I don't think this is a very good argument... I want my car to be as quiet as possible, on terms of the noise it makes and it's soundproofness. That making people harder to hear outside is irrelevant, as I mainly use my eyes to scan the environment for people, not my ears.
Also I think we've done enough damage to the world with the (British accent) "what about the children" arguments...
I think this one in particular ticks me off personally because people I meet are all moralistic about me listening to podcasts while bicycling in traffic, telling me I "shouldn't block the sounds of the environment."
And I'm left wondering: have these people ever been in a modern car? Just sitting in it blocks out more of the environment than my open-backed earbuds. If I can drive one of those without causing an accident, surely I can listen to podcasts on my (much slower) bicycle without causing an accident too!
I see it now where people bellyache about headphones and runners and don't really get it. I don't have the volume jacked up and I know where to look for traffic.
On a bike I don't want headphones just because I'm mixing with so much more traffic and want that awareness.
Also I think we've done enough damage to the world with the (British accent) "what about the children" arguments...