---
title: "Stopping distance at every speed, with the maths"
description: "Reaction distance plus braking distance from 20 to 130 km/h, on dry, wet, snow and ice — computed, with the reaction time and friction figure stated openly."
url: https://atoneplace.net/reaction-time/stopping-distance/
site: "At One Place"
---

# Stopping distance at every speed

**Stopping distance is two things added together: How far you travel before you touch the brake, plus how far you travel after**

The first is proportional to speed. The second is proportional to speed squared — which is why doubling your speed more than doubles the distance.

Published stopping-distance tables disagree with each other, and the reason is always the same: they quietly choose different reaction times and different tyre friction figures. This one uses 1.35 seconds, which is the middle of the measured range for an unanticipated hazard, and a friction coefficient of 0.7 for dry asphalt. Both are stated so you can see what changes when they change.

## The arithmetic

- Reaction distance = speed × reaction time. At 100 km/h that is 27.8 m/s × 1.35 s = 38 metres before the brake is touched.
- Braking distance = v² ÷ (2 × μ × g), the standard constant-deceleration result, assuming the tyres stay at the friction limit — an ABS car in good condition on a level road.
- μ is the tyre-road friction coefficient: about 0.7 on dry asphalt, 0.4 wet, 0.2 on packed snow, 0.1 on ice. g is 9.81 m/s².
- Only the braking term is squared. That is why the total rises far faster than the speed does.

## On dry asphalt

*Reaction time 1.35 s, μ 0.7, level road, ABS, good tyres. Worn tyres or a downhill grade lengthen the braking term substantially.*

| Speed | Reaction distance | Braking distance | Total |
| --- | --- | --- | --- |
| 20 km/h — 12 mph | 8 m | 2 m | 10 m — 32 ft |
| 30 km/h — 19 mph | 11 m | 5 m | 16 m — 54 ft |
| 40 km/h — 25 mph | 15 m | 9 m | 24 m — 79 ft |
| 50 km/h — 31 mph | 19 m | 14 m | 33 m — 108 ft |
| 60 km/h — 37 mph | 23 m | 20 m | 43 m — 140 ft |
| 70 km/h — 44 mph | 26 m | 28 m | 54 m — 176 ft |
| 80 km/h — 50 mph | 30 m | 36 m | 66 m — 216 ft |
| 90 km/h — 56 mph | 34 m | 46 m | 79 m — 260 ft |
| 100 km/h — 62 mph | 38 m | 56 m | 94 m — 307 ft |
| 110 km/h — 68 mph | 41 m | 68 m | 109 m — 358 ft |
| 120 km/h — 75 mph | 45 m | 81 m | 126 m — 413 ft |
| 130 km/h — 81 mph | 49 m | 95 m | 144 m — 471 ft |

## What the surface does

*Total stopping distance, same reaction time throughout. Ice roughly triples the dry-road figure.*

| Speed | Dry asphalt | Wet asphalt | Packed snow | Ice |
| --- | --- | --- | --- | --- |
| 20 km/h | 10 m | 11 m | 15 m | 23 m |
| 30 km/h | 16 m | 20 m | 29 m | 47 m |
| 40 km/h | 24 m | 31 m | 46 m | 78 m |
| 50 km/h | 33 m | 43 m | 68 m | 117 m |
| 60 km/h | 43 m | 58 m | 93 m | 164 m |
| 80 km/h | 66 m | 93 m | 156 m | 282 m |
| 100 km/h | 94 m | 136 m | 234 m | 431 m |
| 120 km/h | 126 m | 187 m | 328 m | 611 m |

## Why the two-second rule works

Following distance measured in seconds rather than car lengths automatically scales the reaction component with speed, which is exactly the part that scales linearly. It does nothing for the braking component, which is why the advice becomes four seconds in the wet and considerably more on ice — you are buying margin for the squared term.

> These figures assume you brake at the friction limit immediately. Real drivers frequently do not brake hard enough in the first second of an emergency, which is what brake assist systems exist to correct.

## Common questions

### What is the stopping distance at 100 km/h?

About 94 metres on dry asphalt — 38 m of reaction distance and 56 m of braking. On a wet road it is about 136 m.

### Why does doubling speed more than double stopping distance?

Because only the reaction component scales with speed. The braking component scales with speed squared, since kinetic energy does. Doubling from 50 to 100 km/h roughly quadruples the braking distance.

### Why do published stopping distance tables disagree?

They choose different reaction times — anywhere from 0.7 to 2.5 seconds — and different tyre friction coefficients. Both assumptions move the answer a long way, and most tables do not state either.

## Related pages

- [Driver reaction to a surprise](https://atoneplace.net/reaction-time/driver-reaction-surprise/)
- [The 2.5-second road design figure](https://atoneplace.net/reaction-time/aashto-perception-reaction/)
- [Reaction times in driving](https://atoneplace.net/reaction-time/driving/)

## Sources

- Typical range compiled from manufacturer specifications — At One Place (/about/)
- A Policy on Geometric Design of Highways and Streets (the Green Book) — American Association of State Highway and Transportation Officials (https://store.transportation.org/)
- Calculated on this page — At One Place (/about/)