Do computer drivers have to be perfect, or is it enough to be much better than human drivers?

Perfection is of course, impossible, the best we can hope for is progress asymptotically close to ideal. The minimum bar should be performance surpassing human performance, but the details do matter. For example, a 99.99% safety rated car which almost never crashes but randomly explodes once every 10,000,000 uses would probably not be accepted.

Or if you see a ball cross the road there are strong odds a child might be chasing after it.

The day a human can distinguish between a plastic bag in the road and a child is the day I'd support them on the road.

My mother once almost killed a child who ran into the road. The kid was on the side of the road and suddenly ran into the street. She did not see him, and he ran into some sharp piece of metal on the car (this was in the early 60s). He bled profusely, and the neighborhood people came out and almost became violent with her, blaming her for the incident.

If she had been in an automated car, it would have almost certainly have stopped before the collision.

And what's wrong with slowing/stopping for a plastic bag? How often do plastic bags really sit in the street shaped like babies?