No problem with that, as long as I don't end up getting flack for it from people again.
Some fair points.
Regarding hearing speech - I guess as most music is focused around the human voice i.e. singing, rapping etc then of all available sources of sound out there this is the most likely to be interfered with. I'd suggest it would be the same as if two people were talking to you at the same time - I'd argue that you would still notice the other person talking - one wouldn't make you completely unaware of the other - but you'd struggle to make out the specifics of what both of them are saying.
Unless you were listening to particularly heavy metal, dubstep or some kind of avant garde noise music, I'd argue that this won't interfere with the sound of general traffic noise, but that even then you would still be aware of both, much as you would be aware of them amongst the general fug of noise that surrounds us in busy urban environments anyway.
I guess I am coming from a different perspective however, I am a sound engineer of sorts by trade, and it is an essential part of my job to be able to listen out for stuff other people might not notice, and maybe that's giving me somewhat of an advantage over others, but the main crux of my argument is that additional noise doesn't stop you hearing others - the main thing is just paying attention, and I guess some people might find that hard when listening to music, but I suspect the issue with 'iPod zombies' is more from their wilful inattentiveness than from the act of listening to music itself. Take the iPod off people like that and I'm sure they'd still ride like bellends.