I started riding with a rear light during the day because I do a lot of long rides with no shoulder or bike lane. Cars tend to give me more space when they pass if my rear light is on. Reading this blog post by Steve Tilford, it makes a lot of sense. We become significantly more visible with the rear light, even during the day.

"I saw the results from a couple different studies that show that the lights reduce chances of collisions with cars somewhere between 19-35%. That seems pretty significant, but you have to realize the odds of getting hit by a car is super low, so you’re just reducing a super low chance that much more."

See the full blog post.