My oldest brother and i got stranded in a deep canyon within Colorado 10 years ago due to an event like this.
It moved in fast and we couldn't see more than 20-30 feet for about 2 hours.
Lucky for us we had a nice flat area to pullover and park my truck.
We were in the middle of the pass, 20 miles either way to get out of winding switchbacks and huge elevation changes.
On the way out we saw about 30 cars crashed off the road.
A big truck and trailer had gone over a 40-foot drop-off and died.
Crazy shit.
Due to this event the state installed railing and barriers to prevent cars going off the drop offs.