What I've Learned

Do people in cities walk faster than people in towns?

And if so, does this scale upward? If a city grows from one million to two million, will people there walk faster than before?

Does location matter? As cities in France or the US or Germany grow, will people walk faster, but as cities in New Zealand or Mongolia or Guatemala grow, will people not walk faster?

Until a couple of days ago, my answers would have been these:

Yes, people in cities walk faster than people in towns, but it does not scale upward. People in a city of one million will not walk any faster when the population reaches two million. That's just silly.

And yes, location matters. If there should be an increase in walking speed, city people in some countries won't be effected as much as city people in others.

Those would have been my answers. For the most part, they would have been wrong.

Let's leave the question of walking speed for a moment and talk about mammals.

You and I and all other mammals, be it a mouse in Missouri, a dog in Russia, a cat in Kenya, a llama in Peru, or an elephant in Indonesia, each have weight.

Each mammal also has a metabolic rate. That is, a certain heart rate, breathing rate, and amount it must eat to have energy enough to move about.

Suppose we plot the weight and metabolic rate of all the mammals in the world on a graph. We'll use up and down on the graph to measure metabolic rate and left and right to measure weight. The higher up on the graph something is, the slower its metabolic rate. The further to the right something is, the more it weighs.

If a mammal has a very fast metabolic rate and doesn't weigh much--a mouse, for instance--it would appear low and to the left on the graph. If a mammal has a slow rate and weighs a lot--say, a hippopotamus--it would appear high and to the right.

What do you suppose the graph will look like? You would think there would be dots all over the place, looking like a shotgun blast. After all, mammals developed at different times and in different environments.

But this is not so. All mammals will fall somewhere on or very near a line that slopes upward from left to right. That line is so regular that given the weight of any mammal from anywhere, its metabolic rate can be estimated by doing some easy math. Also, if the metabolic rate is known, its weight can be figured out.

We mammals are that consistent. Our metabolic rate and our weight are proportional and scale neatly upwards. The larger a mammal is, the slower its metabolic rate. A mouse's heart will beat around 500 times a minute; a human's, around 65; and an elephant's, only 30.

Geoffrey West, a physicist and former president of the Santa Fe Institute, wondered if cities, plotted on a graph, would show the same sort of consistency that mammals do. Are there things that scale upward as a city's population increases?

It turns out, to his surprise and my amazement, that each time a city's population doubles, just about anything you can name increases at a predictable rate of 15 percent. It doesn't matter where in the world the city is, wages, number of gas stations, length of roads, instances of AIDs and flu, cultural events, number of colleges, number of patents awarded, crime rate, waste produced, and, yes, average walking speed, all go up 15 percent.

On the internet there is a brilliant 17-minute talk by West in which he explains his findings. Search for his name and the word TED to locate it.