Vacuous Highway Signs?

Has anyone ever seen fallen rock in a fallen rock area? What about speed enforcement from aircraft? The amount of money in fines seems small compared to what it would cost to put these enforcement aircraft in the air. Never struck me as likely despite the plethora of signs for it one sees.

I've always hated the signs in Pennsylvania that tell you "bridge may be icy." First of all, it's 94 degrees out; no, bridge will not be icy. But the construction itself is all but tautological. Bridge may be icy, then again, it may not. It tells you nothing. Elsewhere they let you know "bridge freezes before road," a fact whose relevance you are left to detemrine, but a fact nonetheless. But "bridge may be icy" not so much.

Are there other signs which warn you of hazards that may be less than hazardous?