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seems like they are some kind of mobile water condenser during the winter, the busier the road the damper it seems. Even off road, riding through a fishing lake along the gravel track it's sopping all the way to the carpark and dry past it.
Alot of water vapour comes out of an exhaust in the cold it condenses and falls to the ground?
Salt from recent gritting.
Always makes the road 'damp'.
Needs a good amount of rain to wash the road clean.
salt
Yeah salt is hygroscopic innit.
Also warm ambient temperatures, plus the sun high in the sky whose radiant heat warms dark surfaces, tends to dry road surfaces during other times of the year.
Out in the country, I've noticed that the road can be dry out on lanes but when you enter a village the road is damp. My theory is that moisture clings to the added air pollution often hanging in the winter air in villages thanks to firesmoke etc, and falls to the road surface where it rarely dries out. Only noticed this ooop North where its colder and damp enough for it to be a thing. The same mechanism as why on a cold crisp day you often see your breath after a car has passed, in its exhaust fumes, but not in the air otherwise. I have no idea what I'm on about but have heard it mentioned that air pollution increases air carrying moisture content as molecules latch on to the particulate.
as said above....salt (technically brine if it looks "damp")