Speed limits, following distance and safe driving
Speed limits, following distance and safe driving are tested with practical scenarios rather than definitions. You need the default limits, why a limit is not a target, and how speed changes stopping distance.
Speed limits
- In built-up areas the default limit is 50 km/h; on the open road it is 80 km/h, unless a sign shows otherwise.
- Posted signs override the defaults, and lower limits apply near schools during school hours and at roadworks.
- The limit is the maximum for ideal conditions, not a safe speed in rain, at night, on gravel or when towing.
- Keep left, and drive at a speed that lets you stop within the distance you can see to be clear.
Following distance
- Use the two-second rule in dry conditions: watch the vehicle ahead pass a fixed point, then count two seconds before you reach it.
- Double it to four seconds in rain, at night, when following a truck or when towing.
- Never tailgate: the driver ahead cannot see you, and you cannot see past them.
Stopping distance
Stopping distance is thinking distance plus braking distance. Thinking distance grows in proportion to speed; braking distance grows with the square of speed.
Overtaking and conditions
- Overtake only when it is legal and you can see the whole way is clear; never across a solid centre line, on a blind bend or crest, or at a crossing.
- Use headlights at night and in heavy rain, and make sure they are actually on.
- Fatigue slows reaction time much like alcohol; stop and rest rather than pushing on.
Worked example
A car travelling at 50 km/h stops in roughly 27 metres in good conditions. At 100 km/h the thinking distance doubles, but the braking distance roughly quadruples, so total stopping distance is far more than double - well over 90 metres. That is why a small increase in speed on a wet road has a large effect on whether you stop in time.
Traps
- Treating the limit as a target on a wet road.
- Counting 'one Mississippi' slowly rather than a true two seconds.
- Overtaking a truck on a bend because the road looks clear for a moment.
- Forgetting that a loaded vehicle needs longer to stop.