Fortunately waymo and others do have years of data.

Also, you can make reasonable inferences about fatal accidents using non-fatal accidents. All fatal accidents are also normal accidents. If waymo has far fewer non fatal accidents, you can reasonably infer it would have fewer fatal accidents. Otherwise you'd have to believe waymo's accidents are more likely to be fatal, but the opposite is probably true because of the locations and speeds where they drive (at least for the passenger lol)

You can also make inferences about accidents based on disengagements or undesirable events (human labeled). It's not as data limited as you might think.