whilst following another game development they have come up with a way of making weather transitions really interesting with the road, ground.
basically materials can be transitioned to a "wet" state by adjusting their material values (at least on their engine) and it doesn't require any texture or material swapping - at least that is the theory anyway...
you can find the source of information in the additional information section.
this method is used more and more now i.e. GTA V (when it rains etc)