Obesity and the Built Environment

Planners have asserted for years that the built environment affects health, claiming statistical correlation between mixed land uses and lower obesity rates. This finding is important, because it casts the blame for American obesity on the suburban landscape. Planners now have another tool in their arsenal arguing for better land use mix (a cause that I support). As the thinking goes, people who are able to walk to activities will do so, and thereby have lower obesity rates than the general population.

I just finished reading a paper (by my advisor and her former doctoral student in Transportation) that critiques the methodology in at least one of these studies, a 2004 study by Frank, Andersen, and Schmid (American Journal of Preventative Medicine) relating obesity to travel patterns in Atlanta. The authors constructed a linear regression model based on a household travel survey of Atlanta residents. They showed statistically significant positive correlation with age (older people are more obese), in-vehicle travel time (obese people have longer commutes), and being black. Statistically significant negative correlation was seen in education (people with higher incomes are less likely to be obese), income, daily walking distance, uniform land use patterns, and being a woman.

My advisor's analysis examined the AJPM model and improved on it by introducing non-linear relationships. For example, people tend to gain weight until they reach about 60, and then lose it again. So using age as a strictly linear variable misses the impact that late age weight loss may have on obesity. This and other adjustments increase the ability of the model to predict obesity far more than improving the prediction of the land use field. So while people who live in mixed-use areas are obese less frequently than the general population, land use by itself is a poor predictor of obesity.

What are needed are studies that carefully measure both travel behavior and Body Mass Index. But the studies have not indicated that health records should be a major focus of travel survey collection. Indeed, theTransportation paper indicates that shifting the focus of a travel survey to health information can be a very risky proposition, because you may end up under-sampling more important variables, like actual travel behavior.