A Tale of 200 Cities

We think we control the size of our cities by restricting outsiders. How vain is human striving!


At the level of individual neighborhoods, there is certainly control over whether outsiders are let in.  Neighborhoods, and even some small cities, can simply refuse to build any new housing for years or decades. But the size of of the city as a whole—the cohesive economic unit that makes up an urban area—is governed by economic laws that cannot simply be repealed by policy.


One of the most interesting is Zipf’s law, which says that there is a linear relation between the log of the population size of a city and the log of its rank—first, second, eightieth, or whatever.


Zipf's Law
There is a strong linear relationship between the log of a metropolitan area’s population size and the log of its rank among cities.


Jeremiah Dittmar has done some fascinating work showing that Zipf’s law has held for cities, not forever, but at least since about 1600. It is close to a universal law in economics as it gets. The strength of this law is stunningly—albeit quite tragically—demonstrated by Japan, in which the size distribution of cities hardly changed at all in the mid-Twentieth Century despite massive, destructive bombing of some of the cities by the US.  Hiroshima, the 9th largest city in Japan in 1899, was the 10th largest city in 2010. Cities are exceptionally resilient.


The reasons for this resiliency are not entirely clear. Size resiliency could come from innate geographic advantages, or perhaps from some initial random advantage.


Whatever the causes, the implications of this work are pretty clear. If you don’t make room for people in your neighborhood, they are likely to move to another neighborhood in your metropolitan area, or just make the whole metropolitan area geographically larger, by moving to the edges. As people move farther out, they rely more and their cars, and congestion will undoubtedly increase, not decrease. Those who are worried about new residents causing new congestion should take a long look at Zipf’s law. Because new residents are inevitable—congestion is not.

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