Speed, following distance and stopping distance
What the exam tests
This domain tests whether you can choose a safe speed and a following distance that leaves room to stop. The exam expects you to apply the two-second rule and to understand that braking distance grows with the square of speed.
Core rules
- The posted limit is a maximum, not a target. Rain, gravel, darkness, bends and traffic require a lower speed.
- Use the two-second rule in good conditions. Pick a fixed point, count two seconds after the vehicle ahead passes it, and ensure you pass it no earlier. In rain, tunnels or poor visibility, increase to at least three or four seconds.
- Total stopping distance is reaction distance plus braking distance. Reaction distance grows linearly with speed; braking distance grows roughly with speed squared.
- At 80 km/h you travel about 22 m in one second. A one-second reaction alone is therefore 22 m before the brakes bite.
- Wet roads roughly double braking distance. Gravel, leaves, painted lines and diesel reduce grip further.
- Keep a larger gap behind large vehicles, which block your view and have long stopping distances.
Worked example
You follow a car at 80 km/h. The two-second gap is about 44 m. If the car brakes suddenly, your reaction distance is about 22 m and dry braking may add around 30 m or more. Your total stopping distance is therefore roughly 52 m, already greater than a two-second gap if the car stops instantly. On a wet road the braking distance can approach 60 m, so the gap must be larger.
Traps
Treating the two-second rule as a target in rain; measuring from the car rather than a fixed point; believing a modern brake or ABS cancels the physics; following a lorry closely to "see past" it; confusing speed limit with safe speed.