Which signs apply only for the period of time when the roadway pavement is wet?

Explanation

Only sign C; the 50 km/h speed restriction together with the "wet surface" plate.

The plate beneath sign C bears the image of a cloud with raindrops, and its job is to limit the time scope of the main sign: the 50 restriction is valid only while the carriageway surface is wet. On a dry road that same sign has no effect and the general limit for that stretch applies.

The safety logic of this works both ways. On wet asphalt the friction coefficient falls sharply and the stopping distance at the same speed becomes far longer; the risk of aquaplaning is added too. But if you applied the same restriction in dry weather, you would create a sign that most of the time looks pointless, and drivers would gradually ignore it — and then they would not observe it on a rainy day either. Making the sign conditional on circumstances preserves its authority.

The main trap is sign A. Its image is a vehicle skidding on a slippery surface, and the mind instantly translates it as "wet road". But slipperiness can also come from ice, mud, leaves or an oil patch; that sign is not conditional and is always in force. The rule to memorise: a limitation to particular conditions is announced only by a supplementary plate beneath the sign, never by the image on the sign itself.

• "Only A and C" is attractive because sign A's image shows skidding on a slippery surface; but that sign has no conditional plate and is in force on a dry icy or oily road too. • "Only B" is not right; sign B warns of gravel thrown up by the wheels and has nothing to do with the wetness of the road surface. • "All" would mean all three signs are conditional on wetness, whereas only sign C has the wet-surface supplementary plate beneath it.

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