When I’m cycling, I’m most concerned about traffic coming up from behind me. I want to do all I can to ensure that distracted motorists will be able to easily see me. To help make sure this happens, I always use a tail light.
One of the best tail lights I’ve found for cycling is the Planet Bike Superflash tail light. It is extremely bright and can easily be seen even in bright daylight. At dawn or dusk, it is incredibly noticeable.
Typically the light will be mounted on your seat post or attached to the back of an under-seat bag. It runs on 2-AAA batteries and battery life is excellent. I easily get a season out of one set of batteries. It’s diminutive size belies it’s amazing performance. The superflash is about $20 and available from Amazon.com