Just render it using layers and passes with overrides to get just the parts you need to keep the alphas. I.e override the primary visibility of the background but keep its shadows and lighting etc on...

Thin blue line around the layers? Could you post up a pic, never had that problem when rendering off myself.

Why are you making the road wet? It looks like its a nice day, you would have to "wet everything up", personally I would just have it dry.

The colours also look a tad too saturated, but that can...

I would ease the movement of the camera quite a bit, its very fast at the start.