Rational Driving Based on Safety Regulations
The core idea
Rational driving means choosing speed, gear, spacing and braking so that the vehicle stays inside the limits of physics, law and visibility. The exam tests whether you can link a rule to a physical consequence.
Concepts to master
- Stopping distance = perception/reaction distance + braking distance. Reaction distance grows linearly with speed; braking distance grows with the square of speed. Doubling speed roughly quadruples braking distance.
- Safety margins: two-second rule in dry conditions, at least double in rain, more on ice or with a heavy load.
- Defensive observation: mirror checks before any change of direction, blind-spot checks, scanning far ahead, anticipating vulnerable road users.
- Engine braking and retarders on long descents: select a lower gear before the slope, not during it. Continuous service-brake use overheats drums/discs and causes fade.
- Ecodriving: early upshifting, steady throttle, avoiding unnecessary idling, using cruise control where appropriate.
Traps in questions
- Confusing reaction distance with braking distance.
- Assuming ABS shortens braking distance on all surfaces - on gravel or snow it may lengthen it, though steering control is retained.
- Believing a retarder replaces the service brake entirely; it supplements it.
- Ignoring that a laden vehicle needs longer to stop and has a higher centre of gravity.
Worked example
A truck travels at 80 km/h. Reaction time 1 s gives about 22 m. If braking distance at 80 km/h is roughly 60 m on dry asphalt, total stopping distance is about 82 m. At 40 km/h the reaction distance halves to 11 m and the braking distance falls to about 15 m - total roughly 26 m. Rule of thumb: halve the speed, quarter the braking distance.