THEORY TEST PASSED
Stopping distances calculator
Thinking distance, braking distance and the total, in metres and in car lengths. Most people underestimate the total at 70 mph by half.
Why the test asks in car lengths
The Highway Code publishes stopping distances in feet and metres, but the question usually asks how many car lengths. A car length is taken as four metres, which is why 70 mph works out at 24 car lengths in the dry.
- Thinking distance is your reaction time, and it scales with speed
- Braking distance scales with the square of speed, which is why 70 is not twice 35
- Wet roads roughly double the braking distance
- Ice can multiply it by ten
Figures follow the Highway Code stopping distance table for a car in good conditions with good brakes and tyres. Real distances depend on the vehicle, the surface and the driver.
The whole table, dry conditions
Worth knowing by heart. The car length column is the one the test asks about.
| Feature | Thinking | Braking | Total | Car lengths |
|---|---|---|---|---|
| 20 mph | 6 m | 6 m | 12 m | 3 |
| 30 mph | 9 m | 14 m | 23 m | 6 |
| 40 mph | 12 m | 24 m | 37 m | 9 |
| 50 mph | 15 m | 38 m | 53 m | 13 |
| 60 mph | 18 m | 55 m | 73 m | 18 |
| 70 mph | 21 m | 75 m | 96 m | 24 |
Based on the Highway Code stopping distance table. Wet doubles the braking distance and ice can multiply it by ten; thinking distance is unchanged by the surface but rises with tiredness, alcohol and distraction.
Stopping distance questions
What is the stopping distance at 70 mph?
96 metres in the dry, which is 24 car lengths: 21 metres of thinking distance and 75 metres of braking. In the wet the braking part roughly doubles.
Why does braking distance grow so fast?
Because it scales with the square of speed. Doubling your speed roughly quadruples the braking distance, which is why 70 mph is far more than twice the stopping distance of 35 mph.
Does thinking distance change in the rain?
No. Thinking distance is your reaction time, so the surface does not change it. Tiredness, alcohol, drugs and distraction do.
How long is a car length?
Four metres, for the purposes of the test. That is what turns the metric figures into the car length answers the questions use.
Knowing the table is a handful of marks
Stopping distances are a few questions. Passing takes the whole bank. Get the full DVSA question bank, every hazard perception clip and unlimited mocks for £4.49.
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