Before I retired, I rode my (acoustic) bike almost every day to the high school where I taught. The challenging part of the ride was a very steep, but short hill. One morning, I went to make the turn starting up that hill, when suddenly I was down due to black ice. No traffic, fortunately, but I tore the knee of some new pants. When I walked the bike up the hill, I discovered that concrete (the sidewalk) can be a lot less slippery than asphalt (the street). I wonder if others have found this to be true?