* NTSB review: Airplanes face higher scrutiny than automobiles (despite the lower overall casualty rate). Part of the reason people have grown comfortable with air travel nonetheless is because the NTSB analyzes what went wrong after each crash and recommends (mandates) certain changes. Moreover, as far as government agencies go, the NTSB does its job reasonably competently and independently.
* Gradual introduction: It's unlikely fully self-driving cars will be introduced all at once. What's more likely is the gradual automation of certain driving functions (sort of like cruise control, automatic parking, and other feature sets). I think any system that hands off control from a robot to a human introduces its own safety risks, but you might see something like a dedicated autonomous lane on a freeway with special parking areas where humans can safely resume control for driving in the city. There's also augmented driving where a car is primarily human driven, but will do things like pre-charge brakes and give advanced warning to a human diver when it believes the human is about to screw up.
* Economies of scale: Large business organizations will probably get comfortable more quickly with autonomous cars than individual consumers. If FedEx and UPS support policies like "no left turns," they'll probably be among the first to get on board with fully automated trucks. And you'll certainly see some support for the insurance business, if autonomous vehicles really do result in lower overall accident rates.