Self-driving cars technically wouldn't have to go anywhere besides the roads - except maybe periodically to charging and a repair station. Of course, by 3 am, it might be cheaper to have them parked. But there's a lot of parking currently available at 3 am.

Of course, these cars occupy space, which means the road they'd be on would be a good deal more occupied. But remember, parking and traffic aren't static per-car, it's all a matter of rates.