Here is our most recent thread on this topic. Do I have to stop at private stop signs? - Factual Questions - Straight Dope Message Board
In that thread, I linked to even more previous threads, and cited some cases from Ohio.
I think we’re talking about two different things here; the laws in parking lots, and what actually happens in parking lots. As I said earlier, I honestly don’t think police around here enforce anything in parking lots. The only way the laws become relevant is, as people have said, you have an accident in a parking lot and have to try to deal with the aftermath. Parking lots are a free-for-all, and the best you can do is try to avoid getting hit.
I actually had a case where my client was ticketed for horn abuse (you’re only allowed to use your horn to warn other drivers of danger–who knew?) in a parking lot. I filed a motion to dismiss based on the fact that the statute only applied in roadways, and parking lots didn’t fit the definition. The case settled before the judge ruled on the motion, so I never got a ruling.