If you want to get technical, black ice should really be called "clear ice." Because black ice forms with very few air bubbles, it's virtually transparent and much harder to see than normal ice. Drivers usually mistake black ice for wet pavement, so they're often totally unprepared to react to the slippery conditions. One study found that driving on black ice was five times more dangerous than driving in normal conditions. Stopping on black ice takes nine times longer than normal [source: Adams]. Studded tires and snow chains can help you stop faster, but not by much.
Making matters worse, black ice can form when you least expect it. A snowfall may have melted days before, but the roads can still have patches of black ice waiting to send your car out of control. Black ice is more likely to cause problems in the mornings and at night when temperatures drop.
You should be particularly wary of driving over bridges and overpasses once freezing winter temperatures kick in, but black ice also can form on shaded sections of the road. Besides driving slowly and carefully, there's not much else you can do when you find yourself on black ice. If you're fortunate, the highway department will have treated the roads with salt or sand, but even then your car's stopping distance and handling will be impaired.