The premium DToplight from Busch & Müller. Two bright LEDs for visibility from behind and from both sides. This is a serious tail light that doesn't need to flash to get you noticed by traffic. In the automatic mode, this tail light will detect when the lights should come on -- even if you forget -- and turn off again when not needed.
The sensor can detect whether the bike is moving, and will turn the light off if the bike is parked.
The LEDs protrude backwards providing side lighting and light that bounces off of the twin reflectors for 220 degrees of visibility.
Up to 60 hours of visibility from two AA batteries.
Mounts to your rear rack, so your rack cargo does not interfere with rearward visibility.
If you don't want a light that's making so many of its own decisions, use the manual on/off mode instead, or get the Busch & Müller DToplight XS Permanent Bike Tail Light -- it's the same tail light, but without the automatic on/off feature.
Busch & Müller DToplight XS Permanent Bike Tail Light