I think anywhere between 80% and 99.999% would cause a lot of accidents... "good enough to not pay attention and be inattentive when it needs your help, not good enough to not need your help."
It depends on where the 80% is. If it's "80% of the roads in the country" that's one thing, if it's "at random points, at any time and on average for 20% of your journey you'll be taking control" that's another.
It's all about predictability. Even just full motorway / major road driving would be pretty awesome, and the car would know when it's coming up to a section you need to drive and you can have plenty of warning. Not "HELP DRIVER I SAW EITHER A LEAF OR A CHILD QUICK TAKE OVER"
If I have to pay attention to the road while the car is on autopilot then it's basically useless -- I might as well drive the car just for the stimulation.
If/When it's good enough that I can sleep during a long highway trip (and pull into the last rest stop before my exit if I don't wake up) then I'll be happy.
It's all about predictability. Even just full motorway / major road driving would be pretty awesome, and the car would know when it's coming up to a section you need to drive and you can have plenty of warning. Not "HELP DRIVER I SAW EITHER A LEAF OR A CHILD QUICK TAKE OVER"