My thanks to Dr Anne Lusk from the Harvard School of Public Health for the intellectual substance in what follows:
Rates of obesity and chronic disease related to inactivity in the course of our days are in the order of 20 to 60%, depending on what study you read. The orthodox view among environmental designers is that clustering apartments around transit nodes and designing districts so they invite us to walk—hmm, yum, cafes!—will gradually lead to a healthier, slimmer population.
This really is a polite fiction and one that architects and urban designers have been all too willing to buy into. Walking a few hundred meters a day between transit hubs and our homes provides such a small dose of movement that I doubt it would even have a placebo effect. Common sense tells us as much and I am sure Jane Jacobs would tell us as well. Our problem are latter-day-Janes, most notably Jan Gehl, promulgating a dogma so entrenched among planners, architects and urban designers, that I’ll have to use facts if I want to have common sense heard. Please read the next paragraph slowly:
Colleagues of mine at the Harvard School of Public Health checked up on 18,000+ middle-aged women whose weights and activity habits they knew from another study 16 years prior. Their general finding was that, once a person has gained weight, they tend to be unable to lose it if all they are doing is walking. By contrast cycling was found to be helping the slim stay slim, and the overweight lose some, even if they were only cycling for a very short time every day.
What is the difference? It is that too few people are able to walk briskly enough to really raise their metabolisms. Joint pains that naturally increase with ageing, as well the weight they may already be carrying, makes brisk walking uncomfortable for too many people. All this means walking cannot be counted on as a strategy in environmental design to control the weight of a population. Bicycling can be. On upright bikes, with their weight relieved from their hips by their saddles, even people with joint pains or excess weight can participate in an activity that cannot help but raise their metabolism into the weight burning range.
Cycling helps control weight because it raises our metabolic equivalent of task (MET) by on average 8 times—compared to a base rate of 1 while we are resting. The only other activity that speeds our metabolisms like cycling is walking up stairs, which few of us are likely to do for any duration. Casual walking only has an average MET score of 3.
What do those numbers mean if we relate them to transport? They tell us that a person will burn four times more calories by riding their bike for half an hour to work, than they would burn if they split that half-hour into 15 minutes of walking and 15 minutes on the tram doing nothing.
If a bicycle city is sounding like some kind of pill, it is worth remembering that pills don’t have to be bitter. Think of a bike city as one of those vitamin-C pills that your kids would eat all at once if you let them. There might be an acidic sting somewhere below, but it is masked by a nice mix of sweeteners. One thing that would make a bike city sweet, is the time it would free up by making small errands far more efficient, and fun.
 Lusk et al., (2010) “Bicycle Riding, Walking and Weight Gain in Premenopausal Women” Archives of Internal Medicine June 28:170(12):1050-1056) http://archinte.jamanetwork.com/article.aspx?articleid=416094
 For MET scores for walking and cycling see: Ainsworth, 2000 Med SciExerc; 32(9Suppl)S498-504. See also: “Corrected METs”, https://sites.google.com/site/compendiumofphysicalactivities/corrected-mets, accessed 15.8.2014.