According to most of the medical studies I've read, the #1 cause for being fat in the Western world is lack of education, followed by the daily use of a car, the lack of a proper stimulating environment that will make you walk. Improving the efficiency of public transportation can have a dramatic effect on the occurrence of obesity.
"The use of light rail transit to commute to work was associated with an average −1.18 reduction in BMI (p<0.05) and an 81% reduced odds (95% CI= 0.04, 0.92) of becoming obese over time."
Walking only 1 mile a day will have similar results: the occurence of become obese over time or dying because of a CVA (cerebrovascular accident) is immediately reduced by a tremendous factor.
This is why the equation suburbia (individual housing) + shopping malls + lack of diplomas = fat people is true most of the time.