The second car is drawn behind yours, braking from the same line.
Downhill is negative. Gravity is helping the car along instead of the tyres.
The Highway Code figures are not a measurement of your car. Work backwards from them and they assume a 0.67 second reaction and 0.67 g of braking — brisk reactions and, by modern standards, modest brakes. On a dry road a current car beats them comfortably. In the wet it does not.
| Speed | Official total | This car, here | Difference | Thinking | Braking |
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Releases in which this page changed, newest last. Derived from the archived copy of every release, not from notes written afterwards — so it reflects what actually shipped. Site-wide passes are left out; they are in the full changelog.
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How far you travel before the brakes are touched: speed × reaction time. Linear in speed, and nothing on the car changes it.
v²/(2μg): proportional to speed squared, because that is how kinetic energy scales.
About 0.90 on dry tarmac, 0.60 wet, 0.12 on ice. Braking distance is inversely proportional to it.
Worked backwards, the official UK table assumes a 0.67 s reaction and 0.67 g of braking. Shown alongside, not reproduced.
Double your speed and the braking distance quadruples, exactly: kinetic energy goes as the square of speed. But the number on the sign is the total, and thinking distance only doubles, so going from 30 to 60 mph roughly trebles the total.
At the Highway Code’s own assumptions a car doing 30 mph stops in 23 metres. A car doing 40 mph arrives at that same point still doing very nearly 30: it has not shed the extra ten miles an hour, it has not shed anything. It is still at the speed limit where the other car was already stationary.
You cannot fit better brakes to your attention. At 70 mph you cover 31 metres every second, so the difference between an alert driver and one reading a message is 73 metres of road before the brakes are touched: more than two thirds of the official stopping distance, spent doing nothing.
Thinking distance is speed times reaction time, so a gap measured in seconds covers it at any speed while a gap in car lengths does not. Two seconds on a dry road, four in the wet, and the arithmetic never needs doing again.
Every figure is speed to zero: the right question for a child, a stopped queue or a fallen branch. Following another car is a different sum, because they are braking too; the moment the thing ahead is stationary rather than slowing, the full distance is back.
Gradient: downhill adds gravity to your momentum and lengthens the stop; uphill shortens it. ABS stops the wheels locking so you keep steering, and on most surfaces shortens the stop. Aquaplaning: standing water lifts the tyre off the road entirely: grip does not reduce, it disappears. g is 9.81 m/s²: a car braking at 1 g sheds 9.81 metres per second of speed every second.
It quadruples the braking distance, because kinetic energy goes as the square of speed. But the number on the sign is the total, and thinking distance only doubles — so going from 30 to 60 mph roughly trebles the total rather than quadrupling it. The rule is right about the physics and wrong about the answer.
Grip falls from about 0.90 dry to 0.60 merely wet, which stretches the braking distance by 50% with no standing water and no drama. Ice takes it to about 0.12 — a factor of seven and a half. At 30 mph that is roughly 10 m of braking dry and about 76 m on ice, and nothing you do with the pedal recovers it.
A time, not a length. Thinking distance is speed multiplied by reaction time, so a gap measured in seconds covers it automatically at any speed, while a gap in car lengths does not. Two seconds on a dry road, four in the wet, and the arithmetic never needs doing again.
Because it is the half you can actually change — you cannot fit better brakes to your attention. At 70 mph you cover 31 metres every second, so the difference between an alert driver and one reading a message is 73 metres of road covered before the brakes are touched. That is more than two thirds of the official stopping distance, spent doing nothing.