Taking mass transit is healthier than driving door to door. Duh! Even a 3 or 4 minute walk to the bus and/or a daily trip up or down some subway stairs is better for you than a daily drive through the Timmy's drive thru.
Access to the easily affordable car makes Americans (and Canadians) lazy. Then we move to subdivisions where we can't function without cars, keep our kids indoors away from the scary strangers and poof! we have an obesity epidemic. But that's bemoaned all over the media.
What I think isn't said often enough is that quick, efficient mass transit is healthy for cities. I happily jumped out of smog-and-traffic-jam laden Toronto, which had a thoroughly adequate transit system, because who wants adequacy when you can have excellence?
Boston's transit system isn't quite excellent all the time, but it gets super-duper points for having many subway lines and decent buses. However, I am uber-impressed at the speed and frequency of San Francisco transit. The F trolley comes about every 4 minutes (or did yesterday) when it was on a holiday schedule. The buses ran quickly, too. And I'm in love with those cable cars.
But one thing was just bizarre - they have two different transit systems interspersed with each other that aren't harmonized well. If you take a bus to the subway or the subway to a bus, you have to pay with two different sets of tickets. It would be like needing separate preloaded Charlie cards for buses and for subways.
I'm sure some San Franciscan policy wonk can tell me why it's so super logical, but I'd say that if your transit system confuses tourists and your city relies on tourists for the economy, you will soon be losing market share to Hertz and Avis.