Does using public transport make the community healthier?

A new US study finds that mode share and accessibility by public transport aren't correlated significantly with health outcomes. If the study's right, how could that be?

Distance walked to rail stations on an average weekday for all trip purposes, Sydney GCCSA (source data: NSW BTS)

Here's a finding that goes against the common wisdom that public transport users are healthier because they walk more. It's an extract from a new working paper, Transit makes you short, by University of Minnesota researchers, Elizera Ermagun and David Levinson:

[The] effect of transit mode share and accessibility by transit on general health, body mass index, and height are investigated, while controlling for socioeconomic, demographic, and physical activity factors… (We) found that the transit mode share and accessibility by transit are not practically significant, and the power of large-sample misrepresents the effect of transit on public health.

Note this finding doesn't mean you and I won't get more exercise from walking to the bus stop or the rail station rather than (say) driving. What it throws doubt on is the claim that the walking inherent in public transport delivers important health benefits across the whole community and, accordingly, that policy on public transport should take those benefits into account.

The link between the physical environment and health outcomes like obesity and diabetes at the social level is a contested issue; I've discussed it a number of times before (e.g. see here, here and here) .

Unfortunately the paper doesn't provide enough information to assess if the methodology is reasonable; but it prompts an intriguing question. Assuming for arguments sake that the authors are right, how could it be that greater public transport use doesn't translate into better community health outcomes? (1)

One reason might be that public transport users are skewed toward the young, who tend to be healthy anyway in spite of their sometimes careless habits. A few hundred or even a few thousand metres walking per day mightn't make a measurable difference to their health status.

Another might be that for many users the amount of walking involved in getting to their nearest stop isn't that much. Dedicated public transport users tend to select housing and jobs near stops; for example, 64% of train passengers in Sydney who walk to the station cover less than 500 metres one-way and 79% less than 1,000 metres.

Even a 500 metre walk in each direction only involves around 1,250 steps a day relative to the recommended daily minimum of 10,000. Bus and tram stops have even smaller catchments (buses carry more people than trains in Sydney). Some travellers "power walk" their way to the station/stop; others adopt a more relaxed gait.

Or it might be that the extra time required to travel by public transport gives users less time for for active recreation (e.g. playing with the kids, gardening) or for intensive exercise (e.g. gym, sport). The average train trip is considerably longer than the average car trip for all purposes e.g. see Is driving quicker than taking the train?