The Two Components of Stopping Distance
Total stopping distance = Reaction distance + Braking distance
Reaction Distance
This is the distance covered while your brain perceives a hazard and your foot moves to the brake. For an alert driver, reaction time is approximately 0.8–1.0 seconds. At 90 km/h that is 25 metres of travel before braking even begins.
Factors that increase reaction time: fatigue, alcohol, distraction, and age.
Braking Distance
Braking distance increases with the square of speed. Double your speed → braking distance quadruples:
- 30 km/h — ~5 m braking distance
- 50 km/h — ~14 m
- 70 km/h — ~28 m
- 90 km/h — ~46 m
- 110 km/h — ~69 m
Total Stopping Distance
- 50 km/h — ~28 m total (about 6 car lengths)
- 70 km/h — ~47 m total (about 10 car lengths)
- 90 km/h — ~71 m total (about 15 car lengths)
- 110 km/h — ~100 m total (about 21 car lengths)
How Conditions Multiply Stopping Distance
- Wet road — braking distance roughly doubles
- Ice — braking distance can be 5–10 times longer than on dry tarmac
- Worn tyres — significantly reduce grip, especially in wet conditions
- Downhill slope — increases braking distance; reduce speed before descents
ABS (Anti-lock Braking System)
ABS prevents wheels from locking under hard braking, allowing steering while braking hard. On ABS-equipped cars: apply firm, continuous pressure — do not pump the brakes. ABS does not significantly shorten dry-road stopping distance but helps greatly on wet and slippery surfaces.
Key Takeaways
- Stopping distance = reaction distance + braking distance
- Doubling speed → quadruples braking distance
- At 90 km/h you travel 25 m before your foot hits the brake
- Ice can multiply stopping distance by 5–10×
- On ABS cars: press firm and continuous — do not pump