My guess is that nothing works in daylight. Car with two bright "day running" headlights get involved in accidents during the day all the time.

If you look at the picture 10 Wheels posted, that headlight blends in pretty well. Unless you were focused on it, you'd never notice it.

Maybe some very bright flashing lights - as in "as bright as auto headlights" and flashing might work. But I don't know if there are any bike lights that are really as bright as auto headlights (despite somewhat optimistic manufacturers claims).

Utility trucks around here do use some kind of very bright strobe light mounted in/very close to turn signals. Those are bright enough to catch my attention in daylight - I assume that they use a whopping amount of power - but I don't know for sure.