The answer is you focus on dealing with what's in front of you, so turn on wipers and demist. If all the drivers do that, then no one needs to worry about what's happening behind them. After you take care of being able to see and control the car, then you worry about rear fogs and hazards.

And if it's really, really bad, pull over. Don't be like those idiots who decide to stop in the middle of a highway with 75mph traffic around them, under an overpass to protect themselves from a hailstorm.