Loose Surfaces Don't Brake Like Tarmac

What Changes Your Numbers treated surfaces as multipliers on your stopping distance. Murram adds a complication that a multiplier can't capture.

Dry murram and gravel give less grip than tarmac — the loose top layer rolls under the tyre, so some of your braking is spent moving stones rather than slowing the car. That much is a longer stopping distance, and you can plan for it.

But the real problem is variability. The surface isn't uniform: a patch of hard-packed murram grips reasonably, and two metres later a drift of loose gravel grips very little. So your braking distance on a murram road isn't a number you can carry in your head the way you can on tarmac — it changes underneath you, sometimes within a single stop. That unpredictability is the thing to plan around, and the only honest way to plan around it is margin: a lower speed and a bigger gap than the surface seems to need, because the surface's behaviour is not consistent enough to trust.

Wet murram is a different surface again. Where wet tarmac loses a predictable fraction of its grip, murram turns to mud — and grip can approach zero, particularly on the smooth, polished sections worn by traffic. A wet murram road after the first rains is one of the most slippery surfaces a Kenyan driver meets, and it combines directly with the first-rain effect from Rain and Wet Roads.