In reply to Lion Bakes:
> Why not doesn't block out road noise any more than car sound proofing does already.
It absolutely does. So does having your car stereo loud mind you, and deaf people are not excluded from driving, so whilst I wouldn't say they make you unsafe to be on the road, they will reduce your awareness.
Probably not as much as, say, being a bit tired or having unruly children to deal with in the back, but all these things can have a cummulative effect so always worth taking into account in your personal risk assessment.
I often cycle with headphones on, particularly if I'm using my phone as a sat-nav - far less intrusive to have a voice in your ear every so often as you approach a junction than to be looking down at a screen, so you can enjoy the environment - but on the odd occasion I do listening to music when training, I am well aware that I'm increasing the chances I could make a fatal mistake, or fail to register another road user doing so in time.