> Surely the number of cars than can do it, and the price, also matters, unless you're going to ban private cars

Indeed, like this: the more cars sold that claim fully autonomous capability, and the more affordable they get, the higher the standards should be compared to their AV predecessors, even if they have long eclipsed human driver's safety record.

If this is unpalatable, then let's assign 100% liability with steep monetary penalties to the AV manufacturer for any crash that happens under autonomous driving mode.