Speed, stopping distance, and road conditions
Domain 6 is the arithmetic of stopping. The exam asks you to compare total stopping distances, explain why speed matters so much, and judge what a road surface does to your braking.
The two parts of stopping distance
Total stopping distance = reaction distance + braking distance.
- Reaction distance: speed multiplied by your reaction time, roughly one second. At 50 km/h you travel about 14 m before the brakes bite.
- Braking distance grows with the square of the speed: double the speed and you need four times the braking distance.
Typical dry figures
- 30 km/h: about 4 m braking, roughly 13 m in total.
- 50 km/h: about 12-13 m braking, roughly 27 m in total.
- 80 km/h: about 31 m braking, roughly 54 m in total.
- 100 km/h: about 50 m braking, roughly 78 m in total.
Road conditions
- Wet asphalt roughly doubles the braking distance; standing water can more than that, and aquaplaning removes braking and steering altogether.
- Gravel, sand, wet leaves, painted lines, metal covers and ice give far less grip and punish heavy front-brake use.
- A passenger and luggage lengthen the braking distance and change the suspension setting and tyre pressures you should run.
- Tiredness, alcohol and medication lengthen reaction time first, which adds metres before you have touched the lever.
Worked example
You ride at 30 km/h past a school where the limit rises to 50 km/h. Braking distance goes from about 4 m to about 12-13 m - roughly three times longer, not twice - because 50 is 1.67 times 30 and 1.67 squared is about 2.8. Total stopping distance goes from about 13 m to about 27 m.
Traps
Thinking total stopping distance doubles when speed doubles. Forgetting reaction distance and calculating only the brakes. Believing ABS shortens braking on gravel - it mainly stops the wheel locking and preserves steering. Assuming a motorcycle stops faster than a car: two small contact patches often need more room, not less.