…wait… how are your cars moving right now?
I expected you to use a nav mesh (which is very much possible for cars as well). You will need to tweak the settings quite a bit and do a bit additional work but in general the navmesh is nothing but an area which is set up for an AI on top of that to use. Basically indexing the area and saving all points one could walk on. What the AI does on top of that is implementation dependent.