One thing that has clearly struck me the last half-year is the amount of distraction on San Francisco sidewalks. I keep my earbuds tuned to just above the ambient noise level, and need to ratchet this up in response to local traffic. (Cars with bass speakers and a certain style of motorcycle riders are painfully polluting.) It's really remarkable, the volumes to which we're actually exposed each day. More people lose hearing than suffer from "secondhand smoke", and yet the true health risk is ignored by our local political structure.
There's another type of noise which imposes costs on bystanders, and that's the lack of attention that many, particularly groups, use while walking. Trying to predict where some unaware person may swerve next removes attention from audio materials. Trying to study while walking city streets on Sundays is nearly impossible... people who don't pay attention to where they are require extra attention from those of us who do.
Audio learning materials always fade in and out of awareness while walking in San Francisco, due to loud noises and erratic traffic. Society will eventually improve, but it's still a great unacknowledged cost here in 2006.