Urban density not denting CO2 emissions, sadly

April 9, 2015

You’ve heard it before: The U.S. is urbanizing, and younger generations are becoming less and less car-reliant. It’s logical that CO2 emissions would drop as people move into the cities and, presumably, opt for less environmentally-damaging transportation. So why are CO2 emissions still climbing? A Boston University paper authored by Conor K. Gately, Lucy R. Hutyra, and Ian Sue Wing, analyzes 33 years of data of CO2 emissions from roads, discovering that urban areas are largely responsible for the growth of vehicle CO2 emissions.

Highway traffic in Ottawa

Image from Peter Blanchard.

Drawing on data from the Federal Highway Administration’s Highway Performance Monitoring System, a database of roadway traffic, Boston University researchers developed an inventory called DARTE (Database of Road Transportation Emissions) to determine how much CO2 is produced by car travel in cities and suburbs. “DARTE reveals that urban areas were responsible for 80% of the growth in vehicle CO2 emissions since 1980, and for 63% of total 2012 vehicle CO2 emissions,” says Gately, a BU graduate student and lead author of the paper, reports

Another interesting discovery, courtesy of DARTE: The population density numbers that experts often rely on to predict local levels of vehicle-produced CO2 emissions may be incredibly inaccurate. Researchers “have been crucially assuming that there’s a linear relationship between population density and driving activity,” Sue Wing, associate professor and a co-author, tells “We raise a huge red flag, saying no, that’s not necessarily the way to go. In major urban areas, DARTE can differ from population-based estimates by as much as 500%.”

It’s largely accepted that “smart growth” approaches, which push for more compact urban residential development, will naturally lead to decreased vehicle emissions. “However,” says Gately, “the most pronounced declines in per-capita emissions are only observed in cities that are already very dense such as New York and Boston, while for low- and medium-density cities the effects of densification are much more varied.”

Consider Salt Lake City, where the population density hasn’t varied over time, but per-capita emissions have skyrocketed, thanks to population growth in suburbs and areas beyond the suburbs. And in cities like Denver, for example, denser housing, more bike lanes and improved transit systems haven’t automatically resulted in a decrease in emissions either, since suburban populations continue to sprawl and suburbanites continue to rely on their cars, explains Quartz.

DARTE suggests that areas must reach a density of approximately 1,650 people per square kilometer, in order for vehicle emissions to drop off quickly. Densities for Atlanta, Houston, Denver, Phoenix and Salt Lake City, for example, dropped below that rate, and each of those cities’ per-capita vehicle emissions rose. Meanwhile, denser cities like Boston, Washington, D.C., San Francisco, New York and even notoriously spread-out Los Angeles experienced drops or flat-lining in their per-capita vehicle emissions. (One notable exception, points out Quartz, is Chicago, where density dropped and emissions rose.)

What’s the solution for lowering emissions in less-dense cities? As Sue Wing explains, there’s “a fight between urban planners on one side, who focus on the design of the urban environment, and economists on the other side, who say, if you make something more expensive people will use less of it.” There are a few options, but not one obvious approach: “Do we tax driving and emissions, and make driving more difficult and more expensive? Or do we change people’s environment, creating inducements or mandates that they will live closer to where they work, walk more, bike more and use more public transit?”

DARTE is the first “nationally consistent map” of vehicle-produced CO2 emissions at a one-kilometer scale over a 33-year-long timeframe, from 1980-2012; similar studies have only covered a particular year, or have just addressed state- or nation-level emissions.

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Category: Transportation

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