Why not headlights blinking on a bike during the day?
We use our Planet Bike Superflash Blinkies taillights during the day, and they are quite visible, except perhaps in the brightest of sunlight. In shade or even slight cloud cover they stand out. When the wife gets ahead of me I notice how attention-getting hers is.
We got ours when someone told me of a study showing that vehicles give cyclists with blinking lights X feet more room, even during the day.
Highly recommended.