It's more complicated than just "good drivers" and "bad drivers". Your driving performance is very different on minute 10 of a trip vs minute 600, or 11am vs 11

It's more complicated than just "good drivers" and "bad drivers". Your driving performance is very different on minute 10 of a trip vs minute 600, or 11am vs 11pm, or happy vs having a bad day. You're also very likely to be in very different environments at all of these times, and have different familiarities. Insurance risk estimates are practical simplifications instead of trying to model how risky an individual driver is on a particular day at a particular time, in a particular area. Waymo's results are trying to compare to a statistically average driver with all other variables controlled.