About ten years ago something like that happened to me. It was quite a problem because: It was rush hour (a bit after 17:00) so lots and lots of traffic; It was pitch black so even more disorienting; Vehicles all around were spontaneously starting to slide from stationary, even on fairly modest slopes; Even people who could get away from the worst bits had severe difficulty getting home - so lots of abandoned vehicles; Absolutely no gritting in the entire area.