How 'green' is public transit, really?

Slate e-Magazine's Green Lantern takes a look at environmental questions, and this week they took a look at just how environmentally-friendly public transit travel really is. At first glance it looks like a simple question, but when you factor in the infrastructure costs and the wasted resources for low-ridership (but still necessary) trips, it gets a little foggy. Here's part of what the Green Lantern had to say:

"A train produces more emissions per trip than any car, bus, or truck; it makes up for that fact environmentally because it carries a lot more people. It stands to reason, then, that if you ride in a full sedan on a day when the train is pretty empty—and, in particular, if you are in a fuel-efficient car—the car could conceivably be greener per passenger mile. (The study says a car would need to have about three passengers—double the average—to break even environmentally with the typical train.) The numbers are even more striking for buses, which can experience extreme variability in ridership between peak and nonpeak hours. At peak hours—with 40 riders onboard—the Berkeley researchers find that buses often look like the greenest option, producing fewer greenhouse-gas emissions than even the average train per passenger mile. At off-peak hours, a bus looks a lot worse, performing even more poorly than a gas-guzzling pickup truck.

"Does that mean we shouldn't run buses or trains during off-peak hours? No. If you want people to ride public transportation at rush hour, you need to make it possible for them to get around the rest of the day, too. (Not to mention the fact that some people—for either physical or economic reasons—simply can't drive.) And as long as those buses and trains are kept running, it's better—environmentally speaking—to take public transportation, since the marginal impact of your trip will be very low."