Speed limits, following distance and stopping
Speed is examined as a decision, not a number. The questions check whether you can choose a speed from which you can stop within the distance you can see to be clear, and whether you can estimate following and stopping distance in real conditions rather than reciting a table.
The limits
- Built-up areas: about 60 km/h unless signs say otherwise, and lower near schools, markets and crowded roadside activity.
- Outside built-up areas: about 90 km/h on ordinary roads, lower on unpaved, eroded or laterite sections.
- Posted limits override the general figure; conditions override the posted limit. Rain, dust, darkness, a live load or a crumbling shoulder all require less speed.
- The overriding rule: you must be able to stop within the distance you can see to be clear. On a blind crest or through a flooded dip that distance may be 20 m.
Following distance
Use time, not metres. Leave at least two seconds in dry weather; four seconds in rain, on gravel, or behind a lorry that blocks your view. Count 'one thousand and one, one thousand and two' from the moment the vehicle ahead passes a fixed point such as a sign, and check that the count finishes after that point.
Stopping distance
Total stopping distance = reaction distance + braking distance. Reaction distance grows linearly with speed: roughly 17 m per second at 60 km/h. Braking distance grows with the square of speed, so doubling speed roughly quadruples the braking distance.
Worked example
At 90 km/h you cover 25 m in one second of reaction. Braking on dry asphalt takes about another 55 m, so you need roughly 80 m to stop. In heavy rain the braking distance roughly doubles, taking the total past 100 m. If the downpour cuts visibility to 50 m, no speed near 90 km/h is legal or survivable: you must slow until the clear distance ahead exceeds the stopping distance. The reasoning, not the arithmetic, earns the mark.
Traps that cost marks
Treating the limit as a target; quoting a memorised stopping distance without adjusting for rain or load; following a lorry closely because it seems to show the way; and forgetting that an overloaded pickup or a vehicle with worn dampers needs noticeably more road to stop.