Headlights serve two main purposes:
1. They make your bicycle visible to others
2. They enable you to see where you are going
What we immediately noticed, is that one headlight cannot do a good job at serving both of these purposes simultaneously. If the light is angled for optimal visibility (straight ahead), it does not provide optimal road illumination. If the light in angled for optimal road illumination (downward), it does not make you maximally visible to others.
If you live in an area that is reasonably illuminated at night, you do not really have a need for #2 and one headlight is perfectly sufficient. However, if you live in the country, in the suburbs, or in any other area with stretches of unlit road, you will need to see where you are going. This is especially important for avoiding pot-holes, objects on the road, patches of black ice in the winter, pedestrians dressed in dark clothing, and those cyclists who travel with no lights on (in the Boston area this is alarmingly common!).
Pictured here is a set of Cateye Opticube LED bike lights, attached to the brackets on the front tire of my Pashley Princess, pointed downward. In the first photo on this post you can see how far the beams illuminate the road in the dark.
With rear lights things are simpler, because visibility to others is the only requirement. Still, because we like to travel through poorly lit areas, we added something extra to what Pashley already provided.
We attached these Cateye Bike LD610 tail lights to the stays of our rear racks. These can be set to be solidly lit, or blinking, or lighting up top to bottom and back, like Christmas tree lights. The effects can be quite fun.
And here the same Cateye tail light (just one) is used as a rear light on Marianne (photo on the right). It attaches to the rear rack with a special mounting bracket. In a pinch, you can also use electrical tape. The photo on the left shows the enormous SunLite Low Rider Bullet headlight that I've permanently mounted on Marianne. Since we use the Motobecanes for recreation rather than transportation, the lighting set-up is not as extensive as on the Pashleys.
Having a good lighting system on your bicycles makes cycling in the dark considerably safer and more enjoyable. Rather than rush to get home as fast as we can once it gets dark, we can take nocturnal joy rides on bike trails and country roads. Let there be light!