Flashing lights in the front during the day especially if you are using annoying lights like the MagicShines or the knock-offs annoy the jay-walking pedestrians coming off the curbs in NYC so much they have no choice but to see you.

In cities with less pedestrian traffic it may not be as beneficial.

I voted "sometimes". I ride on bike lanes with shady trees planted alongside. My cheap lights blink during the last hour or so before dark. Once I can see a difference between cars with running lights and cars without as they pass through the shade, on mine go.

My tail light is always on.
Front light & helmet light are on whenever it is cloudy, rainy, snowy, foggy, and of course dark.

Some comments compared riding w/o light to driving a car with headlight off in daylight, but I think that's not a good comparison. Car headlights are very bright, so it is effective in daylight. But most bike lights are not bright enough to make much difference in daylight. (Of course it can make a difference if you use an expensive >2000 lumen light during daytime.)

Something to think about - the daytime running lights used on automobiles are actually LES bright than either the low beam or the high beams. Thats particularly true in Europe where light pollution issues has resulted in very low powered daytime running lights being the norm.

As far as bike lights go, its apparently just as easy to pick out a 300 lumen light as a 900 lumen light in the daytime. Partly because unlike auto headlights, bike lights have no cutoff to shield the emitter. I certaily agree a 50 lumen light wouldn't be easy to pick out during the day - but at night it wouldn't be much better either. I'm currently experimenting with a pair of amber 300 lumen running lights to see if motorists pick out the color easier.

Burtom,

Could you provide more information on 300 lumen amber lights? Do they flash?
Thanks- David Oglesbee Vancouver, BC Canada

I don't know if that's true in the US. First, running lights are generally high beam lights, so they are aimed higher up (and this can mean that even if the output of the running lights is less than low beams, more of it is reaching your eyes). I googled a bit trying to get a decisive answer one way or the other and couldn't find anything. As far as I can tell, running lights in the US aren't regulated(??).

Without trying to start a US/Europe flame war, I'm not terribly impressed with European running lights. In Europe (France, Switzerland), I drive with my headlights on and this tends to freak out the locals...

Cheers,
Charles