Personally, I use my ears too much and am not comfortable in traffic with my iPod going. Between doors and cars and pedestrians, I'm not good enough without my hearing. On trails, the music can help me drive my pace and move faster and enjoy the ride more. Since I always make sure to look all around before I make any moves, it's only the sudden unplanned moves where I'm significantly less aware of my environment and might have a problem because I haven't heard someone coming up from behind. But those situations are so rare, it's a calculated risk I am comfortable with and take.