Here’s a nice catch, from alert reader Callie Jordan:  transportation guru Reid Ewing discussing his new, comprehensive study of the links between community design and transportation.  Here’s Ewing:

The best way to minimize driving appears to be to develop in existing centers near the core of the metropolitan area, in areas of high destination accessibility where there are a whole lot of jobs near by…[O]ther factors like mixed-use and intersections and block size…fall into a second group that is less important than destination accessibility…

Density turns out as less important than land-use mix…If you’re trying to minimize vehicle miles traveled and maximize walking and transit, you’re better off emphasizing mixed-use and destination accessibility than just bumping up density. A dense development in the suburbs, far from transit and employment centers and stores, is probably not going to buy you much in the way of walking and transit use. Almost any development in the central city is going to be more efficient from a transportation standpoint.  [Highlights added.]

Hm.  That mostly makes sense, but the discussion of density seems a bit circular.  Density doesn’t matter much, but putting lots of people really close to lots of jobs and stores in the urban core is critical.  Huh?  How, in practice, do you do the latter without the former?

But Ewing’s point is that residential density, by itself, isn’t enough to relieve car dependence; while commercial density without people nearby is a wasted opportunity.  And that means that complete communities—where people live really close to jobs, stores, and services—really do help curb car-dependence.  And the best places to find that kind of “destination accessibility” are, unsurprisingly, close to major urban centers. 

In short, Ewing’s findings give a big analytical boost to efforts like Vancouver, BC’s EcoDensity initiative and Seattle’s Center City Strategy, both of which are designed to create new housing close to major urban job and commercial centers.  Those kinds of efforts, according to Ewing’s findings, are the best tools that cities have to curb car dependence.

  • Still, I have to wonder about the idea that density by itself doesn’t matter much to travel patterns.  Consider the research of Australian researchers Peter Newman and Jeffrey Kenworthy—here’s a recent example.  Their research also looks at car dependence—but considers entire metro areas, not just individuals.  And they find a clear and strong relationship between metro-area density and how much driving people do.  Take a look, for example, at the following chart—with metro-area density on the horizontal axis and transportation energy on the vertical axis. 

    Density and energy chart -  Newman and Kenworthy

    Newman and Kenworthy believe that density has such a strong effect on driving because of two fundamental travel constraints.  The first is the so-called Marchetti Constant—a long-standing and perhaps natural human desire to keep one-way commutes to about half-an-hour, a value that has held steady since Medieval times and perhaps longer.  Once people have to travel more than half an hour to their workplace, they do something to get to work faster—by changing where they live, where they work, or how they get from place to place. 

    The second constraint is the inherent speed of different modes of travel—that is, how far you can travel in half an hour on foot vs. bike vs. transit vs. car.  In big, sprawling cities, people just can’t get from homes to jobs in under half an hour without a car.  Muscles and buses just don’t move fast enough!  So low-density land use inherently drives up both car-dependence and energy use for transportation.

    Because of these constraints, Newman and Kenworthy argue, density is a fundamental determinant of transportation patterns at the metro level.  Whenever the aggregate density of a metro area falls too low, people’s only option for keeping their travel time under the Marchetti Constant is to use cars for most trips. 

    This perspective—that density has a fundamental impact on how people get around in a city—is almost the inverse of Ewing’s view that density is at most a side note to individual choices. How to reconcile the two?  Maybe it’s not all that hard.  Density, as Ewing points out, is commonly associated with the features that really do affect transportation choices:  big clusters of jobs and services, central locations, and short blocks and lots of intersections to make walking trips more direct.  When lots of people live in or near complete, compact city and town centers, you get both density and “location diversity” in one package—and, hence, lots of access without a lot time in cars.