UK stopping distances explained: the Highway Code table and the formula behind it
Most pages on stopping distances just hand you the Highway Code table and tell you to memorise it. That works for exactly six speeds — 20, 30, 40, 50, 60 and 70mph — and falls apart the moment a theory test question, or a real road, gives you 35mph or 55mph instead. There's a simple formula behind the table that produces any speed's stopping distance on demand, and it also explains a detail most guides get wrong: wet and icy roads don't lengthen your reaction time, they lengthen your braking distance. Knowing which part of the number actually changes is what turns "safety margins" from a memorised table into something you can reason through.
The official table (Highway Code Rule 126)
Stopping distance is thinking distance (how far you travel before your foot reaches the brake) plus braking distance (how far you travel once you're braking). The Highway Code publishes both, in good, dry conditions, for an alert driver in an average car with good brakes and tyres:
| Speed | Thinking distance | Braking distance | Total stopping distance |
|---|---|---|---|
| 20 mph | 6 m (20 ft) | 6 m (20 ft) | 12 m (40 ft) |
| 30 mph | 9 m (30 ft) | 14 m (45 ft) | 23 m (75 ft) |
| 40 mph | 12 m (40 ft) | 24 m (80 ft) | 36 m (118 ft) |
| 50 mph | 15 m (50 ft) | 38 m (125 ft) | 53 m (174 ft) |
| 60 mph | 18 m (60 ft) | 55 m (180 ft) | 73 m (240 ft) |
| 70 mph | 21 m (70 ft) | 75 m (245 ft) | 96 m (315 ft) |
Look at how the two columns grow: thinking distance rises in a straight line with speed (double the speed, double the distance), while braking distance rises much faster. That difference in shape is the whole story, and it's what the formula below makes explicit.
The formula hiding behind the table
The Highway Code's imperial version of the table comes from one expression: for a speed v in mph, stopping distance in feet is v + v²/20. The two terms map directly onto the two distances above:
- Thinking distance (feet) = v. This falls out of the Highway Code's assumed reaction time of just under 0.7 seconds: at that reaction time, distance travelled in feet works out to almost exactly the speed in mph. That's why 30mph gives 30ft of thinking distance and 70mph gives 70ft — no separate calculation needed.
- Braking distance (feet) = v²/20. This is why braking distance grows so much faster than thinking distance: double the speed and you quadruple the braking distance (v²), not just double it. Going from 30 to 60mph roughly triples your total stopping distance for exactly this reason.
Because it's a formula rather than six fixed lookups, you can use it for any speed a question throws at you, not just the six in the table. At 35mph: thinking distance is 35ft, braking distance is 35²/20 = 61ft, giving a total of about 96ft (roughly 29m). At 55mph: thinking distance 55ft, braking distance 55²/20 = 151ft, total around 206ft (roughly 63m).
Worked example: 50mph on a dry road
A car is travelling at 50mph on a dry, level road when the driver spots a hazard. Using the table (or the formula: 50 + 50²/20 = 50 + 125 = 175ft, matching the Highway Code's 174ft after metric rounding):
- Thinking distance: 15m (50ft) — the car still travels this far before the brakes are even applied.
- Braking distance: 38m (125ft) — the distance covered once the brakes are on, until the car stops.
- Total stopping distance: 53m (about 174-175ft) — roughly thirteen car lengths, from the moment the hazard appears to a complete stop.
That figure assumes good brakes, good tyres, a dry road and an alert driver. Change any one of those and the number gets worse — which is exactly what "safety margins" as a test topic is testing you on.
Wet and icy roads: what actually changes (and what doesn't)
This is the detail most stopping-distance guides skip past: bad weather doesn't make you think slower. Your reaction time is a property of you, not the road, so thinking distance stays the same figure as in the dry table. What changes is grip, so it's the braking distance that stretches — sometimes dramatically. The Highway Code's own guidance is that the safe following gap "should be at least doubled on wet roads and up to ten times greater on icy roads," which in practice means: at least double your braking distance in the wet, and allow for it to be as much as ten times longer on ice or snow.
Worked example: 60mph in the wet
Same hazard, same 60mph speed, but now the road is wet:
- Thinking distance: unchanged at 18m (60ft) — your reaction time hasn't moved.
- Braking distance: at least double the dry figure of 55m, so at least 110m.
- Total stopping distance: at least 128m — nearly double the dry-road total of 73m, even though the speed hasn't changed at all.
On ice, the same 55m dry braking distance could become 550m in the worst case (the Highway Code's "up to ten times" ceiling) — which is why the guidance for icy roads is less "leave a bigger gap" and more "consider whether you should be driving at all."
Turning the table into a live check: the two-second rule
You can't measure metres while driving, so the Highway Code gives a practical proxy: pick a fixed point the vehicle ahead passes (a sign, a bridge, a lamppost), and count at least two seconds before you reach the same point. That two-second gap is a rough proxy for a safe following distance in the dry at normal speeds. Following the wet/icy guidance above, that becomes at least four seconds in the wet, and considerably more on ice — the same doubling and "up to ten times" logic applied to a gap you can actually judge in real time, rather than to a table you'd have to memorise metre by metre.
Why you'll sometimes see slightly different numbers
Cross-check two sources and the 40mph total sometimes reads 118ft, sometimes 120ft. Both are "correct" — they come from two different starting points that were never fully reconciled. The clean imperial formula, v + v²/20, gives 40 + 1600/20 = 120ft directly. But the Highway Code's official table is defined in metres first (36m at 40mph), and 36m only converts to about 118ft. The 2ft gap is a rounding artifact from converting between metric and imperial, not a real difference in how far your car travels — a discrepancy noted in detail by Chalkdust magazine's analysis of the Highway Code's figures. If a theory-test question and a revision app disagree by a couple of feet at the same speed, this is almost always why.
Common mistakes worth avoiding
- Assuming wet weather lengthens thinking distance. It doesn't — only braking distance is affected, because grip (not reaction time) is what changes.
- Treating the two-second rule as the whole answer. It's a proxy for the dry-road table at normal speeds, not a fixed safe distance at every speed and in every condition — it still needs to become four seconds or more in the wet.
- Forgetting that braking distance is roughly proportional to the square of your speed, not your speed itself — a small increase in speed costs you more stopping distance than it looks like it should.
Try these numbers against real Highway Code scenarios with PassDrill's safety margins practice questions, which cover following distances, weather conditions and the reasoning behind them.
Source: The Highway Code, Rule 126 (gov.uk) for the official stopping-distance guidance and the wet/icy following-gap wording; figures cross-checked against The AA's and RAC's published stopping-distance tables; the imperial formula and the metric/imperial rounding discrepancy verified against Chalkdust magazine's analysis of the Highway Code's stopping-distance figures. This is educational study material for the UK driving theory test, not a substitute for practical driving instruction.