Or do you mean headphones that completely shut off the environment? That would be pure stupidity and neglect. But also something far different from just listening to music on a bike.
Based on this, I think joggers and cyclists with headphones are significantly more dangerous to themselves and others than those without.
Of course, it's possible that you are different and can hear fine through your headphones, but I'm skeptical. (Just as I'm skeptical of people who claim they can drive fine while talking on the phone, or while drunk.)
I CAN hear fine with my volume setting. Believe me.
Listening so loud or with headphones that cut out other sounds is dangerous, of course. I was not talking about that and I thought I had made that clear.
Just don't tell me that I cannot listen to music while biking. Unless you want to argue about the possible distracting itself, then we could go on about car radios.
Sometimes people will yell at you when they see a possible accident happening but you wouldn't be paying attention through your music. You need to hear cars accelerating quickly in case you need to get out of the way or avoid them.
There is no greater fallacy than wearing both headphones on your ears while biking. What is more important you, music or your life?
Like other posters have stated, it's better to be alert and safe rather than trying to listen to your tunes while in traffic.