Minnesota and Wisconsin laws require a bicycle to have a front light visible at 500 feet and a rear reflector when riding after dark. I think it is common sense to have both front and rear lights!
There is an overwhelming variety of bicycle lighting out there. This category of product is the fastest changing in our world of bicycles. Lights are front or rear, with a few exceptions. There are many different power sources. There are lights intended for traffic to see you and more powerful front lights that enable you to see where you are going.
The purpose of taillights is to be seen. All modern taillights do a good job of this. Some have more side illumination, alerting your presence to drivers on your side. There are different brightness levels. Brighter lights can be from seen further away.
Attaching a taillight directly to the bike lets you aim it correctly. I get upset when I see a cyclist with a bright taillight on a pack pointing straight up. The only traffic seeing this is a helicopter! If your taillight is aimed above the horizon by a mere 4 degrees, the center of the beam will be 4 feet higher than the light at a distance of 50 feet. Depending on where the taillight is mounted, this could be up to 8 feet above the road, which is way above the eye level of most drivers. If you aim is off by 15 degrees the beam could be up to 9 feet above the road only twenty feet behind you! Be safe; be seen. Aim your lights!
Front lights have more variety. They range from low power lights just to be seen to multi beam lights intended to really light up a path in the dark woods. Things to consider when deciding on a headlight are light output and beam patterns. True light output is measured in lumens, which is a metric version of candlepower. Wattage is not useful for evaluating modern lights. It is a measure of electrical power, but light output per watt varies considerably depending on the source. Most modern bike lights use LEDs, which require low wattage for hi output. The number of lumens a light gives off is not the end of the story for how well you can see the road or trail.
Light beams can provide a very even lighting pattern up ahead, or have bright and dim spots. A tight well-focused beam will give you a bright small patch of light ahead of you, but little light to the side, in front of, or behind the center of the beam. A wide, diffuse beam of the same lumen value will give you a big pattern of light, but much less in the center. Many high quality light systems will have a spot and flood beam.
The amount of light that you need to see where you are going is fairly subjective. Some of the factors are the amount of ambient light from streetlights; your eyes-some people have better night vision; and your expectations.
Bike lights are be powered by one of several sources. Less expensive lights use disposable batteries. These have the advantage of being able to replace them from a convenience store. They have disadvantages from the economical and environmental perspective. Mid priced lights are often powered by rechargeable batteries. These are generally more economical in the long term. A generator hub powers many modern high quality lights. They have the convenience advantage of never replacing or recharging batteries. These hubs produce more power with less drag than the generator hubs of decades past. This is the power source preferred by most of our staff at County Cycles. Bottle shaped generators which rub on the tire are old technology and not very satisfactory. A few lights are powered by magnetic induction with a magnet in the spokes and a coil of wire producing the current. These lights typically have no switches are constantly on when the bike is moving. Better models have a capacitor that acts as a small battery keeping these lights on while you are at a stop light. These are low output lights intended for being seen only.
Get some lights that fit your needs. Enjoy exploring night riding.
Be Safe: Be Seen!