Stopping distance calculator

Overall stopping distance means the length of road a car covers from the moment the driver sees a hazard until it stands still. It has two parts. Thinking distance is covered at full speed while the driver reacts, so it grows in step with speed. Braking distance runs from the first press of the pedal to standstill, and it grows with the square of speed. The Highway Code adds the two in its diagram: at 30 mph a car travels 9 m before braking and 14 m while braking, 23 m in all. At 60 mph thinking distance doubles to 18 m, but braking distance roughly quadruples to 55 m. That is why the total climbs much faster than the speed. A tired or distracted driver lengthens the first part. Rain, ice, worn tyres and weak brakes lengthen the second.

This stopping distance calculator shows the overall figure, both parts, the length in feet and car lengths, the time to stop (2.7 s at 30 mph) and the average deceleration. A table repeats the method from 20 mph to 70 mph.

The Code prints one diagram of typical stopping distances, for a car in dry weather from 20 mph to 70 mph (Highway Code, typical stopping distances). Each row gives the three distances in metres, then the total in feet and in car lengths of 4 m.

Rule 126 points to this diagram: the safe rule, it says, is never to get closer to the vehicle in front than the overall stopping distance. The same figures turn up in the theory test, whose questions are drawn from The Highway Code, Know your traffic signs and Driving: the essential skills.

The feet memory aid

Driving instructors teach a memory aid in feet that reproduces the diagram exactly. Thinking distance in feet equals the speed in mph; braking distance in feet is the speed squared, divided by 20. At 30 mph that gives 30 ft plus 45 ft, 75 ft in all. DVSA publishes no formula, so the aid is a teaching shortcut, not an official rule. Converted to metres, it drives the calculator's Highway Code method; the diagram's own figures are under checked against known answers.

In rain, stopping distances will be at least double those on dry roads, and on an icy road they can be 10 times greater (Highway Code, Rules 227 and 230). The Code groups snow with ice under icy and snowy weather.

In the calculator, the Highway Code mode applies that factor to the whole distance: thinking stays as it was and braking takes the rest. At 30 mph a wet road gives 45.7 m: 9.1 m of thinking and 36.6 m of braking. On ice the same speed needs 228.6 m.

The physics mode changes only the grip of the road surface, so only braking distance grows. It takes a friction value of 0.8 for dry asphalt and 0.4 for a wet one. Snow at 0.2 and ice at 0.1 are typical textbook values rather than official figures. At 70 mph, a wet surface takes the physics total from 83.4 m to 145.7 m.

Fog shortens the road that can be seen instead. Rule 235 asks drivers in fog to be able to pull up well within the distance they can see clearly. With 20 m of clear view, the diagram's dry figures allow no more than 27 mph.

In GCSE physics each part of stopping distance comes from a formula rather than a diagram. Worked at 30 mph on a dry, level road, the steps run as follows.

1. Convert the speed to metres per second: 1 mph is 0.44704 m/s, so 30 mph is 13.41 m/s.
2. Thinking distance = speed × reaction time. Reaction time is the delay between seeing the hazard and pressing the brake; at 0.67 s it gives 9 m.
3. Braking distance = v² ÷ 2a, where a is the deceleration. On level ground a is the coefficient of friction times g, so 0.8 × 9.81 m/s² makes 7.85 m/s² and a braking distance of 11.5 m.
4. Add the two parts: 20.4 m, against 23 m in the diagram.

The coefficient of friction (μ) measures how well the tyre grips the road; it has no unit. Energy explains the square. A moving car carries kinetic energy of ½mv², and the braking force removes it by doing work, force × distance. Braking distance is therefore mv² ÷ 2F. With the same force, twice the speed means four times the kinetic energy and four times the distance, so speed squared drives the result.

A gradient changes the deceleration to g × (μ + G), with G the slope as a fraction, positive uphill. On a 10 % downhill the dry figure falls from 7.85 m/s² to 6.87 m/s², and braking from 60 mph grows from 45.8 m to 52.4 m. When a slope is steeper than the grip, the car cannot stop and the calculator says so.

Used as a braking distance calculator, the physics mode rarely matches the diagram. The diagram's braking column implies a steady 6.56 m/s², gentler than 7.85 m/s² on dry asphalt. Its thinking column implies about 0.67 s, which is quick, and real reaction times are often longer. At 1.5 s the physics total at 30 mph rises to 31.6 m. How both methods are built and tested is set out in the methodology.

Yes, when the whole stopping distance fits inside the clear road ahead. Rule 126 sets a stricter aim: a speed that allows a stop well within the distance you can see to be clear (Highway Code, Rule 126).

With 30 m ahead at 30 mph, the Highway Code mode shows "Stops in time" with 7.1 m to spare. The highest speed that still stops in that space is 35 mph. At 40 mph the car needs 36.6 m, so it reaches the hazard still doing about 21 mph. The physics mode, at 0.67 s, needs 32.4 m and arrives at 14 mph.

The impact speed is the speed left on reaching the hazard. It equals the starting speed times the square root of the braking still needed over the full braking distance. A hazard inside the thinking distance is reached at full speed, before the brakes act.

The two-second rule measures the gap to the car ahead in time rather than metres (Highway Code, Rule 126). The driver picks a fixed point, such as a lamp post or a bridge. If the car behind reaches it 2 s or more after the car in front, the gap meets the rule.

At 70 mph, a gap of 2 s covers about 63 m, well short of the 96 m overall stopping distance. The gap works because the car ahead also needs road to stop, and it runs out when that car stops dead. Rule 126 asks for at least double on wet roads, more on icy ones, and four seconds for large vehicles in tunnels.

Tailgating, following another vehicle too closely, has no offence of its own in Great Britain. It is prosecuted as careless driving, meaning driving that falls below what a competent and careful driver would do (Road Traffic Act 1988, section 3). The fixed penalty is £100 with 3 penalty points, and a court can impose more.

Anything that slows the driver lengthens thinking distance, and anything that weakens grip or braking lengthens braking distance.

• Tiredness, alcohol, drugs and distractions such as a phone delay the reaction. In the physics mode this is a longer reaction time.
• Worn tyre tread, low tyre pressure and a wet, icy or loose surface reduce grip, which a lower friction value represents.
• Brakes in poor condition lengthen the braking part, and so does extra vehicle weight from passengers or luggage.
• A heavier load also raises fuel use, which the fuel cost calculator prices per journey.

The calculator models a car only. Lorries, motorcycles and bicycles, the mass of a load, tread depth, anti-lock brakes (ABS), trains and trams, and working back from skid marks after a collision all stay outside it.

For cars, the national speed limit is 60 mph on a single carriageway and 70 mph on a dual carriageway or motorway (Highway Code, Rule 124). Built-up areas with street lighting default to 30 mph, and to 20 mph in Wales, unless signs show otherwise.

• 20 mph, the Welsh default on lit streets: 12 m, about 3 car lengths.
• 30 mph, most towns in England and Scotland: 22.9 m, which the diagram rounds to 23 m.
• 60 mph, the national limit on a single carriageway: 73 m.
• 70 mph, dual carriageways and motorways: 96 m, the longest row.

Rule 125 calls a speed limit the absolute maximum, not a speed that is safe in every condition. The rows show how much dry road each limit takes; weather multiplies them.