Steady lights don't help in daytime. Several times on daytime rides I've seen pinpoints of light in the distance, or through the foliage on the MUP, and couldn't tell for certain whether it was a bicycle or a glint from a reflective surface.

And this weekend I attended a group ride on a long, flat open road that I've ridden solo many times before, but never with a group. Under the steel gray overcast morning sky the cyclists blended in with the road and sky -- particularly because we wore blue, as requested for the "Back the Blue" theme. The blue jerseys blended in with the steel gray sky and road. But the handful of cyclists who ran blinking white front and red rear lights were much more visible, even from hundreds of yards away.

Cyclists running flashers are much more visible -- particularly those irregular, pseudo-random flashers. It's an attention getter. Anything out of the norm will help.

On the one hand, it sounds like the proving a negative fallacy. On the other hand, I haven't seen any studies that researched exclusively the effectiveness of lights.

I'm satisfied from my own experience that flashing lights are effective, including in daytime. I've had drivers and passersby tell me so. While I usually prefer evidence and facts, it isn't always necessary to confirm the efficacy of something. I don't expect lights to protect me from all situations, particularly from negligent or hostile drivers. But I am satisfied lights will improve my odds of being seen. And there are zero consequences for running appropriate lights, other than the relatively small personal cash expense.