Being visible from behind is also important, but doesn't take anything extravagant: A five-red-LED blinker on your bike and a blinker on your helmet or large reflective strips on your bag is almost certainly sufficient. As long as you're not weaving from lane to lane and not terribly unlucky (e.g. being followed by a drunk or a car full of reckless teenagers), it's unlikely you'll get hit from behind.
(Fog/glow under the bike would be awesome, though.)