There’s a spot south of Flagstaff (AZ) where this periodically happens... You’re on an 8,000+ foot mountain with just a little guard rail to keep you from going over the edge. The fog came up on us so fast that we could barely get over to the edge of the road. Luckily, someone was pulled off ahead of us too and we just had enough shoulder. It was the scariest thing ever. We kept our seatbelts on and just prayed we didn’t get hit (well, not like the seatbelts would have helped much going over that much mountain, but still).
Yeah, I’ve had that happen to me just south of Flagstaff too.
But the worst case I ran into was just south of Denver on I-25 on Monument Pass. Not much of “pass” by Colorado standards, actually pretty similar to interstate just south of Flagstaff, just windy enough that there’s some blind corners where you can enter the corner in full clear and run into a zero visibility fog bank in the middle of the turn.
And just like Flagstaff, just a little band of Armco between you and a several hundred foot drop.