I don't understand. You seem to think that the car can do something a human can't. Any driver hitting TULE fog immediately slows down, but as long as they're moving there's a chance they'll plow into the car in front. We just drive on, hoping the car ahead will drive on too. Stopping in the road almost guarantees you'll be rear-ended. These are all situations where a human is going to risk everything just as much as a car on autopilot will. Humans don't have radar, yet they've been navigating through these scenarios for years, vision only.
In low visibility situations, I would expect the car would set off an alarm and insist that the driver take over as it begins to slow.