Bicycling at night can be dangerous, particularly if you don't put much effort into making yourself visible to drivers. Alas, many people don't. This video describes the construction of an Arduino-controlled rear light meant to make a cyclist more visible by throwing a sequence of red spots on the ground adjacent to the bike. It's not a perfect insurance policy by any means, but it's better than what many people are doing—riding about on bikes at night with little or no light to advertise their presence.
Read more: Build an Attention-Grabbing Bicycle Light