I find that many walkers as well as the slower bike riders (with squeaky chains) on MUPs listening to MP3 players are a big hazard. When I call out to your left, most are STILL startled or plain do not see/hear me as I pass them. I have used my MP3 player on long solo rides and have had the volume so that it did not interfere with hearing cars well behind me. In some of the headwinds I have ridden into, I could have had closed back studio headphones piping in Slayer Raining Blood at full blast and it would not have made a difference because of the loud wind noise in my ears.