In CA there is the Tule Fog. (
https://en.wikipedia.org/wiki/Tule_fog) It is an impossible fog, or should I say impassible. Very dense and it comes up on you before you realize that you can't see 500 feet, or less, much less sometimes. You may slow down but you worry now that the truck behind you just disappeared and can't see you.
What to do and/or where to go? Well, in the time it takes to think that much you could be being wapped by your air bag. It's a Catch 22.
The only real defense is to listen to the radio or your phone and get off the road. It's patchy and catches those that keep going totally off guard. There is no safe speed. Just don't.