A child steps out between parked cars. Before your foot even reaches the brake, the car is still moving at full speed. Then it needs more road to slow down and stop. Together, those two parts make up your stopping distance.
The UK Highway Code puts the typical stopping distance at 23 metres at 30 mph and 96 metres at 70 mph. That jump is much bigger than the jump in speed, and the reason is simple physics.
Want to try your own numbers? The free VoltReflex stopping distance calculator lets you change speed, reaction time and road conditions and see the result instantly.
Key takeaways
- Stopping distance = thinking distance + braking distance.
- At 30 mph the Highway Code gives 9 m + 14 m = 23 m.
- Thinking distance grows in line with speed. Braking distance grows with speed squared.
- Gaps should be at least doubled in the wet and up to ten times greater on ice.
- A slower reaction adds metres before you even start braking.
What Is Stopping Distance?
Stopping distance is the total distance a vehicle travels from the moment the driver sees a hazard to the moment it stops. It has two parts.
- Thinking distance. The distance covered while you notice the hazard, decide to stop and move your foot to the brake. The car is still going at full speed.
- Braking distance. The distance covered from when the brakes start working until the car stops.
Thinking distance depends on you. Braking distance depends mostly on the car, the tyres and the road.
Highway Code Stopping Distances
The UK Highway Code publishes a chart of typical stopping distances for an average car on a dry road. It calls these figures a general guide.
| Speed | Thinking | Braking | Total |
|---|---|---|---|
| 20 mph (32 km/h) | 6 m | 6 m | 12 m |
| 30 mph (48 km/h) | 9 m | 14 m | 23 m |
| 40 mph (64 km/h) | 12 m | 24 m | 36 m |
| 50 mph (80 km/h) | 15 m | 38 m | 53 m |
| 60 mph (96 km/h) | 18 m | 55 m | 73 m |
| 70 mph (112 km/h) | 21 m | 75 m | 96 m |
Look at the thinking column. It rises by 3 m for every 10 mph. That works out to a reaction time of about two thirds of a second. Real drivers are often slower, as we explain below.
Why Speed Matters So Much
Thinking distance is easy. It is speed multiplied by reaction time. Double your speed and you double the thinking distance.
Braking distance is different. A moving car carries energy that depends on the square of its speed. Double the speed and the brakes must remove about four times as much energy, so the braking distance roughly quadruples.
Braking distance = speed² ÷ (2 × deceleration)
You can see this in the Highway Code chart. Braking at 30 mph takes 14 m. At 60 mph it takes 55 m, almost four times as far, even though the speed only doubled.
Test yourself Open the stopping distance calculator, set 30 mph and then 60 mph, and watch the red braking bar grow far faster than the white thinking bar.
How Reaction Time Changes Thinking Distance
The Highway Code figures assume a quick, alert response. Real reactions vary a lot. As we cover in average reaction time while driving, an alert driver expecting to stop may take about 0.7 seconds, while a surprise hazard can take about 1.5 seconds.
Road engineers in the US go further. The City of Los Angeles design manual notes that AASHTO uses 2.5 seconds for perception and braking, so roads stay safe for slower drivers too.
| Reaction time | Thinking distance at 30 mph | At 60 mph |
|---|---|---|
| 0.67 s, Highway Code | 9 m | 18 m |
| 1.5 s, surprise hazard | 20 m | 40 m |
| 2.5 s, road design value | 34 m | 67 m |
Tiredness, alcohol and phones all push your reaction time toward the slow end. See sleep and reaction time and alcohol and reaction time.
Wet and Icy Roads
Rule 126 of the Highway Code says the gap to the vehicle in front should be at least doubled on wet roads and up to ten times greater on icy roads. Less grip means the tyres cannot slow the car as quickly.
The same rule asks drivers to leave at least a two second gap on high speed roads. In bad weather that gap needs to grow.
Real world note Worn tyres, poor brakes, a heavy load or a downhill road can all add to the figures above. Treat any chart or calculator as a rough guide, not a promise.
How to Shorten Your Stopping Distance
- Slow down. Even 10 mph less makes a big difference to braking distance.
- Look further ahead. Spotting hazards early is like getting a head start on your reaction.
- Leave a bigger gap. Use the two second rule and double it in the wet.
- Stay alert. Rest before long drives and put the phone away.
- Look after your car. Good tyres and brakes help the car stop faster.
Curious how sharp you are today? Take the reaction time test or the choice reaction test, which is closer to deciding on the road.
Frequently Asked Questions
What is the difference between thinking and braking distance?
What is the stopping distance at 30 mph?
Why does braking distance grow so fast with speed?
How much longer is stopping distance in the wet?
How does reaction time affect stopping distance?
The Bottom Line
Stopping distance is thinking distance plus braking distance. Your reaction time sets the first part, and speed rules the second. Because braking distance grows with speed squared, small cuts in speed save a lot of road.
Play with the numbers in the free stopping distance calculator and see how much a slower reaction or a wet road really costs.
VoltReflex results and calculators are for learning and self-benchmarking only. They are not driving, legal or medical advice.