I'm no where near positive, but I think it's this:
1. Shared drive - a drive that's shared with your neighbour that doesn't have a something separating it from your neighbour like: Fence or boarder.
2. A shared surface maybe a space where anyone can park. On my road I have my own drive, there are neighbours that have shared drives, but across from me there are two parking spaces that anyone can park in. Like friends and family that come to visit.
Or in my case, neighbour who has a double drive puts his work van in there and leaves his drive free. It annoys me, not really sure why but it does!
I could also be completely left field on this answer.