For low-light vis I strap a Black Diamond Icon 3w headlamp on my helmet. I set it to strobe for sunset, and 3w beam for darkness. This way, it shines wherever I look, and I can take the light with me whenever I make a stop, without having to un-strap it from handlebar hardware; plus I can use a headlamp for more than cycling (e.g., rummaging through my panniers or fixing a flat in the dark). It was a fraction of the cost of ridiculously overpriced cycling headlights, as well.