The Swedish National Road and Transport Research Institute has a (extraordinarily slow-loading) report out (in Swedish but with an abstract that comes very close to being in English) on tram and bus use. Here's
the PDF of the report, but in a TechTalk public service, here's the salient bit, with a couple teeny edits, from the abstract:
...public transport makes much more efficient use of the street area than car traffic. Trams are more than twice as efficient as buses, when the number of passengers carried is considered. IN the period 1986-1996 the European cities which based public transport on trams show a bigger growth in the number of passengers carried than do cities which rely on buses.
So that's interesting. San Fran's in the process of extending a light-rail line (I'm assuming that's what the report means by trams, but happy to be corrected) down 3rd St, extending from the SBC Park baseball stadium downtown all the way past some of the town's rougher neighborhoods. A good thing.
But then again (when I worked for a national newsweekly magazine at one time we used to joke about every story having a "to-be-sure graf," a paragraph that began "To be sure," and then went on to undercut every point you'd already made in the story...so this is a little bit of a to-be-sure graf) even light rail is inflexible. Once it's in, it's really hard to change the route. Buses, on the other hand, go where they need to go, responsively. And I often wonder, especially as I ride the quite nice Transbay Express bus to and from work, whether all the good money cities spend on subways and light rail might be better spent building really cushy, well-maintained bus nets.