>human operator is paying attention

Think of how silly this is though. You offload driving to a car, then...demand passenger give absolute attention every second that car operates itself. For one, this is a pointless tool if you have to monitor it all the time. For another, it is impossible that humans will be able to focus their attentions fully when they aren't really doing anything to drive the car. The better self-driving gets (say, safe and effective 99.999% or any incremental level higher), the less the human monitor will actually be watching. Thus the more likely any error will be catastrophic.