I will try to explain you how would i go to solve problem. if you road looks like on image, you can use pixel collsion only for part of texture (blue circle) no need to check whole image. And then you check collsions only on edges (protruding parts) of car tiny green circles.
other way could be that you draw road using vectors (to create a polygon). then you can check if your car intersects iiner and outer polygon. all between those two polygons are road.
solved How to make moving car on the limiting mask?