one of the environmental benefits of public transportation is fuel efficiency. sure, the vehicles themselves use way more fuel per mile than cars and light trucks, but there are also a lot more passenger-miles being served. if you want the in-depth physics of it, kinetic energy equations and all, david mackay wrote this book (completely free download) about it.
if you just want to know how to get more miles out of whatever it is you're driving, what amounts to a handy guide was just put out by some researchers at the university of michigan. the biggest impacts you can have are driving a high-efficiency car, keeping it tuned up, avoiding traffic and making fewer hard starts and stops. interestingly, choosing as hill-free and flat a route as possible makes a big difference too.
all that's interesting, but the real reason this otherwise not-so-well-written-paper makes a transit blog such as this one is that it emphasizes, once again, that driving alone is a terribly resource-intensive way to get around. even accounting for all the late-night bus routes that barely have any people on them, the average passenger-mile on a bus uses about 25% less energy than a car (table 1) and trains about 50% less. of course, this assumes that reducing energy use is, you know, important, which apparently isn't agreed upon. that doesn't mean i won't still be smug about it.