Taking the bus or train is greener than driving, right? Well, the answer might not be so black and white.

The popular economics blog Freakonomics recently dug into the issue and found some pretty surprising numbers. As contributor Eric Morris wrote, riding on a bus actually uses more energy per person that driving a car.

"According to the Department of Energy’s Transportation Energy Data Book, in 2010 transporting each passenger one mile by car required 3447 BTUs of energy," Morris wrote. "Transporting each passenger a mile by bus required 4118 BTUs, surprisingly making bus transit less green by this metric." Trains, by comparison, required 2520 BTUs of energy per passenger mile.

But that alone doesn't tell the whole story. Since many buses are powered by natural gas, and most trains are powered by electricity, they still produce significantly fewer greenhouse emissions than gas-powered vehicles.

Of course, the more you dig, the more confusing the picture gets. Where do trains get their electricity? Depending on the state, it might be coming from a coal-fired energy plant, one of the dirtiest sources of electricity. That pushes back some of the gains from that mode of transportation.

Another location factor: public transportation works best — i.e., is the greenest — where there is both a mass of potential travelers and an appropriate infrastructure to implement it, according to Morris. New York City's subway, for example, produces two-thirds less CO2 per passenger mile than your average car. Light rail systems in other states, which are not as heavily used, actually generate more CO2 than cars.

Morris writes that although environmental activists praise public transportation, most of the "low-hanging fruit" for future systems is already in place and new systems might not make that much of a difference. He suggests that "probably the best thing to do is be very skeptical about adding new transit service and even to discontinue some service we are currently providing (sorry, liberals). Simultaneously, we should raise fees and taxes for driving (apologies to you conservatives)."

Confused? I am, too. I think I'll take a walk to clear my head.

Related green transportation stories on MNN:

Is public transportation really greener than driving?
Taking the bus or train is greener than driving, right? The answer might not be so black and white. Freakonomics digs into the green transportation issue.