The worest blizzard conditions I was in was in SE Minnesota. It was at night, the flakes were the size of half-dollars, we were going 20 mph towing a four place, and I had my window open enough so I could shine a flash out it watching for the edge of pavement to give him course corrections. When we got to an intersection, we go out of the SUV to clean off the signs in order to read them.
I couldn't actually see the edge of the pavement, I could make out tire tracks from previous vehicles. I could also determine where the ditch was. We figured as long as we stayed in existing tire tracks and did drive down any slopes, we where still on the road.