What is the name given to the distance a vehicle travels to safely come to a stop after the driver has spotted a hazard?

Every day people and vehicles travel at different speeds and are exposed to large accelerations and forces.

We're all sometimes guilty of driving too fast for the conditions and too close to other cars. (Although we hate it when it happens to us. Tailgating is the top peeve of drivers, according to our Member surveys.)

But to be a safe driver, it's important to understand stopping distances. Whether you're studying for your theory test or you passed years ago, it's worth revising.

Leaving enough distance between you and the car in front will:

• Give you a better view of the road ahead.
• Let you react and stop in time if cars ahead suddenly brake.
• Help with fuel economy - you'll drive more smoothly and won't be braking every time the car in front slows down.

Read on to learn how much stopping distance you should leave.

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What is stopping distance?

Stopping distance is the time that it takes to bring a moving car to a complete stop. This includes

• The time it takes you to react to the hazard (thinking distance), and
• The time it takes for the brakes to stop the car (braking distance)

You can calculate it with this stopping distance formula:

Stopping distance = thinking distance + braking distance

How much stopping distance should I leave?

When driving, you should leave enough clear distance in front of you to be able to come to a stop. This is in case the traffic suddenly slows down, causing you to brake.

However, stopping distances vary depending on factors like the weather and your driving speed.

The Highway Code shows this by splitting the typical stopping distance up into thinking distance and braking distance. You'll need to remember the distances for your theory test.

The distances are based on the average car length in the UK and assume the road is dry.

Stopping distances at different speeds

The stopping distance at 20mph is around 3 car lengths. At 50mph it's around 13 car lengths. If you're travelling at 70mph, the stopping distance will be more like 24 car lengths.

What is thinking distance?

This is the distance your car travels between you spotting a hazard and starting to brake.

If the car in front slams on their brakes, then no matter how hard you try, you won't be able to brake immediately. It'll take you time (and distance) to react to what's happening, decide to brake, and then hit the pedal.

The Highway Code bases its thinking distances on a thinking time of just under 0.7 seconds. The faster you're going, the further you'll travel in that time.

The thinking distance at 50mph is 15m, nearly the length of 2 London buses. At 70mph, the thinking distance will be about 21m.

What can affect thinking distance?

Besides your speed, other factors can affect your reaction time too:

1. Drugs and alcohol

• Drinking and taking drugs both slow down your reaction time.
• Slower reactions increase the distance covered before you react to danger ahead.

2. Distractions

• If you're not completely focused on the road ahead then it'll take you longer to react.
• Sat-navs, mobile phones and other in-car tech can distract drivers.
• It's illegal to hold a mobile to call or text while driving. But even a hands-free call can take your mind off the road.
• Talking to passengers and fiddling with the radio or heating can also divide your attention.

3. Tiredness

• Lack of sleep severely affects driver attention, awareness and reaction time.
• On longer journeys, you should take a break every couple of hours.
• Research has shown that after driving for 2 hours you'll be less able to concentrate and slower to react.

• All these factors affect how quickly you react and hit the brakes when you spot a hazard. Once you brake, your stopping distance will depend your car's upkeep as well as the road and weather conditions.

What is braking distance?

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. That's why the Highway Code gives typical braking distances for a range of speeds.

The braking distance at 50mph is 38m - almost twice as long as a cricket pitch. The braking distance at 70mph is a huge 75m, which is about 9 London buses.

What can affect braking distances?

You should leave at least the recommended distance when driving a well-maintained car with good road and weather conditions. However, many factors can increase braking distance:

1. Brakes:

• The condition of the car's brakes will affect braking distance, so keep them in good working order.
• ABS brakes won't significantly reduce braking distance (they can actually increase it on snow or gravel). But they do allow you to keep control and steer while braking.
• Worn suspension will increase the distance, as weight transfer while braking affects performance.

2. Tyres

• Different tyres have different wet and dry grip depending on their tread pattern and the rubber used.
• All new tyres include a wet grip rating on the label, which goes from A (best) to G (worst).
• Tests have shown that tyres with only 3mm of tread travel about a third further before stopping than new tyres. So check your tyre tread to make sure they're not too worn.
• Braking is also affected by tyre pressure. Both under- and over-inflation will increase braking distance.

3. Weather conditions

• If the road is wet or icy, this will significantly increase braking distances.
• Double the gap between your car and the car in front when it's wet.
• Leave an even bigger gap if it's icy - some advice says 10 times bigger.

4. Road conditions

• A damaged or muddy road surface will increase braking distance.

5. Weight

• The braking distance will also increase if the car is heavier.

The 2-second rule

The 2-second rule is a good rough guide to check that you're leaving enough stopping distance. Here's how it works:

• Choose a fixed point on the road ahead.
• Watch when the vehicle in front of you passes that point.
• Make sure it's at least 2 seconds or more before you pass the same fixed point.
• That way, you're probably keeping a safe distance.

This is a good rule of thumb for car stopping distances in dry conditions, but if it's wet you should double the gap to 4 seconds.

Remember that it's only a rough guide and there's a margin for error. At lower speeds, 2 seconds will see you further back than the Highway Code stopping distances. But at higher speeds, you'll be considerably closer.

Do stopping distances need to be updated?

Theory tests have used the same stopping distances for decades, even though cars and their brakes are more advanced now. 

Some people think we should reduce stopping distances to account for the improvements. Others say we should increase them because drivers face more distractions these days.

Either way, these guidelines have proved effective so far. And there's no sign of them changing anytime soon.

Published: 11 Aug 2017 | Updated: 23 Jan 2020

Stopping Sight Distance (SSD) is the viewable distance required for a driver to see so that he or she can make a complete stop in the event of an unforeseen hazard. SSD is made up of two components: (1) Braking Distance and (2) Perception-Reaction Time.

For highway design, analysis of braking is simplified by assuming that deceleration is caused by the resisting force of friction against skidding tires. This is applicable to both an uphill or a downhill situation. A vehicle can be modeled as an object with mass \(m\) sliding on a surface inclined at angle \(\theta\).

While the force of gravity pulls the vehicle down, the force of friction resists that movement. The forces acting this vehicle can be simplified to:

\[F=W(sin (\theta)-fcos(\theta))\]

where

• \(W=mg\) = object’s weight,
• \(f\) = coefficient of friction.

Using Newton’s second law we can conclude then that the acceleration (\(a\)) of the object is

\[a=g(sin(\theta))-fcos(\theta))\]

Using our basic equations to solve for braking distance (\(d_b\)) in terms of initial speed (\(v_i\)) and ending speed (\(v_e\)) gives

\[d_b=\frac{v_i^2-v_e^2}{-2a}\]

and substituting for the acceleration yields

\[d_b=\frac{v_i^2-v_e^2}{2g(fcos(\theta)-sin(\theta))}\]

For angles commonly encountered on roads, \(cos(\theta) \approx 1\) and

\[d_b=\frac{v_i^2-v_e^2}{2g(f \pm G)\]

Using simply the braking formula assumes that a driver reacts instantaneously to a hazard. However, there is an inherent delay between the time a driver identifies a hazard and when he or she mentally determines an appropriate reaction. This amount of time is called perception-reaction time. For a vehicle in motion, this inherent delay translates to a distance covered in the meanwhile. This extra distance must be accounted for.

For a vehicle traveling at a constant rate, distance \(d_r\) covered by a specific velocity \(v\) and a certain perception-reaction time \(t_r\) can be computed using simple dynamics:

\[d_r=(vt_r)\]

Finally, combining these two elements together and incorporating unit conversion, the AASHTO stopping sight distance formula is produced. The unit conversions convert the problem to metric, with \(v_i\) in kilometers per hour and \(d_s\) in meters.

\[d_s=d_r+d_b=0.278t_rv_i+\frac{(0.278v_i)^2}{19.6(f \pm G)}\]

A Note on Sign Conventions

We said \(d_b=\frac{v_i^2-v_e^2}{2g(f \pm G)\)

Use: \((f-G)\) if going downhill and \((f+G)\) if going uphill, where G is the absolute value of the grade

Passing Sight Distance (PSD) is the minimum sight distance that is required on a highway, generally a two-lane, two-directional one, that will allow a driver to pass another vehicle without colliding with a vehicle in the opposing lane. This distance also allows the driver to abort the passing maneuver if desired. AASHTO defines PSD as having three main distance components: (1) Distance traveled during perception-reaction time and accleration into the opposing lane, (2) Distance required to pass in the opposing lane, (3) Distance necessary to clear the slower vehicle.

The first distance component \(d_1\) is defined as:

\[d_1=1000t_1 \left( u-m+\frac{at_1}{2} \right)\]

where

• \(t_1\) = time for initial maneuver,
• \(a\) = acceleration (km/h/sec),
• \(u\) = average speed of passing vehicle (km/hr),
• \(m\) = difference in speeds of passing and impeder vehicles (km/hr).

The second distance component \(d_2\) is defined as:

\[d_2=(1000ut_2)\]

where

• \(t_2\) = time passing vehicle is traveling in opposing lane,
• \(u\) = average speed of passing vehicle (km/hr).

The third distance component \(d_3\) is more of a rule of thumb than a calculation. Lengths to complete this maneuver vary between 30 and 90 meters.

With these values, the total passing sight distance (PSD) can be calculated by simply taking the summation of all three distances.

\[d_p=(d_1+d_2+d_3)\]