The answer is freezing rain! Freezing rain creates an instant layer of ice on roads and sidewalks---and ice is the most treacherous for both drivers and pedestrians. Second would be sleet, because this can also cause icing on roads and sidewalks, although sleet tends to form a rougher surface which can provide more traction than the flat ice produced by freezing rain. Snow is also slippery and can cause problems---especially when it gets packed and develops icy patches---but generally, you get more traction on snow than on ice.
---David Murphy
