Road hogs

By The Economist online

A striking relationship between obesity and driving

AMERICANS are getting fatter: obesity rates have risen 74% in the past 15 years to nearly 28% of the adult population. And they are driving more: the number of miles driven by each licensed driver (VMT/LD), excluding commercial vehicles, increased by an average of 0.6% a year between 1988 and 2008. Academics at the University of Illinois have found a striking correlation between these two variables—but with a large time lag. They noted that previous research had found that changes in diet had an effect on body weight only after some six years. Therefore VMT/LD in 2004 is correlated with obesity in 2010 (see left-hand chart). This near-perfect correlation (99.6%) permits predictions about obesity rates. Since VMT/LD fell in 2007 and 2008, America's obesity rate could fall to as low as 24% in 2014 (see right-hand chart). These predictions come with a strong caveat: correlation does not equal causation. And it should be noted that the authors did not control for factors such as diet, income and lifestyle. Additionally, they did not explore the possibility that the larger, and thus more immobile, people become, the more they drive.