Are cars bad for you?

One of the problems with looking for health benefits of active transportation is that people who walk or cycle are self-selected weirdos. It’s a free country. You can’t just randomise people to owning a car or not.  You’d think.

As Alex Hutchinson, of the Globe and Mail reports, based on a research paper in the BMJ,

 Because of mounting congestion, Beijing has limited the number of new car permits it issues to 240,000 a year since 2011. Those permits are issued in a monthly lottery with more than 50 losers for every winner – and that, as researchers from the University of California Berkeley, Renmin University in China and the Beijing Transport Institute recently reported in the British Medical Journal, provides an elegant natural experiment on the health effects of car ownership.

The researchers interviewed a sample of 40,000 people across Beijing and asked them questions. Because of the lottery, the results should be more reliable than useful usual.

It’s not quite as simple as that.  First, people who respond to the survey may be unrepresentative (just over 20% responded). Second, the impact of winning the lottery seemed relatively small:

Our results indicate that those individuals winning a lottery permit to purchase a car reported transit use 45% lower than those who did not win…Differences in physical activity became apparent over time. About 2.6 years after winning, winners spent 7% less time walking or bicycling than losers. At 5.1 years the reduction in walking or bicycling rose to 42%.

This may be less surprising if you’ve been to Beijing and seen the traffic congestion. Anyway, the impact on physical activity was very small initially, though it did increase over time.

The main outcome variable measured was weight:

Average weight did not change significantly between lottery winners and losers.

If you look just at people over 50, and wait until five years after the lottery, there’s an estimated 10kg weight difference, but the statistical evidence is pretty weak and the uncertainty is large. The effect could easily be pretty much zero, and that’s without worrying about picking just one age group.

The basic message here is that it’s hard to do experiments on driving — even in one of the world’s biggest cities, the data end up being consistent with anything from no effect to a huge effect.