We know there is a correlation between car vs. bicycle collisions and failure to use lights, even during the daytime. According to one Wikipedia article, it has been estimated that 75% of bicycles used in the United States at night do not meet the legal requirements for lighting. In California, a white headlight and reflectors are required by law if riding when it is dark.
Daytime running lights (also known as daytime running lamps) are lights on the front of an automobile or bicycle that automatically switch on when the bicycle is moving forward. Daytime running lights are not required by law, but some cyclists believe (and some evidence suggests) that they may make cycling safer.
While studies are limited and there is a disappointing lack of data, there is one study that supports this, and says running lights give about a 19% reduction in crash rates. According to Safety effects of permanent running lights for bicycles: A controlled experiment. (Madsen JC1, Andersen T, Lahrmann HS.), permanent bicycle running lights significantly improve traffic safety for cyclists.
Do you have experience with daytime running lights? Do you think they improve safety, or is there a difference? Share your opinion in the comments below.