> For one thing, we don’t have access to all the data

In the US, we do have access to all the data [1]. They're required to report every incident with an injury or any amount of property damage, and it's all available for download as CSV.

> For another, it at best shows that Waymo is safer than average.

No, it shows that Waymo is 6 to 12x safer than average.

[1] https://www.nhtsa.gov/laws-regulations/standing-general-orde...