I've just got to say I'm so impressed with both Waymo and Cruise. Last year they did 4.5 million miles put together in California with zero fatalities. This is an incredible achievement.
The US rate is 0.57 fatalities per 100 million passenger miles.
So humans drive average of 74 million miles before a person gets killed, and Waymo's driven somewhere above 20M miles but probably fewer than 74M. At this point, I'd expect no self-driving car traffic fatalities, statistically, if the SDC fleet is an average driver.
In some sense Waymo and Cruise are basically waiting around for a fatality (not that they want one) so they start having a denominator that isn't zero.