It is just a matter of what you feel comfortable with. When skiing, I am totally wired up and can't hear anything, but the rules of the road are clear on snow. When commuting in London, IMHHO, things are a bit different. Yes, I look over my shoulder all the time, but I really do want to know if there is a HGV or double decker five yards behind me testing the brakes. Being able to hear it helps, perhaps. My commute takes me through residential areas with quite a lot of inner-urban mini roundabouts. The sight lines are not at all good. I can hear cars coming into them a few seconds before I could possibly see them. That might help me stay alive a bit longer. Old and boring perhaps, but MP3 players stack the odds against you.
Any suggestions for music on the Col de Mines?