Aeroplane has hit on the real issue: It's about how you define "dangerous." Riding with headphones (with music playing in them, of course) increases the chances you'll get in <i>some sort</i> of accident, but the probability that it will be a serious or injurious accident is the same. Wearing no helmet, on the other hand, doesn't affect your chances of getting into an accident, but makes it so that when you do, you'll be more likely to suffer a bad injury.

So if the question is which will create a greater risk <i>of death</i>, the answer is probably no helmet. If the question is which will create a greater risk of <i>some sort of injury</i>, the answer is probably listening to music.

None of this, naturally, takes into account the people who, like me, face a different choice: Exist in a constant state of inebriation, or bridge the periods between drinking with soothing, early 90s hip hop. For me, it's not a question of riding with or without music, it's a question of riding with music or riding with bourbon. I choose the music and make sure to adjust my helmet properly.