I live approximately four kilometers from work, a distance where individual choices really matter. It is far enough to go by car, close enough to take the bike, and public transport is frequent and smooth. So everything, save helicopter, is a realistic choice for me. I do read a bunch of ecologist blogs and dress in Patagonia wear so it might not be a surprise that I take the bike.
Now imagine a fictive colleague of mine who also lives four kilometers from our workplace. Let's call him Charlie. Charlie thinks of my lifestyle as a slightly hypocritical way of being hip through pretending to worry about 1000 and one catastrophes. He takes the car, always did, and tells himself that whatever is true about global warming, his driving or not driving is very unlikely to change the bottom line. I might change his own bottom, though.
I eat a breakfast consisting of about 3-400 k cal Is it unreasonable to presume that Charlie does as well? Not so. At work we eat together, and both consume about 500 k cal for lunch. Since Charlie and I are both bon viviants in hiding we usually consume about 650 k cal for dinner. To sum up: in a day we both consume 1450-1550 k cal
These numbers are completely arbitrary. My point is this: it is not unreasonable to imagine two individuals with the same intake of energy and the same job, one of them bicycling and the other one driving a car. I could even invent a scenario where the car driver eats more than the cyclist, without being unreasonable.
Obviously these 1450-1550 k cal is enough to transport me to work and back without feeling dizzy. It is also enough to take me through a working day with the same kind of tasks as Charlie. The question is what happens to them in Charlie's body.
We differ incredibly much, and conclusions could not be made about a single individual. Some of us can ride cars without growing a stomach, some of us can not. But unless bicyclists eat more, and car drivers work out more in their spare the result on the population at large can only be one: fat car drivers. And this is what nation after nation turn into as modernity spreads around the globe. There is a very strong correlation between car driving and obesity. It is shown in statistics, and self evident.
It follows that investing in highways and making it easier for people to commute by car, as governments around the globe do, will have adverse effects on the nation's health. And that anyone who drives a car on a daily basis must come up with some kind of plan of how to use the energy he or she consumes as food during the day.
The problem is not only related to car driving, of course. It is a general problem in a society where human work is replaced with automation. We are likely to eat the same amount of food as we always did - so what do we do with the energy we do not need any more?
The alternative to obesity is to exercise for the sake of exercise. That can be relaxing at times, but if you think of it as energy usage it is rather wasteful - like sending people to dig a whole and refill it, like the first French Republic did to curb unemployment. Wouldn't it make more sense to use up that energy for productive work, and then have a rest?
In other situations we tend to think that working hours and energy should be used for something useful. Why do we not think the same about the energy we consume as food? The enormous amount of human power in a 7 billion world could be harnessed for something better than gymming, right?