Scariest I've experienced was before they built the Eisenhower Tunnel taking the Loveland Pass when the road was packed with falling wet snow one spring in the early 1970s. I had four studded snow tires on my Mercedes 250SL, and after sitting still on a fairly steep decline for a few minutes because of trouble ahead, the car started to slide. The studs did nothing on that ice. Fortunately, I was able to creep forward toward the inside to stop the slide toward the cliff side.
A few years ago I drove down from Powder Mtn. outside Ogden, UT, in a fog that was so heavy the patrol shut down the ski operation. That pavement is really steep and narrow, but not too twisty. I imagine a little ice would make that road quite treacherous.