I think wearing headphones while cycling is outright dangerous and should never be done. I think to lessen the danger, turning the music on low from a cellphone in your jersey pocket is much safer..I said much safer, but the safest option, the one I choose is NO MUSIC.The fact that as you say "the headphones are really loud" makes matters worse. Think about these few things
-without headphones on, you can hear a car coming from quite a distance away
-You can hear wildlife moving around along side of the road. I was riding yesterday and saw a bear 200 feet or so ahead of me. Can you imagine being lost in your tunes and smacking straight into an animal like that?Regularly, I hear deer moving around on the side of the road and they sometime run parallel to me. With music on, id probably have hit one already.
-If you get into an accident, regardless of fault, your music distraction will kill some credibility. Wether right or wrong, it will be an excuse against you.
-If you use a phone for music, it will greatly diminish the batter life. That battery life could be important if a situation arises.