[jsmith45]

The realistic definition would be that if a human could not reasonably be expected to drive in it (fog/smog with less than 6 inches visibility, hurricane, blizzard, military invasion) then the cars are not required to drive it in. And perhaps the cars can accept a circumstance or two where humans can't drive, in exchange for one or two where humans could, but the cars can't. But it would need to be circumstances that humans can predict/understand. Like if the car won't drive if the countrywide car-to-car comms network is down or something.