The size of cities

2 recent NYT columns about the size of cities caught my eye.

Edward Glaeser explains why India's (and emerging economies') cities grow so large. And Steven Strogatz explains the power law that describes the exponential distribution of city sizes within a country - and relates it to similar distributions observed within living organisms.

Here's a particularly interesting insight from Strogatz's column:

For instance, if one city is 10 times as populous as another one, does it need 10 times as many gas stations? No. Bigger cities have more gas stations than smaller ones (of course), but not nearly in direct proportion to their size. The number of gas stations grows only in proportion to the 0.77 power of population. The crucial thing is that 0.77 is less than 1. This implies that the bigger a city is, the fewer gas stations it has per person. Put simply, bigger cities enjoy economies of scale. In this sense, bigger is greener.


The same pattern holds for other measures of infrastructure. Whether you measure miles of roadway or length of electrical cables, you find that all of these also decrease, per person, as city size increases. And all show an exponent between 0.7 and 0.9.