Speed limits, stopping distance and safe following distance
This domain tests the numbers and the reasoning behind them: Danish speed limits, how stopping distance is composed, and how big a gap you must leave. Exam questions rarely ask for a limit alone - they set it in rain, behind a lorry, or on a blind crest.
Limits in Denmark
- Built-up area: 50 km/h unless signed otherwise (30 km/h zones and lower local limits exist).
- Outside built-up areas: 80 km/h.
- Expressways (motortrafikvej): typically 80 km/h, 90 km/h where signed.
- Motorways (motorvej): 110 or 130 km/h as signed.
Limits are ceilings, not targets. The stronger rule is that you must be able to stop within the distance you can see to be clear - which is why 80 km/h can be too fast over a blind rural crest.
The arithmetic of stopping
Total stopping distance = reaction distance + braking distance.
- Reaction distance: speed in m/s × reaction time. At one second: 50 km/h ≈ 14 m, 80 km/h ≈ 22 m.
- Braking distance: v² ÷ (2 × μ × g). It grows with the square of speed, so 100 km/h needs about four times the braking distance of 50 km/h.
Following distance
Two seconds minimum in the dry, measured from a fixed point the vehicle ahead passes. Three seconds in rain, four or more on gravel, wet leaves or ice. At 80 km/h two seconds is roughly 45 m. Drop further back behind lorries and buses, which block your view and hide you from oncoming traffic.
Worked example
You ride at 80 km/h on dry asphalt (μ ≈ 0.7). Reaction: 80 ÷ 3.6 = 22.2 m/s, one second = 22 m. Braking: 22.2² ÷ (2 × 0.7 × 9.81) = 493 ÷ 13.7 ≈ 36 m. Total ≈ 58 m, about fourteen car lengths. In rain (μ ≈ 0.45) braking becomes 493 ÷ 8.8 ≈ 56 m, total ≈ 78 m. ABS and good tyres do not change the arithmetic; they only make reaching the figure more likely.
Traps that cost marks
- Doubling speed more than doubles stopping distance - the braking part squares.
- Quoting 50 km/h for an unsigned country road.
- Treating ABS as permission to brake later.
- Believing a two-second gap is generous; it is a minimum, and wet weather adds a second.
- Forgetting that speeding also carries klip on the licence, not just a fine.