Does it make you safer to use lights during the day? My bike light, the Cygolite Metro 360, has a "Daylighting" mode which flashes very brightly and is intended to be used during the day. Similarly, many tail lights have flashing modes. Obviously one should bike with lights at night. But is it advisable to bike with lights on (in flashing mode) during the day as well, to alert cars to your presence? Or does it not make much of a difference? If there are any scientific studies or data on this, please reference.
According to Safety effects of permanent running lights for bicycles: A controlled experiment. (Madsen JC1, Andersen T, Lahrmann HS.) they give about a 19% reduction in crash rates. There's a copy of the paper in Scribd as pdf. Every reference I've been able to find appears to refer to this one study.


  The incidence rate, including all recorded bicycle accidents with personal injury to the participating cyclist, is 19% lower for cyclists with permanent running lights mounted; indicating that the permanent bicycle running light significantly improves traffic safety for cyclists


Wikipedia has a few articles, starting with daytime running lights (reports largely positive results) and bicycle lighting which gives a link in Danish that appears to refer to the Odense study above but claims 32% reduction.


  Daytime running lights improve cyclist safety significantly A large-scale experiments with daytime running lights on bicycles among 4,000 cyclists in Odense shows 32% reduction in cyclist accidents. The result is so striking that Councilman


Wikipedia Daytime Running Lights links to Google books copy of OECD, International Transport Forum (2013), Cycling, Health and Safety, but I can't copy'n'paste out of it so here's an image:



A BicycleForums anecdote resulted in a largely positive discussion, and there's this rant that concludes they're essential. The Australasian College of Road Safety amongst others is strongly support of them for motor vehicles so it's fair to assume bicycle riders would gain something from the practice.

FWIW I'm convinced. I have dynamo lights on my commuter bike that I never turn off (they have standlights so they go off a few minutes after I stop moving).