Humans are mostly terrible at handling unknown complex situations on the road.

For many human drivers on the roads right now, the response to a sudden weird situation is either going to be "brake heavily" or "swerve dangerously."

You don't have to get "undefined behavior" in response to an unknown situation. The AI can simply default to braking to a stop in the absence of any better programmed alternative. This will already put it on par with most drivers. Better, in fact, because the AI can begin braking instantly.

I personally find that the majority of driverless vehicle talk vastly overestimates the average human driver, and demands unreasonable perfection from the computers.