Murram: The Surface That Won't Hold Still

A huge share of driving happens on the unpaved Class D and E roads from the road-classification lesson, and the friction framework explains why murram earns its reputation.

• Dry, compacted murram can grip respectably. But a skin of loose gravel or dust over a hard base behaves like ball bearings: the wheel locks the loose layer, not the road, and f drops toward 0.3 with no visual warning.
• Wet murram ranges from merely slick to genuinely mud-like — f 0.2–0.3, braking three to four times the dry-tarmac figures.
• The defining property is variance, not any single value. Tarmac's grip is roughly uniform for kilometers; murram's changes meter to meter — compacted, loose, rutted, wet — faster than you can re-estimate f.

So the planning answer isn't a better coefficient. It's a speed low enough that the worst plausible patch is survivable — the margin-for-the-invisible principle, applied to the road surface itself. You're not driving for the grip you have; you're driving for the grip that might vanish under the next wheel.

One thing to carry into the braking-technique lesson: ABS — the system that saves you on tarmac — can actually lengthen stops on loose gravel. Murram is the reason that caveat matters.