One of the more interesting things Waymo discovered early in the project is that the actual incidents of vehicle collision were under-counted by about a factor of 3. This is because NHTSA was using accident reports and insurance data for their tracking state, but only 1/3 of collisions were bad enough for either first responders or insurance to get involved; the rest were "Well, that'll buff out and I don't want my rates to go up, so..." fender-taps.
But Waymo vehicles were recording and tracking all the traffic around them, so they ended up out-of-the-starting-gate with more accurate collision numbers by running a panopticon on drivers on the road.