From The Professor, Jacob VanDyke:

Many folks have told me over the years about the necessity of being lit from behind to avoid being hit by a car. While I don’t dispute the practicality and safety of a rear light, some of these same people will happily cycle after dark with naught but a blinking rear, safe in their assurance that they are protected from all other traffic. For the sake of being seen at night by other road users, the most important light on a bicycle is a forward-facing light. This is true regardless of whether one is riding with or against traffic, to which I will return later.

Though it is true that a rear light will help one avoid getting hit from behind, such collisions comprise a small statistical part of all bicycle-automobile collisions. The vast majority of cars that hit bikes hit them during a change of direction (either a right or left turn, for example), before they have a chance to properly see the cyclist’s position on the road. In spite of everything else about them worthy of forgiveness, motorists do fairly well identifying objects directly in front of them, even at night. Periheral objects, including cyclists, benefit far more from enhanced visibility as motorists wield their machines on the road.

Still, this engrained preference for rear lights belies a general cultural fear of getting rear-ended by an automobile while riding a bicycle. This fear is of such a magnitude and power that entire communities of cyclists have been motivated to cycle in the opposite direction of automobile traffic in the mistaken belief that they will encounter fewer wayward cars. This behavior, which informal surveys have led me to believe me is motivated by this fear, is a unique example of a population voluntarily exposing itself to increased risk in an attempt to reduce risk. Put in this context, perhaps excessive faith in the protective abilities of rear lighting may not be the most dangerous manifestation of this preoccupation with getting rear-ended.