Check out this short piece from the Streetsblog network:
Why Isn't Traffic Reduction a Top Public Health Concern
As anyone who has ever been in a car accident can attest to: driving is dangerous. In addition, cars spew all sorts of particulate matter into the air (y'know that stuff that we breathe to stay alive). It doesn't take a scientist to bike or walk down the street and realize that the exhaust coming out of most cars' tailpipes is not the healthiest mixture. If you get caught behind a big enough/poorly maintained vehicle, your gasping for breath will be all the evidence you need.
So why do we take traffic for granted and just assume that it is a constant that we need to live with? Why is it so hard to imagine urban and suburban places with severely decreased automobile and truck traffic?