How to use it
- Enter your Speed in km/h and your Reaction time in seconds. The reaction time is 1 second by default.
- Choose the Road surface, and enter the Slope, % if the road climbs or falls.
- Read the Total stopping distance: reaction plus braking. The table lists the distances from 30 to 130 km/h.
- Compare with the Two-second gap and the Highway Code table at the bottom.
How the stopping distance is calculated
The stopping distance has two parts. The reaction distance is covered before the brakes start to work: speed times reaction time. The braking distance is covered while the car slows: speed squared divided by twice the deceleration, which is gravity times the friction. The formula d = v² ÷ (2μg) is the standard one (see the braking distance article linked below).
The friction values are the coefficients between tyre and road. The table of tyre friction coefficients after J. Y. Wong (1993) gives peaks of 0.80 to 0.90 for dry asphalt, 0.50 to 0.70 for wet asphalt, 0.20 for hard-packed snow and 0.10 for ice. The dry and wet values used here are below those peaks, so the calculator leans towards longer stops.
Reaction distance (m) = speed (m/s) × reaction time (s)
Braking distance (m) = speed² (m/s) ÷ (2 × deceleration)
Deceleration = g × (μ × cos θ + sin θ), with g = 9.81 m/s² and tan θ = slope ÷ 100
Total = reaction distance + braking distanceThe Highway Code comparison
Rule 126 of the Highway Code says to drive at a speed that lets you stop well within the distance you can see to be clear, and that the safe rule is never to get closer than the overall stopping distance shown in its typical stopping distances diagram.
The diagram's figures for a dry road are 53 metres at 50 mph (about 80 km/h) and 96 metres at 70 mph (about 113 km/h), as Oxfordshire County Council quotes them. The Highway Code also says the gap should be at least doubled on wet roads and up to ten times greater on icy roads.
- The Highway Code figures are a guide for typical drivers. This calculator uses your own reaction time and friction, so the two will differ.
- The comparison table uses a dry, flat road and your reaction time. It does not change with the road you choose.
What this calculator does not include
- Tyre and brake condition, the car's weight and whether it has ABS.
- Your reaction time: the figure you enter is an assumption; real reaction times vary with fatigue and distraction.
- Distance covered while the driver is distracted or looking away from the road.
Worked example: 50 km/h, 1 second reaction, dry road, flat
Speed in metres per second: 50 ÷ 3.6 = 13.89 m/s. Reaction distance: 13.89 × 1 = 13.89 m.
Deceleration on a dry flat road: 9.81 × 0.7 = 6.87 m/s². Braking distance: 13.89² ÷ (2 × 6.87) = 14.05 m. Total: 13.89 + 14.05 = 27.93 m.
| Part | Metres |
|---|---|
| Reaction distance | 13.9 |
| Braking distance | 14.0 |
| Total stopping distance | 27.9 |
Frequently asked questions
How far does a car travel before it stops at 50 km/h?
On a dry flat road with a 1-second reaction it is about 28 metres: 13.9 m before the brakes work and about 14 m while the car slows. On ice the braking part alone is about 98 m.
Why does the braking distance grow so fast with speed?
It grows with the square of the speed. On the same surface, twice the speed needs four times the braking distance, because the energy to remove grows with the square of the speed.
What is the two-second rule?
It is the gap a following car covers in two seconds. Article 107 of the Turkish Traffic Regulation accepts that gap as an alternative to half the speed in metres: at 50 km/h, 27.8 m against 25 m.
Does the reaction time include the time the car needs to stop?
No. The reaction time is the time from seeing the hazard to pressing the brake pedal. The braking distance starts then and is the car's slowing, not the driver's response.
Why does my car's stopping distance differ from this calculator?
The calculator is a model with one friction value per surface and no figures for your car. Tyres, brakes, load and the real grip of the road all change the result, so use it for planning, not as a measured figure.
Are the Highway Code figures the same as this calculator's?
No. The Highway Code table is a published guide for typical drivers and conditions. This calculator uses your reaction time and the friction you choose, so compare the two only with similar assumptions.
Sources
- The Highway Code, Rule 126 (stopping distances), 2021 revised edition, UK Department for Transport
- Oxfordshire County Council: Don't drive too close (Highway Code dry-road figures for 50 and 70 mph)
- Wikipedia: Braking distance (the v² ÷ (2μg) formula)
- HPWizard: tire friction coefficients (table after J. Y. Wong, Theory of Ground Vehicles, 1993)
Figures and text last checked on Tuesday, November 10, 2026.