Driving and Obesity.. coincidence
The Economist has a nice chart showing the correlation between increase in miles driven and obesity. Now this is hardly ground breaking, many have seen this correlation and one can look at studies that show that individuals in cities are on average thinner than suburban fold, and that they drive less.
I think you can look at many corroborating reports that tie, more driving is more sedentary and this leads to greater obesity. The Freakonomics blog has a post showing causation of his age with obesity. A bit cheeky, since the sample size of one is ridiculous, where the larger population suggests a pattern. Granted some normalization for average vehicle miles and maybe normalized BMI may be more convincing. But not sure, I'm not a statistician.
Freakonomics is taking a pot shot that can be used to describe any trendline going up.