We all knew that, but this study is randomized.
Indeed, we have been arguing about this for years. Is it the urban design and sprawl that makes us fat (the case that I have made many times?) Or is it the car itself? Or, as economist Ryan Avent wondered, are people self-selecting where they live?
But the bottom line is that there is a very strong correlation between living in sprawl and being obese. And while it isn't necessarily the case that sprawl makes people fat, it is the case that obese people move to the 'burbs because it's so easy to be fat there, what with the not having to walk and the many drive-through restaurants and so on.
Now Hutchinson points to a really interesting study that seems to clarify the issue, titled Physical activity and weight following car ownership in Beijing, China: quasi-experimental cross sectional study. It's randomized, solving the self-selection question.
Since 2011 the number of new car permits issued in Beijing has been capped to control congestion, with a lottery being held every month. Since winning the lottery is completely random, the researchers were able to track what happened to winners and losers over five years. These are not urban vs suburban, but regular people who live in Beijing, some who are lucky at lotteries and some who are not.
As you might expect, the winners walked or biked a lot less, 42 percent less after five years, averaging 24.18 minutes per day less. Over five years the weight piled on, an average of 5.2kg for people under 40 years old, and 10.34 for those older than 50.
Hutchinson writes, "The randomized assignment of cars in Anderson’s Beijing cohort finally shows that it’s the car itself, not simply being the type of person who wants a car, that influences behaviour and ultimately health."
He spoke with researcher Michael Anderson who concludes:
“The public-health impacts of automobile travel are really important,” he says. “While cars have saved trillions of hours of travel time globally, they’ve also likely shortened lifespans by trillions of hours in aggregate via traffic accidents, pollution and obesity-related disease.”
We go on about how cars are a public health crisis because of the 40,000 people killed every year in the US by cars, but they are clearly deadly even if you are just sitting in them. Perhaps instead of building more roads to handle more cars, we should be doing everything we can to get people out of them.