I'm not sure humans have reached Level 5 autonomy either; many of them can't handle the full range of conditions humans drive in and occasionally need other humans to step in and help them.

Level 3 or 4 is more than enough to get most of the commercial benefits.

IMO the "failure rate" is obvious enough to be tracked, but I worry that we won't be paying enough attention to differences in error type/impact/recovery.

We've learned a lot of ways to predict, mitigate, and recover from human errors, and the equivalent ecosystem may not exist for nonhuman ones.

Level 5 means "people inside are purely passengers and never have to take steering wheel". For humans, being level 4 means that because of some road condition, driver gets out of the car and asks someone else to drive - either one of the passengers, or some sort of emergency 3rd party service.

I never did this myself (except while learning how to drive). And I don't think I've heard about my friends doing this either. Turn around because the road was impassable? Sure. Pass the wheel to someone else? nope.

That means a lot (most?) human drivers are "level 5 autonomous".