Car exclusion zones around schools have grabbed the headlines, or rather the page nines, of many papers. The exclusion zone proposal was, however, one proposal extracted from a 69-page report. Please understand, I'm not complaining as we've been fairly inundated with media interest. Only a collection of murders over the weekend knocked us off a slot on Monday's Today programme.

At the heart of our report was something very fundamental: walking has been replaced by car use as more people attain car access. Indeed, this is in some senses hardly newsworthy. To paraphrase one BBC radio presenter's introduction to an interview, people are being paid to tell us that walking is good for our health: what's going on?

Yet, this behavioural change over the past three decades is very important as it is a major driver, excuse the pun, of obesity and climate change. Let me explain.

Surprisingly to some, we are not consuming more calories but have substantially reduced calorie expenditure through mechanisation and particularly transport. The substitution of walking for car use, which we track through National Travel Survey data from 1975/76, shows time spent travelling by car each day rising while walking declines, and households without cars falling from 41% to 19% between 1975 and 2005. If a typical British adult were to walk for just one hour more a week - returning to the average walked by people without cars - it would prevent them gaining two stones over a decade and make a major contribution to halting the obesity crisis.

Main drivers walk half the distance of those in non-car households. Why? Because walking is often not seen as an acceptable means of transport. Habitual car use (what social psychologists call "automaticity") means repetition of yesterday's behaviour. And, this transfer from walking to car adds to carbon dioxide emissions too.

So what are the solutions? Practically 38% of all journeys under two miles are by car and many of these could be replaced by walking. Yet, even with good intentions, many people remain locked into sedentary lifestyles through perceptions of time and physical barriers. Despite gym membership popularity (not gym use) the solution must be to walk (or cycle) for short journeys, as recommended by the chief medical officer. School exclusion zones are one means to achieve this, as well as improving the built environment, but we desperately need an overarching policy and programme to integrate transport and health. A national walking strategy would be a start.