Conclusions These data indicate that vehicle ownership in a rapidly growing global city led to long term reductions in physical activity and increase in weight. Continuing increases in car use and ownership in developing and middle income countries could adversely affect physical health and obesity rates.

Results Of 937 people analysed in total, 180 had won a permit to purchase a new vehicle. Winning the permit lottery resulted in the purchase of an additional vehicle 91% of the time (95% confidence interval 89% to 94%; P<0.001). About five years after winning, winners took significantly fewer weekly transit rides (−2.9 rides (−5.1 to −0.7); P=0.01) and walked and cycled significantly less (−24.2 minutes (−40.3 to −8.1); P=0.003) than those who did not win the lottery. Average weight did not change significantly between lottery winners and losers. Among those aged 50 and older, however, winners’ weight had increased relative to that of losers (10.3 kg (0.5 to 20.2); P=0.04) 5.1 years after winning.

Obesity rates are rising worldwide, including in China, but the underlying causes are poorly understood. 1 One possible culprit is the long term decline in active forms of transportation. Previous research has disclosed substantial cross sectional or longitudinal associations between reported physical activity, body mass, and use of cars. 2 3 4 5 6 7 8 9 10 11 12 13 To understand how car ownership affects physical activity is important for both public health and environmental policy. 14 The relation between car ownership and physical activity remains unclear, given the confounding inherent in observational data. In this study we used the random assignment of permits for automobile purchase to households in Beijing, China, to estimate the relation between car ownership, physical activity, and weight.

Methods

In January 2011, to deal with the problem of congestion, Beijing capped the number of new vehicles allowed at 240 000 each year and introduced a vehicle permit (licence plate) lottery. After that date, only residents who entered and won the lottery were entitled to a licence plate. The lottery was drawn monthly, and winners had to purchase a car within six months of winning. By mid-2012 the probability of winning fell below 2% a month.

The sample was drawn from a group of households collected by the Beijing Transportation Research Centre. The research centre conducted a transportation survey of a random sample of 40 000 households from Beijing in the autumn of 2014, stratifying by district. The survey included questions on whether a household had entered the vehicle permit lottery. Among households with lottery entrants, a stratified random subsample was further surveyed in early 2016 with questions about the permit lottery. Because the lottery had many more losers than winners, sampling was stratified by winning status, with only one third of losers sampled. All lottery entrants in a household were surveyed.

The average household response rate in the 2016 survey was about 22%. Response rates were similar for winning and losing households—21.8% and 22.2%, respectively—suggesting that there was no response bias related to the exposure itself. The resulting sample comprised 180 winners and 757 losers. The sample size was considered sufficient to detect an effect of about 0.2 standard deviations in the primary outcomes—namely, transit rides, minutes spent walking and bicycling, and weight, with 80% power. The 2016 survey asked questions about car ownership, use of public transportation, minutes spent walking or bicycling daily, and weight, which are the key outcome variables for this analysis.

We used data from the 2014 Beijing Transportation Research Centre transportation survey, and found that 2016 survey respondents were broadly similar to 2016 survey non-respondents for many characteristics: household size, number of adults, number of working adults, number of children, household income, sex, age, and education (supplementary appendix). Usually, the mean differences between respondents and non-respondents were small, and in all cases were of the order of 0.1 standard deviations or less.

All outcomes were based on survey responses. Previous work in the Chinese population has found that self reports of physical activity have good reliability and moderate validity, and self reports of weight are generally reliable.1516 Nevertheless, the accuracy of self reported weight has been shown to be inversely related to changes in weight and when weight is changing quickly it may not be reliable.17 In that case the bias would attenuate our estimates, which could thus underestimate the magnitude of activity or weight changes if individuals do not quickly update their weights or activity levels.

Data were complete for most outcomes and key predictors. In general, we omitted missing data, rather than attempting to impute a value. We made exceptions, however, for two variables: the month of initial participation in the lottery and the month of winning the lottery. Although data on the year of lottery entry and year of winning were complete, the exact month of entry (or month of winning) was missing for 251 (26.8%) of 937 individuals. In these instances we replaced the missing month with 6.5 (that is, the midpoint between 1 and 12).

Stata (StataCorp, College Station, TX), version 15.1, was used to estimate intention to treat linear regressions of different outcomes on an indicator for winning the lottery, with standard errors clustered at the household level. Lottery winners were intended by the policy to be treated (with car ownership), while lottery losers were intended not to be treated. Intention to treat analysis exploiting the lottery was important because most licence plates in Beijing were issued before the lottery and thus not randomised. Furthermore, trading or rental of licence plates on the secondary market has occurred.18 Thus winning the lottery is the only valid source of randomisation. Although trading in the secondary market is widely recorded, there has never been any suggestion that the lottery itself was corrupted. Owing to the public nature of the lottery, buying or borrowing a licence plate from an existing vehicle owner was a much safer strategy than attempting to corrupt the lottery administrators.

Winning the lottery was random, conditional on date of entry—participants who entered earlier had better odds and greater chances of winning. Thus all regressions controlled for a cubic function in date of entry. We checked lower and higher order polynomials in date of entry to ensure that results were not sensitive to polynomial order (supplementary appendix).

Because the survey date was approximately constant across households, variation in the date on which winning occurred generated variation in the time between winning and outcome measurement. Because the associations for some outcomes could evolve over time, we allowed interaction between winning and years since winning and reported predicted changes for an individual surveyed 0.1 years, 2.6 years, and 5.1 years after winning (see supplementary appendix for details). These values represented the minimum, average, and maximum times that had elapsed, respectively, between winning and being surveyed. Weight gain is strongly related to age.19 Thus we estimated separately activity and weight regressions for individuals over 40 or over 50 years of age (corresponding to the top half or quarter of our sample’s age distribution) and tested whether an interaction between age and winning the lottery differed from zero. We also tested whether an interaction between male sex and winning the lottery differed from zero. Two sided hypothesis tests were conducted, with Pvalues under 0.05 considered significant.