Check your local laws to find out the requirements. In Houston the requirement is for a (red) reflector or light on the rear. I use a Dahon rack mounted light and 2 PlanetBike blinkies on each side of my rear tire. Reflectors and non-flashing lights are generally ineffective at a distance but as a driver approaching from the rear gets closer it helps to establish a safety zone around you. A driver will first notice a flashing light at a distance then notice the non-flashing light. The further away the driver is when they see you the more time they have to react to your presence. For this reason I also use a flashing white light on the front in addition to my headlights. My signature is a comment I made to a driver who told me to put on brighter clothing when I was riding at night. I had been out all day and it and been cold that day so I was dressed in mostly dark colors. MY experience is that drivers do not notice clothing at night and when they do notice brightly colored clothing they have little lime to react. Several factors to consider is the drivers mental state, impairment, speed, weather conditions, lighting conditions all affect not only how well you are seen but how much time the driver has to react. None of these things are within control of the bike rider so the best rule to follow, IMHO, is to light up my a**.