One time my wife went down hard enough on black ice that a car pulled over and the driver got out to see if she was OK, and he immediately went down off his feet and slid down the hill toward her. Fortunately she was close enough to home to be able to hobble back. She no longer rides past daylight savings time. In general on black ice days it becomes apparent before making it off our block so it's never an issue to be too far from home when you go down that hard and fast. Which I have not done since getting studded tires. I suppose one studded tire could prevent too many wipeouts but you really need two to both stay upright and keep it moving in the direction you want.