Freakonomics revives a tried, and tired, debate.

It's Travel Day in America, so before we fight over the Thanksgiving wishbone, let's fight over who hurt the environment more while getting home. The subject comes courtesy of Clemson planning professor Eric Morris, who recently wondered at the Freakonomics blog whether cars might actually be greener than mass transit. Last week Morris's breakdown was echoed over at Marketplace radio under the headline: "Save the earth, drive your car?"

Morris offers essentially a three-pronged skepticism. Transit vehicles are environmentally friendly when they run full, he says, but quite damaging when they run empty. As a result, while systems in major cities like New York's have a low per-passenger carbon rate, those in Cleveland, Pittsburgh, and Memphis have a comparatively high one. In conclusion, people should be "very skeptical" about adding new transit services and instead focus on attracting riders to those that already exist.

Transit advocates aren't buying it. Angie Schmitt at Streetsblog accuses Morris of an "intellectually dishonest argument." The blogger Cap'n Transit has even harsher words. Transit planner Jarrett Walker, at his Human Transit blog, calls Morris's position "confused" and says he's given up responding to this tried — and tired — debate. Each makes several good counterpoints to Morris's flawed case; we'll summarize and expand on the best ones here.

Full vs. average capacity. Morris is right to point out that an empty bus is a lot less friendly to the environment than a full one, and he's right that the average transit bus carries only about ten passengers. However he's wrong to conclude that transit supporters intentionally skew these numbers when they study the mode's environmental impact. They don't have to, and a quick review of Morris's own research shows why. 

Morris points to a 2009 study of American transit buses [PDF]. The research indeed reveals that a full bus (70 passengers) emits only .14 pounds of carbon per passenger mile, leagues better than the .89 pounds for a typical car with one occupant. It also shows that even buses with 11 passengers — right around the average stated by Morris — still have a slightly better emissions rate than cars at .87 pounds. The full chart: 

Empty vs. wasteful buses. Even empty buses aren't necessarily wasteful buses, as Morris implies. As Walker points out in his recent book, if a city only wanted to run full buses, it could do so quite easily by abandoning low-density routes and running the remaining lines at peak hours. But many metro areas choose to design systems that favor coverage over capacity, knowing full well that will mean running some empty buses, because suburban or low-income residents need them. What's "wasteful" to Morris is quite useful to these folks in particular, and these metros in general.

Walker explained the coverage-capacity distinction to Cities in March

So when transit agencies do run that low-ridership service, as most do, it doesn’t mean they’re failing, as anti-transit conservatives often assume. It just means they have a goal other than ridership. Coverage goals, often expressed by a policy like “95 percent of population will be within walking distance of service” cause service to be spread out over large areas despite low ridership. 

There’s nothing wrong with either goal, but they’re competing goals.

NYC vs. everywhere else. New York City transit is at or near the top of metro rankings when it comes to carbon emissions per passenger mile for various modes — measuring .17 pounds for heavy rail, .50 pounds for buses — but other, less-dense cities have impressively green rates too. On average, in fact, America's heavy rail systems (.24 pounds), light rail systems (.41) and bus systems (.65) each emit less than a typical car trip (.96 pounds for a single occupancy trip, .85 for a commute). General car trips (.59) and four-person carpools (.24) are more competitive but represent a much smaller mode share. (These figures from a 2009 report by the Federal Transit Administration [PDF], which Morris knows exists, because he linked to it.)

Even Cleveland, Pittsburgh, and Memphis, which Morris cited as examples of poor environmental performance, don't quite hold up under scrutiny. He cherry-picked light rail emissions from these metros, and while those figures do exceed that of single-occupancy cars (1.01, 1.38, and 2.85 pounds, respectively), the bus figures from these areas tell a different story. Cleveland's bus system has an efficiency of .71 pounds per passenger mile, and Pittsburgh's stands around .69. (Memphis didn't report one, but Milwaukee, which is only slightly more populated as a metro, measured .73.) Saying that only big cities can run efficient transit also ignores the high figures from Boston's bus system — though at .91 it was still below the single-occupancy car rate. 

Of the ten largest systems (by share of American bus mileage), only three exceeded the general car trip emissions rate of .59, and all fell below single-occupancy auto emissions. (It should also be noted, from the fine print, that these bus figures were based on an average of nine passengers per transit vehicle, not buses running at full capacity):

Full auto v. transit life cycle. Even starting from scratch, transit's environmental efficiency fares well compared to that of cars. If you consider greenhouse gas emissions from the full life cycle of each transport mode — including operations, construction, and maintenance — the only mode that does more harm than cars is a bus with about five passengers. As soon as you reach the average of nine passengers the benefits become clear (via the 2009 F.T.A. report):

This chart doesn't even take into consideration the fact that transit often begets sustainable land development. Highway building tends to promote a vicious transport circle: the more lanes you create, the more latent demand you invite. Transit creation, meanwhile, tends to partner with high-density, mixed-use development that promotes a culture of walking, biking, and alternative transport in general. Non-rider residents of a transit-oriented area still often end up driving less than they might otherwise, because their desired destinations are built closer together.

To be fair, some of Morris's cautions bear consideration. Not every city in America needs (or can sustain) a new light rail system, for instance, and most should think long and hard (and about much more than the environment) before building one. It's also quite possible he was simply being provocative. Toward the end of the post he backtracks a bit, acknowledging transit's efficiency potential and recommending incentives that would help balance the transit-auto playing field: congestion pricing, proper gas taxes, and market-rate street parking, to name a few. These topics alone are great to pick a fight with — why poke at one that's already on the canvas?

Figures from the 2009 reports "Manufacturing Climate Solutions" (top) and "Public Transportation’s Role in Responding to Climate Change" (middle, bottom).

Top image: lianxun /Shutterstock

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