There was an article in Reader's Digest at least 20 years ago. They studied the rate of traffic accidents for people who didn't have their headlights on during the day, vs. those who did. No headlights showed a huge leap in accidents, even in good weather. Most modern cars have a setting for daytime running lights, which can be turned off if necessary, such as the person who doesn't want to blind their neighbors while backing into their driveway. It's a safety thing.