Is it always greener to take public transportation?

Illuminating answers to environmental questions.
Nov. 25 2008 7:02 AM

Trains vs. Planes vs. Automobiles

Is it always greener to take public transportation?

On Wednesday, I'm heading back to my grandparents' house for Thanksgiving via Amtrak. (They live near Philadelphia, I live in Boston.) Compared with the alternatives—either flying or braving the holiday-weekend traffic—I imagine this is the greenest way to go, since the trains will be packed. But it got me thinking: A few weeks from now, the train will have many more empty seats. Will it still be a more eco-friendly way to travel?  

Last year, the Lantern pondered how you could make your turkey dinner greener—and even contemplated the heretical idea of eating Thanksgiving chicken instead. But while cooking a more carbon-conscious meal is a good step, the steps you take to get to the table in the first place can have a much greater environmental impact.

To answer the question of how to best make your trip home, the Lantern calls your attention to a recent study conducted by Mikhail Chester and Arpad Horvath, researchers at the University of California-Berkeley. When we typically think of the environmental impact of driving, we focus on the energy and emissions associated with moving a car, say, 30 miles. In reality, that sort of analysis is incomplete: How the car is made, how the road is built, and even whether the roads have been salted because of ice all have some effect, too. And while those effects are spread out over many cars and many different trips, they still take a toll. When we start thinking about train travel, the infrastructure matters even more, since getting a rail line up and running requires enormous amounts of construction and manufacturing.

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The UC-Berkeley analysis tries to get a more complete picture of how we travel by taking all these variables into account—down to the impact of planting grass on the side of the road. Chester and Horvath's data suggest that riding in the average train is a significantly greener choice than the average car or plane. For example, they find that Caltrain (a system similar to Amtrak, averaging 155 passengers per train) produces less than half as many greenhouse-gas emissions or particulate matter per passenger mile compared with driving a sedan (average passengers: 1.58). * (The sedan comes out better when it comes to sulfur dioxide but much worse on volatile organic compounds.) And on Thanksgiving weekend, when trains are certain to be full and cars are likely to spend a long time idling in traffic, rail is easily a better option.

But you can come up with examples in which driving a car looks better. 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. (For more on this point, click here.)

Like any sophisticated environmental accounting, these evaluations have pitfalls. They rest on a lot of uncertain assumptions—how long a vehicle will last, for example—and require using data from a wide range of sources that may not always be reliable. (It's also worth noting that the Berkeley center where this research was conducted is sponsored by a Volvo-funded foundation, although that funding isn't directed toward specific projects.) Depending on the assumptions you make, similar data can be used to make contradictory arguments—see, for example, these arguments for (PDF) and against expanding rail systems.

But the Lantern thinks there are a few basic lessons that these life-cycle analyses can teach us. First, no matter what data you use, two very simple variables make a big difference: how far you travel and how many passengers are in your vehicle. Air travel is much maligned as a source of CO2 emissions, and the Berkeley research confirms that airplanes do emit more than trains or buses per passenger mile. But the differences aren't as large as you think, and the real reason air travel contributes so much to our collective carbon footprint is that we use planes for longer trips. That's not to say you shouldn't go to your Grandma's house for Thanksgiving, but if she lives across the country, any means of getting over the river and through the woods is going to have a hefty carbon footprint. Likewise, designing bus routes and train schedules that fit rider demand—along with encouraging urban development that gives transit more appeal—makesa big difference, owing to the environmental downsides of traveling alone.

Secondly, you can't discuss the environmental impact of getting around without considering the infrastructure that makes travel possible. We have a tendency to focus on the environmental impact of the things that move—the cars, trains, and planes we see getting from point A to point B. But Chester and Horvath found that in some cases, construction is the biggest polluter. Roads were responsible for more particulate matter than tailpipes, for example. For rail travel, operating the trains actually accounts for less than half of a system's greenhouse-gas emissions. The implication: Making concrete and asphalt in a more environmentally friendly way can be just as important as getting vehicles to run more efficiently. In other words, it's not just the road you take, but what it's made out of, too.

Is there an environmental quandary that's been keeping you up at night? Send it to ask.the.lantern@gmail.com, and check this space every Tuesday.

* Correction, Nov. 25, 2008: This article originally misstated the relationship between the emissions produced by Caltrain and those produced by a sedan. The train generates less than half as many greenhouse-gas emissions or particulate matter per passenger mile as a sedan, not less than twice as much. (Return to the corrected sentence.)

Jacob Leibenluft is a writer from Washington, D.C.

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