Originally Posted by cyccommute

...and they don't work if light doesn't fall on them. There are lots to situations where a reflector isn't going to have anything to "reflect". If you are coming to an intersection and the car is at 90 degrees to the bike, the reflector is going to be useless. Angled intersections are going to do the same. In fog or snow, the light can't travel very far and even straight in front of the car, the reflector can't reflect the light back. If the reflector is dirty, it won't reflect.

On the other hand, laziness isn't an excuse. Every state law that I have looked at requires a front white light. Some require a reflector as well but all of them require active lighting for the front. Some, like my state, require a reflector for the rear but not an active light. Because of this, I carry a red reflector but I only do so because the law requires it. It's useless and I would rather have a light required but that's going to take legislation to change.