Should I have some sort of backup plan beyond "well, if things get close we just call an ambulance and I birth on the freeway"? There are hospitals closer but not significantly, and for various reasons I have enormous trust and a need to continue with my OB, and he only delivers at the one hospital.
Sounds good to me . Trust your instincts the whole way through. Ive had long births and short births. First was 3hrs 20mins, 3rd was 49hrs, 7th 42mins. It all depends. I wouldnt hang around when you start getting contractions but you cant predict how fast or slow this labour and birth will be.

Is homebirth an option? Something to consider maybe if you are up for it?