Reaction Distance

Reaction distance grows in direct proportion to speed: double the speed, double the reaction distance. Five times the speed, five times the distance. Simple, linear, intuitive.

What stretches it — all human factors, previewing Human Factors:

• Needing to decide. A hazard demanding a choice — brake or swerve? — measurably lengthens reaction time versus a hazard with one obvious response.
• Alcohol, drugs, and medication.
• Tiredness.

What shrinks it — and this is the empowering half:

• Anticipation. A driver who has already spotted the developing hazard reacts from a prepared position — the perception step is already done.
• Preparedness. Foot hovering over the brake near a school gate is reaction distance pre-paid.

This is why reaction distance is the stage most within your control: you can't change physics, but you arrive at every hazard either prepared or surprised, and that choice was made seconds earlier.

Calculating it — the mental-math method:

1. Remove the last digit of your speed. (50 km/h → 5)
2. Multiply by your reaction time in seconds. (× 1 = 5)
3. Multiply by 3. (= 15 metres, at 50 km/h with a 1-second reaction)

The precise formula: distance = (speed × reaction time) ÷ 3.6 — the 3.6 converts km/h to metres per second. At 50 km/h × 1s: 50 ÷ 3.6 = 13.9 m. The mental method rounds slightly high, which for a safety margin is the right direction to round.