Think of the implications of having an automated driving system... the onboard computer is collecting and analyzing data in real time, and it will likely store that information, at least temporarily. So if a Google car is involved in a crash, a full report will be generated, detailing exactly what happened and liability will be very easy to determine in most cases. "Car A has had a faulty motion sensor on the front bumper that the driver failed to have replaced" or "Car B drove through a red light to hit Car A".
I also think that automated cars will observe all safety rules to the letter... like only driving the speed limit (or slightly below), always maintaining a safe distance behind other vehicles, stopping for yellow lights, and having a generous braking distance. Remember, Google could be held liable if the system is reckless, and they aren't going to want that when human lives are at stake.
I think that making automated, passenger-less cars legal will be a very easy decision for legislatures, and will pass quickly. Like I said, I believe automated cars will err far more on the side of caution, like the most grandmotherly of drivers.