The sentiment that transit, especially bus transit, isn't really all that green seems to be popping up quite a bit in the livable streets blogosphere lately. The argument goes something like this: On a passenger-mile-per-gallon basis, buses are really only more efficient than cars when they're well-loaded, and since transit systems (especially bus systems) often run rather empty outside of rush hours, they should be dismantled and the energy put into them re-directed into making more environmentally friendly cars.
Now, I haven't done the fuel efficiency calculations on my own. The promulgators of this myth may indeed be right- I'm not certain, and I wouldn't bet on it (especially considering that there are so many inefficient SUV's and pickups being driven around alone), but I can't speak from a place of expertise here. However, they're wrong in a larger sense, and this is because of three different points.
First, as I've mentioned before, cars aren't only an environmental disaster because of their tailpipe emissions and energy consumption. They enable a pattern of development and a lifestyle with disastrous social and environmental implications, and they will continue doing so even if they were to all be solar-powered tomorrow. Not to mention the waste of space in manufacturing and parking. Every bus in Riverside parks in one small building on 3rd Street. Think how big a building you'd need to park every car in Riverside.
Second, one of the most frequent reasons I hear for people not using transit for their work trip is schedule flexibility. Sure, 95% of the time they only need to travel just before 9 and just after 5, but that 5% of the time they absolutely NEED to be able to get home- it's a sick family member, or a very important meeting, or simply a half-day furlough where they'd rather not stick around at the office all evening. Those mostly-empty buses running around the city all day are a key component in filling the mostly-full buses during commuter hours, in the same way that the last bus of the night may be empty, but it's critical in putting passengers on the next-to-last bus. Distributing the environmental benefits of a system across all of its bus trips, rather than on a per-trip basis, would provide a more realistic picture of the environmental benefits of transit. If you accept that trains are more efficient, you have to think about the cost of the feeder buses that got the passengers there as well.
Lastly, in most suburban bus systems (which are most vulnerable to this sort of critique), public transit is not usually run for environmental benefits, but as a social service. There will always be a class of people who cannot drive, either through poverty, age, disability, or judicial sanction. The buses will run anyway, regardless of their efficiency, because they serve a different social purpose. (I'm willing to bet that RTA's services in outlying areas like Hemet and Canyon Lake are probably inefficient compared to small, fuel-efficient automobiles.) Since the buses are running anyway, it's much more environmentally friendly to ride them, rather than add a new vehicle trip to the environment and traffic system. Even if the bus itself is rather inefficient on a passenger-miles-per-gallon basis, your individual share of that trip is significantly better than what your footprint would be if you drove for that trip. This critique about efficiency is only an issue when people don't use public transit- if anything, it is an argument to use transit much, much more than we do.