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Road Rules Foundation
Road Signs & Markings
Speed & Stopping Distances
Load Safety
Adverse Conditions
One Variable to Rule Them: Friction
The precise braking formula from the physics lesson — distance = speed² ÷ (250 × f) — makes one thing explicit: every surface condition in this lesson acts through a single number, the friction coefficient f. Dry tarmac is about 0.8. Halve the friction and you double the braking distance; quarter it and you quadruple it.
That formula is a reasoning tool no memorised table can give you: when you can estimate what a surface does to grip, you can estimate what it does to your stop.
The reference ladder:
| Surface | f(approx) | Braking vs dry |
|---|---|---|
| Dry tarmac, good tyres | 0.8 | x1 |
| Wet tarmac, steady rain | 0.4 | x2 |
| Mud / first-rain film / loose murram (plan for) | 0.2-0.3 | x3-4 |
Key takeaways
- Nearly every condition acts through one variable, friction: distance = speed² ÷ (250 × f) — halve the grip, double the stop
- The ladder: dry 0.8 · steady wet 0.4 · mud/first-rain/loose murram 0.2–0.3 · standing water at speed ≈ 0 — and it touches braking only, never reaction
Quick check
A surface cuts your grip in half compared with dry tarmac. What happens to your braking distance?