Getting fat? Blame the car

Rising trends in adult obesity in the US parallel very closely the trends in annual mileage travelled per licensed driver in cars and light vans, according to a paper in the June issue of Transport Policy.

Correlation is not causation, as the authors freely admit. But in this case the two trend lines are astonishingly well-matched, if you allow a six-year delay between changes in vehicle miles travelled and those in adult obesity. Such a delay is plausible, the authors argue, as it takes some time for changes in behaviour to have an effect on body weight.

If they’re right, obesity rates in the US should be about to plateau, as the vehicles miles travelled per licensed driver has been flat or declining since about 2005. The chart below, taken from the paper, shows how closely obesity rates, lagged by six years, followed vehicles miles.




Can something as complex as obesity be explained by people spending more time in their cars? It sounds unlikely, though it’s undeniable that the authors – led by Sheldon H. Jacobson of the computer science department at the University of Illinois – have produced a good match between 1995 (when the obesity time-series began) and 2007.

Their model predicted obesity rates of 26.5 per cent in 2008 and 2009, and the actual figures, subsequently published, were 26.7 per cent and 26.9 per cent – relatively close.

It is possible that miles travelled is simply a marker for GDP growth per adult, but the authors show the fit is much better for mileage than it is for GDP. What is hard to believe is that relatively modest increases in car use could have such a huge effect on obesity – an increase of one mile a day per licensed driver is associated with a 2.16 per cent increase in adult obesity six years later – or if they do, that the effect should be delayed for so long.

An increase in miles travelled per licensed driver in the US  from 11,700 a year in 1989 to 13,500 a year in 2003 has corresponded to a near-doubling of obesity six years later, from 15.9 per cent in 1995 to 26.9 per cent in 2009. Put that way it doesn’t sound very likely.

I played around with UK figures to see if a similar relation could be teased out of them. Data on miles driven in cars and vans does exist, though information on the number of licensed drivers is patchier. But, subject to those limitations, there appears to be been almost no increase in miles driven per licensed driver between 1990 (8,390 miles) and 2000 (8,490 miles) while adult obesity went up between 1996 and 2006, six years later, from 17.5 per cent to 23.9 per cent. So I’m unpersuaded.     

Reference: A note on the relationship between obesity and driving, by Sheldon H Jacobson, Douglas M King and Rong Juan, Transport Policy 18 (2011) 772-776