Green Transport Not Always So Green

Passenger transportation is responsible for 20 percent of U.S. energy consumption. So you'd think that if more people rode the subway, the country could have a smaller environmental footprint.

In general that may be true. But figuring out which mode of transportation is greener is more complex, suggests a new study by Mikhail V Chester and Arpad Horvath in the Department of Civil and Environmental Engineering at the University of California, Berkeley.

The researchers say, for example, that an SUV fully occupied may be greener than a subway that's practically empty. And the overall impact can depend on what the train runs on — electricity generated by the burning of fossil fuels vs. cleaner electricity. Other factors to consider include, for example, the materials that go into building train stations and the power used to operate them and run the A/C.

"We find that total life-cycle energy inputs and greenhouse gas emissions contribute an additional 63% for onroad, 155% for rail, and 31% for air systems over vehicle tailpipe operation," the researchers write in the journal Environmental Research Letters. "Most current decision-making relies on analysis at the tailpipe, ignoring vehicle production, infrastructure provision, and fuel production required for support."