Every year there are some pretty spectacular accidents and pile-ups due to ice, especially "black ice". Black ice is the invisible and very slippery ice that usually thinly coats the entire surface of a road. It's particularly prevalent in the Central Valley during winter and the Sierra late autumn to early spring.

Black ice often forms on the top of bridges and causeways, and is virtually invisible. You won't know it's there until you've hit it, at which point it's probably too late to do much except use all your ice-driving skills to keep control, slow down, and try to avoid hitting anyone else. The best defensive measures are to be extremely careful when approaching bridges, causeways, or other stretches of road that aren't embedded on firm ground (the ground keeps things warmer so that, in California at least, the road itself doesn't normally freeze too often if it's actually touching ground). You should also be careful after freezing rain, or during and after heavy fogs in mid-winter.