: put a stop to the guessing game and know your stopping distances.

According to a recent AA members survey, one of the most annoying things on the road is people driving close behind them, most commonly known as tailgating, and it isn’t hard to see why. Not only does tailgating restrict the rear view of the driver in front, particularly at night, but it is incredibly unnerving. Drivers have reported not knowing whether the driver behind is trying to overtake, which distracts them from their own driving, feeling pressured to speed up, and of course the fear that they will crash into the back of them.

That is the main danger of tailgating: Even though most drivers would argue they are completely competent behind the wheel, hazards can crop up at any time and if you are too close, you simply don’t have the distance required to avoid a collision. This distance that it takes to bring your car to a complete stop is the stopping distance and it is incredibly important for both your safety and that of other drivers.

This is a part of the UK driving theory test, so at some point all drivers have had to learn this, but let us refresh your memory on what this distance is, how to calculate it, and give you some tips for how to make sure you always leave enough space to stop.

What is stopping distance?

What is stopping distance

One common mistake that people make is to confuse the vehicle braking distance for stopping distance, but that is only half the story. Stopping distances for cars are actually a combination of two different distances, thinking distance and braking distance, and can be worked out with a stopping distances formula.

Thinking distance is the time it takes for a driver to react to a hazard or obstacle in the road or around them and to realise that they need to apply the brake and then push down on the pedal. There are a vast range of factors that can affect a driver’s thinking distance (we’ll get to these later), but it is estimated that under normal conditions for every mile per hour that the driver is travelling, the thinking distance is 1 foot (0.3m). So, if you are travelling at 20mph, the thinking distance will be 20 feet (or 6 metres).

Braking distance is the distance that the vehicle will travel in the time it takes to come to a complete stop after the brakes have been applied, i.e. the time it takes for the brakes to stop the car. The man factor in these, of course, is how fast your car is travelling when the brake is applied. The faster you are travelling, the more momentum you have and the greater braking distance will be.

Again, lots of things can impact braking distance, but to help with stopping distances Highway Code have provided these estimates for braking distances at certain speeds:

Speed (mph)
Braking distance (m)
20
6
30
14
40
24
50
38
60
55
70
75

To give some examples of complete stopping distances, at 30mph the braking distance of 14m (45 feet) would be added to the thinking distance of 9m (30 feet), to give a total stopping distance at 30mph of 23m (75 feet).

The stopping distance at 70mph would be 75m braking distance and 21m thinking distance, meaning overall 96m.

To act as your braking distance calculator, the stopping and braking distance chart below will give you all the information you need to work out safe braking distances. All of these, as with the examples above, are based on the driver not being distracted or impaired, driving a well-maintained vehicle, and normal, dry road conditions:

Speed
Thinking Distance
Braking Distance
Overall Distance
20 mph (32 km/h)
6 m
6 m
12 m (40 feet)
30 mph (48 km/h)
9 m
14 m
23 m (75 feet)
40 mph (64 km/h)
12 m
24 m
36 m (118 feet)
50 mph (80 km/h)
15 m
38 m
53 m (175 feet)
60 mph (96 km/h)
18 m
55 m
73 m (240 feet)
70 mph (112 km/h)
21 m
75 m
96 m (315 feet)

Why is it important to know your stopping distance?

The main reason is of course to help you avoid accidents if cars in front of you suddenly brake. If you know that your car will travel 38m before it comes to a complete stop ( can you work out how quickly were you travelling?), then you can anticipate how far behind the car in front you need to be for it to be a safe distance. Similarly, if you know a car behind you is only 10m away, but you are travelling at 40mph, you can see that there is not enough distance between you and if you brake sharply it is likely an accident will occur. This is a hard situation to be in, but some solutions here could be to apply the hazard warning lights in an attempt to make the driver aware, gradually decrease speed by stepping off the accelerator until the distance is acceptable and the over driver is also forced to decrease their speed, or use your indicators to signal and pull over to the side of the road when it is safe to do so and let the driver pass.

Knowing stopping distances also gives you a better view and understanding of the road ahead. There’ll be an appropriate distance between you and the other car and it will allow you to think about the distance before crossroads, junctions, or corners in a different way and be able to predict better what might happen, how you will have to adjust your speed, or what potential hazards may arise.

It will also help with fuel economy. Since you know how long it takes to stop and to slow down, you’ll be able to drive more smoothly and won’t be braking every time the car in front slows down.

What can affect your stopping distance?

  • Weather and road condition: The most common influence on stopping distance is the condition of the road. Adverse driving conditions such as rain or snow will mean that your tyres have less traction on the surface of the road and less friction is generated when the brakes are applied, leading to a much greater braking distance. Experts have suggested that the stopping distance in rain is double the usual stopping distance and this should be taken into account, especially when travelling at high speeds or in darkness. Braking distances on ice would be even worse and drivers should be extremely cautious when driving on ice. Whether you are travelling on a level road, going up or down a hill, and the surface of the road (whether it is gravel, tarmac, dirt, etc.) can all affect traction and speed and make your stopping distance more unpredictable.
    Weather and road condition
  • Drugs and alcohol: It should go without saying, but drinking and taking drugs are going to have a negative impact on your ability to drive safely. Both of these slow down your reaction times and make it much harder for you to react to or even perceive possible hazards in the road. Slower reactions increase the distance covered before you react to danger ahead and increase the likelihood of a collision.
  • Distractions: There can be all kinds of distractions on the road, both inside and outside of the car. Whether it is children arguing in the back seat, talking to passengers, listening to the radio, phones, sat-navs or other in car tech, or even a gorgeous sunset, anything that takes your mind off the road is going to decrease your reaction time and increase your stopping distance. Since it is illegal to hold a mobile to call or text while driving many people use a hands-free system to phone people while driving, but even this can take your mind off the road.
  • Tiredness: Lack of sleep severely affects driver attention, awareness, and reaction time.
    It is recommended that on longer journeys (more than 2 hours), you should take a break every couple of hours for at least 15 minutes to let your mind regain focus and to relax. Research has shown that after driving for 2 hours you’ll be less able to concentrate and slower to react.
  • Tyres: The condition of your tyres can also play a part in how long it takes to stop your car. Are they high-quality tyres and properly inflated, or worn or badly inflated? Maximum contact is needed between the tyre and the road surface in order to provide the best possible stopping distance. When tyres are over or under inflated, the tread contact patch is reduced and this is bad news for your stopping distance. The depth of the tread is also an important factor to consider. Most tyres start with around a depth of 8mm tread and legally need to be changed when this drops to 1.6mm, though this is an absolute minimum. Tread depth will start affecting your braking distance well before because as the tread reduces, so too does the tyre’s ability to grip the road. At 30mph on a wet road, a car with brand new tyres with 8mm of tread will come to a stop after 25.9 metres. The same car travelling in the same conditions but fitted with tyres with just 3mm of tread remaining would come to a stop in 35 metres. That is nearly 10m and 35% more, despite there being no legal issue here. If it drops to the legal limit of 1.6m? Tests show that the distance then is something around 43 metres, longer than a car with new tyres travelling at 50pmh would take to stop.
    The condition of your tyres can also play a part in how long it takes to stop your car
  • Braking systems: Since ABS is commonplace in most vehicles, drivers seem to think that this offers them some sort of protection against brake issues. The fact of the matter is though, ABS deals mainly with making sure vehicles are stable in maneuvering and do little in terms of reducing your stopping distance. Abrasion and wear to essential elements like brake discs and brake pads which all start to wear down over time or from intense braking, increases your stopping distance. Wet roads and standing water can also leave moisture between the pads & discs that reduce the friction in the braking system and make it harder to stop your car. Pumping the brake slowly after going through lots of water can help them to dry out.
  • Visibility: The more time it takes for you to notice hazards in front of you, the more time will have passed before you can press the brake pedal. Things like damaged windscreen wiper blades can reduce your visibility by smearing dirt across your windshield and a lack of fluid can make it hard to shift grime as well. If the morning is cold and frosty, you should wait until the windscreen is completely clear before setting off.

The 2-second rule

The 2-second rule

Stopping distance can be hard to work out when you are driving, but there is one handy tip that can help you get braking right: the 2-second rule. First, choose a fixed point on the road ahead. This could be a signpost or marker on the side of the road, or text or marker on the road (a chevron or writing). Watch for when the vehicle in front of you passes that point. From then, it should be at least 2 seconds before you pass the same fixed point. If it isn’t you need to decelerate and increase the distance between you and the car in front. As we said earlier, in wet conditions this time should be doubled to 4 seconds and remember that this is only a rough guideline.