Last few days have been very foggy in northern India. Fog often condenses and we find that the ground/roads become wet due to settling of fog. I recently noticed that roads with even a little bit of traffic (1-2 vehicles per minute) do not have a wet surface (expected from settling of fog) whereas surrounding ground and roads with almost no traffic are wet.

Any explanations for this?

A very interesting observation indeed!

I think the explanation could be that the wetness of surfaces is caused by the condensation of water present in the fog. The absence of this wetness on roads (with some traffic) can be because the particulate matter in the automobile exhaust would accelerate formation of larger drops of water (hence removal of moisture from foggy air) which do not condense on the road surface because they are blown off because of the velocity of the exhaust

I think it is not only the hot gas blast from the exhaust pipes of the automobiles passing through the road that are responsible for water not condensing on the roads, the hotness of the underbelly of the vehicles as well as the air drag because of their movement are equally responsible.

If that is true, then the dry stretch of a road would be limited to the portion that is more used by the traffic!

fog is essentially water droplets of various sizes. first one must understand why these don't fall to the ground at a very short time scale. then the answer to the puzzle will become clearer.

Jasjeet

There is a related phenomenon that is explained by the observations made above. After a light rainfall a road with some traffic dries faster than the surrounding ground. Exhaust heat plus the frictional heating of the ground by the tires would vaporize any moisture on the road. An additional factor is that water on the ground would stick to the tires. The vehicles enter from dry patches far away from the rainfall and carry the water from the wet road to dry patches beyond the wet patch. Similarly a wet bathroom would dry faster if lot of people enter and leave it.