Why must you drive well below the posted limit when the road surface is wet or icy?

Why D is correct

A posted limit is calculated for ideal conditions and states the maximum permitted rather than the speed that is actually safe today. On a wet surface available grip falls sharply, so brake pressure that stops you comfortably on dry pavement locks wheels much earlier, and a steering input that turns the car crisply in the dry now demands more grip than the surface has to give. Winter adds a visibility penalty too: spray from other vehicles, fogged glass and salt film all reduce how far ahead you can plan. Together they mean the safe speed is whatever lets you stop inside the distance you can genuinely see, and on ice that can be comfortably under half the posted number.

Why the other options are wrong

A. Because posted limits stop applying in poor weather

The limit does not switch off in bad weather — it is simply that exceeding it was never the requirement. Playing by the posted number withthe understanding that the number assumes dry roads is the point; treating it as legally void is a different and wrong idea entirely.

B. Because headlights are more effective at lower speeds

Headlight performance is unchanged by your road speed in any way relevant to grip. Visibility does degrade in poor weather, but it is a reason alongside traction rather than instead of it, and slowing mostly buys you stopping distance.

C. Because other drivers will expect everyone to slow down

Other road users' expectations carry no weight against physics. Even driving alone on an empty icy road, the grip available to your tyres is the same, and so is the distance you need to stop.

On the road

Freezing rain overnight leaves a sheen on a bridge deck that nobody can see from the driving seat. The limit says fifty-five. A driver doing forty finds the lane drifting wide through a gentle bend and learns from a stationary car that the surface offers almost nothing at all.

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