As an alternative you could create your own simple square mesh in wings3d, blender or whatever modeling app you like, UV map it and apply the texture to that.
You said your game is a top-down view... if the camera is looking straight down at the cars then I wouldn't be too concerned with conforming the shadow to the road unless you're doing jumps, crashing and flipping over, etc?
You said your game is a top-down view... if the camera is looking straight down at the cars then I wouldn't be too concerned with conforming the shadow to the road unless you're doing jumps, crashing and flipping over, etc?