Headphones and biking are a dangerous combination. A bicyclist was recently found near a train track where he had been struck by a train killing him. The bicyclist was wearing headphones at the time. Police are not saying that the headphones were the reason why he died, but it is a big clue. Even if this biker was not killed because of his headphones, it is still dangerous to combine headphones and biking.
It is obvious why headphones and biking do not mix. Headphones diminish a bikers ability to hear. Hearing is an important part of safely navigating streets. With a reduced ability to hear, the bicyclist is less likely to hear things like horns, sirens, verbal warnings or of course, trains. Earphones cause an even greater reduction in ability to hear than loud music in a car. This is because the headphones not only produce loud noise, they effectively serve as earplugs. Earplugs would never be recommended while riding a bike.