How Much Does Density Really Cut Down on Driving?

It is a given among environmentalists and planning professionals that higher density reduces carbon emissions from transportation.  Denser communities allow for more walking, biking, and transit use.  And even when you drive, a shorter trip burns less fuel.  So we can assume that vehicle emissions per capita will drop as a city grows denser, but we’ve known very little about exactly how much — until now.

According to a new study by Boston University environmental scientists, published in the Proceedings of the National Academy of Sciences, climate pollution drops dramatically when very sprawling places become more dense and when already-dense places become denser still, but there’s much less of a pollution drop at the places in between.  In the words of Conor Gately, the study’s lead author and a PhD candidate in earth and environmental science at BU, the relationship between density and emissions is “nonlinear.”  That’s scientist-speak for a wiggly curve rather than a straight line.

Gately and his coauthors, BU professors Lucy R. Hutyra and Ian Sue Wing, examined the data on transportation emissions and residential density in American cities since 1980.  “What we found is that at really low densities you get a lot of improvement in per capita emissions with increasing density, and then it flattens out,” says Gately.  “There’s a zone between a few hundred and 1,200 or 1,300 people per kilometer at which emissions decline at a much slower rate than before.  And then it speeds up again.”

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