Busy streets put a rear bike light to work, and the SeeMee 100AD Rear Light Auto-Detection Alert uses radar to sense a vehicle approaching from behind. The light switches to a bright flash once that vehicle comes within about 82 feet. Separate buttons control the tail light and the detection alert, so each runs on its own. What Our Gearhead(R) Experts Are Saying:"On busy city streets, the steady tail light switched to a bright flash on its own whenever a car closed in from behind. The radar handled the timing, so my hands stayed on the bars through every block."