Jun 17 2009

Planes, Trains and Automobiles

Posted by: Jonathan Marshall

Rail buffs, gird your loins: travel by train can actually produce more greenhouse gas emissions than flying, according to a new study of lifecycle energy use and pollution by cars, buses, trains and airplanes.

The study, by U.C. Berkeley engineers Mikhail Chester and Arpad Horvath, takes into account the full range of environmental impacts from raw materials extraction, manufacturing, construction, operation, maintenance and decommissioning of vehicles, fuels and  infrastructure (roads, rails, airports) needed to run them.

Perspectives change a lot when you look at the big picture. For example, aircraft have big fuel requirements for every passenger-mile traveled, but relatively low infrastructure costs. Building and operating railroad stations and rail lines, on the other hand, takes twice as much energy as operating rail cars.

Their analysis shows that some light rail systems, like San Francisco's Muni, may use more energy per passenger mile than large aircraft. What saves Muni from having a larger carbon footprint is the relatively clean source of electricity it uses. Boston's Green Line, powered mostly by dirty fossil fuels, emits more greenhouse gases per passenger mile than large or mid-sized aircraft with typical occupancy rates.

Lead author Mikhail Chester told me he's cautious about declaring any one mode of transportation the "best" or the "worst." A lot depends on actual ridership. For example, he notes, "the worst performer could easily be the midsize aircraft flight I took recently with 5 passengers." Most aircraft, however, are much more fully loaded.

Buses illustrate his point especially well. The worst energy hogs shown in the paper's charts--and the worst greenhouse gas emitters--are urban diesel buses running off peak, with only a few passengers. But the most energy-efficient and generally cleanest vehicles are also urban diesel buses, running full at peak periods. (All comparisons are per passenger-mile traveled.)

Conventional cars, SUVs and pickup trucks are the worst energy users and greenhouse gas offenders, apart from near-empty diesel buses. But adding one or two passengers can dramatically improve their relative performance. The same point goes for other forms of transportation: higher occupancy works magic.

Strange, isn't it, that we invest so much money and talent into engineering more fuel-efficient vehicles, but not into finding simple ways to make them more people-efficient.

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