Tuesday, February 14, 2006

Fat City?

Does sprawl make one fat?

It appears that suburban residents are heavier on average, all things considered. That leaves the question of causality, which can be asked at least two ways. Do heavier people tend to move to the suburbs, or does sprawl makes you/me gain weight?

In statistical language, as recently put by Matt Kahn's blog, is the outcome of heavier people in the suburbs explained by selection (due to sorting) or treatment (caused by sprawl), or both?

There are loads of studies in this area, with RWJ funding for the topic helping to change the focus of recent land use/transportation scholarship from motorized travel to "active" travel, such as walking and biking. And yet, so far as I know, we have no clear evidence on the influence of treatment vs selection.

I recently had the genuine pleasure of serving on a National Research Council Committee (joint IoM/TRB) on this subject, along with a bunch of smart planners (Don Chen, Robert Cervero, Gen Giuliano, Pat Mokhtarian, ....) and public health scholars led by chair Susan Hanson. Our charge was to assess the literature so we commissioned review papers from the best and the brightest. The report released early last year includes this finding:

The available empirical evidence shows an association between the built environment and physical activity. However, few studies capable of demonstrating a causal relationship have been conducted, and evidence supporting such a relationship is currently sparse. In addition, the characteristics of the built environment most closely associated with physical activity remain to be determined. (p. 7)

That is, we had no confidence about either the details of these relationship or their causation.

One problem is that most research is cross-sectional, comparing the features of individuals (or averages of individuals) and their circumstances at any one point in time. These data are often aggregate (e.g., county-level), which is problematic, or from new household surveys of particular places, which are harder to generalize from.

To test for sorting, it would be better to track people over time and place. Matt Kahn also mentions a draft paper by economists Jean Eid, Henry Overman, Diego Puga and Matthew Turner, "Fat City: Obesity and Urban Sprawl in the United States," which uses panel (cross-sectional and longtitudinal) person-specific data from the National Longitudinal Survey of Youth. I gather they find some evidence of sorting. (Update: A complete October 2006 version of this paper is linked to a Planetizen brief here. After controlling for sorting on a national sample, they find no evidence that sprawl is associated with weight gain in the U.S.)

Any other data sources that track weights and city/suburban location over time with sufficient controls?

1 comment:

Anonymous said...

I've got a blog post over at Cascadia Scorecard going thru the edit process about Larry Frank's work on this very topic - he was in Seattle recently talking about his results.

I'll make sure I put a hyperlink to this post for when it goes up, hopefully soon.

Keep up the good work!

Dan Staley
Planning Director
City of Buckley


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