The problem is if there's any correlation with safe driving and early adoption, a median solution could also lower overall road safety.
For example, teenagers are generally more dangerous drivers than most, safety actually peaks per mile around 60 years old. 60 year olds are more likely to be at peak earning, more able to afford the latest car features.
You're probably right at 20℅ though? This all depends on the curve.
And there's a herd immunity, once most people are driverless, you can probably be less strict as roads become less hazardous overall.
We'd honestly probably make the biggest dent tomorrow by just finding the worst ten percent of drivers and giving them free uber/lyft for life. That would probably save us money as a society, money we could use on driverless tech.
Those aren't teenagers. Judging anecdotally from my time as an attorney, those are serially unemployed middle aged males with revoked licenses who somehow still own an old heavy truck and drive it on the sly with a bottle of something in the glove box.
If you read through case law that profile is weirdly common.
The standard shouldn't really vary per-person.
If it's as good as a 50th percentile driver, it's good enough.
If it's as good as a 20th percentile driver, that's also probably fine.
You're right not to touch the mean at all, though.