Say you're a city, the wannabe green kind, full of citizens who want to sustain the planet. You've built public spaces full of plants and trees, you've installed rain barrels and helped homeowners get into solar. So now you're looking to hack away at the transportation sector, which contributes a quarter of this country's total emissions output. The answer seems easy: Let people get around without gas-guzzling cars. Maybe even dedicate yourself to a large-scale public transportation project, like a new subway line. Despite the dizzying costs, cities like New York, Los Angeles, and Washington, DC, are hoping to do just that.
Problem is, that's not always the best choice for emissions-conscious cities. After taking a close look at the total greenhouse gas emissions of one subway line in Toronto, University of Cambridge and University of Toronto researchers say the system's total environmental cost is not as green as it might seem.
Yes, as a subway rider, you should feel secure your commute is more environmentally friendly than that of the guy who drives his SUV, alone. But if you're a city planner, you must know that day-to-day operations count for just one part of a transit system's greenhouse gas cost. “We have to pay more attention to the transport-land use connection,” says Shoshanna Saxe, a civil engineer at the University of Toronto, the study's lead author. Adding up the total environmental cost of a public transportation project is much harder than you might think---but definitely worth crunching the numbers.
The researchers wanted to pin down exactly how much time it would take a transit line to “pay back” its cost in greenhouse gas emissions. They focused on the local Toronto Transit Commission’s Sheppard line, which hits five stops in 3.4 miles and opened in 2002. That makes it small enough to study easily, new enough to be built with modern construction techniques, and old enough to have accrued detailed ridership data.
The project amounted to a very complicated algebra problem, one with pain-in-the-butt integers that were a challenge to uncover, much less add up and multiply. Want to try this at home? Take the amount of emissions released by the production of the materials used in construction, like concrete, steel rebar. Add that to the emissions cost of getting those construction materials to the site, sometimes from places as far away as China. Throw in the emissions created by the machines that put those newly arrived materials together, along with whatever the completed line spews out once it's up an running. Then account for the cars the subway takes off the road, but also the cars that return to the road when they realize the new train has cut down traffic. (That's what economists call "induced demand.") Oops, almost forgot: Take a big look at the entire region, and how the subway influenced its growth and density. Did developers add housing and offices closer to stops, to make it easier for people to walk there? Or did they stick with spread out, single-family homes and office parks, so even die-hard train commuters had to drive and park before riding? That was just the start of the researchers' calculations.
Yep, exhausting stuff. In all, the researchers determined it took the Sheppard line a full eleven years to “pay back” its emissions cost---and that’s an optimistic view. It might take more than 35 years to make it up, depending on how ridership and the the area around it continue to grow. Saxe and team suggest officials could help their city along by relaxing zoning laws, and letting taller buildings full of offices and homes spring up around newer transportation options. They might even create tax incentives to help construction along. The big takeaway: If your goal is reducing emissions, just plopping down a subway line is not going to cut it.
Given this painstaking work, it's not hard to see why this kind of lifecycle analysis can be scarce in government circles, even in the most well-meaning, greenest ones. “Lifecycle analysis has been around for decades, so from an academic perspective, it’s well-vetted,” says Mikhail Chester, an infrastructure energy and environment scientist with Arizona State University. “But from a policy and decision-making perspective, we’re still in the early stages of using lifecycle analysis to inform recommendations.” Part of the problem, he says, is that there aren’t many government workers trained to do it.
But transit agencies are getting hip to the overall costs of building big infrastructure, whether that’s a subway or a a new highway. (Transport for London, which runs the city’s Underground, is particularly good at this.) “We’re becoming more aware that transportation is not just what comes out of the tail pipe of vehicles," says Chester. Factoring in that information can make a huge difference, and up the project's emissions price by more than a factor of two.
All things told, it's likely that a public transportation system is doing nicer things for the planet than a highway. According to Chester's work, your standard gasoline-powered sedan is going to spit out about 375 grams of carbon dioxide per passenger mile traveled, nearly 40 percent of which is attributable to manufacture and maintenance, energy production, and building the infrastructure it drives on. A subway system like San Francisco's Bay Area Rapid Transit, by comparison, will only produce about 125 grams of CO2 per passenger mile traveled, even when you factor in infrastructure. A diesel-burning public bus working the night shift, when there are fewer riders, might give off an astounding 665 grams of carbon dioxide per customer mile---but just 85 when it's packed during rush hour. A holistic emissions analysis should help cities pick between transportation options, whether it's a subway line or gondola or protected bike lane.
“I still think absolutely that public transportation, and that heavy rail, has a real opportunity to reduce greenhouse gas emissions,” says Saxe. “I think we just need to be more conscientious about how we build and deploy these systems.” City officials: Time to break out your calculators.
