Speed, Following Distance and Stopping
This domain is arithmetic in motion. The exam expects you to combine speed, reaction time and braking distance into one figure, and to know how that figure changes with weather and load.
Following distance
The minimum gap is two seconds: pick a fixed point, and count the time between the vehicle in front passing it and you passing it. Three seconds in rain, and more at night, with a passenger, or behind a large vehicle that blocks your view. Two seconds at 80 km/h is roughly 45 metres, not two car lengths.
Stopping distance
Stopping distance = reaction distance + braking distance.
- Reaction distance: about one second of travel, so speed in km/h divided by 3.6 gives the metres covered. At 100 km/h that is about 28 metres.
- Braking distance on dry asphalt is roughly (speed ÷ 10)² metres: 100 km/h gives about 100 metres.
- Wet road roughly doubles the braking distance; ice can multiply it many times over.
Worked example
You ride at 100 km/h on a dry motorway. Reaction: 100 ÷ 3.6 ≈ 28 m. Braking: (100 ÷ 10)² = 100 m. Total stopping distance ≈ 128 m. In heavy rain the braking part doubles, giving roughly 228 m - nearly the length of two and a half football pitches.
Choosing a speed
Good speed is the speed at which you can stop within the distance you can see to be clear, which on a narrow country road or in a 30 km/h zone is often well below the posted limit.
Traps that cost marks
Measuring the two-second gap as front bumper to front bumper; believing braking distance rises in step with speed rather than with its square; and forgetting that a tired rider's reaction time is well over one second.