Advanced Training in Rational Driving Based on Safety Regulations
The idea behind "rational driving"
Rational driving means choosing speed, gear, road position and braking so that the vehicle stays inside both the law and the limits of physics, while using as little fuel and brake wear as possible. The examiner wants to see that you can link a rule to a physical consequence.
Concepts you must know
- 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.
- Kinetic energy rises with the square of speed, so a heavy lorry at 80 km/h carries far more destructive energy than a car at the same speed.
- Brake fade and overheating: continuous braking on long descents overheats drums/discs. Correct technique is to select a low gear and use the engine/retarder, applying the service brake in short firm applications.
- Aquaplaning, jack-knifing, load transfer in bends and on wet or icy surfaces; the effect of a high centre of gravity on rollover risk.
- Defensive observation: mirror use, blind spots, following distance in seconds rather than metres, anticipation of vulnerable road users.
Traps in questions
- Questions that offer "brake harder" as the answer for a long descent - the correct answer is normally engine braking plus a lower gear.
- Statements implying braking distance doubles when speed doubles - it does not.
- Confusing reaction distance with total stopping distance.
- Options that treat ABS as a guarantee of a shorter stop on ice; ABS mainly preserves steering control.
Rule of thumb
Use the two-second rule in dry conditions, at least double it in rain and ten times on ice, and remember that a laden lorry needs noticeably more than a car at the same speed.