It depends on how much you ride in the dark. A full bike light set is optimal if you find yourself regularly riding at night, at dawn or dusk, or in bad weather conditions. The Garmin Varia is the favorite matching bike light set that we tested, though it is perfectly suitable to mix and match between front and rear lights of different brands or models.
Studies have confirmed that bike lights do help make drivers notice cyclists. The National Highway Traffic Safety Administration has found that the highest incidence of cyclist fatalities happens between 6 p.m. and 9 p.m.
If you live in a well-lit area and rarely venture out on the bike in the dark, a rear light (and perhaps a headlamp for the occasional short night commute) will be plenty. The rear light is critical for making yourself visible to cars coming up behind you.
A front light can also help you stay visible to cars that might be veering off course, as well as to pedestrians who may run or walk opposite the flow of traffic. It can also keep you from crashing on the road. Legally, you might be obligated to have both.