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The Surprising Math Behind Airports and City Size

The population of a given metro area affects airline travelers more than you might think.


Earlier this week, I looked at the role of airports in the economic development of cities and regions. Today, I turn to a fairly obvious question whose answer is less obvious: is city and metro size a function of airport size? The short answer is “yes,” but the relationship is not one to one—and the mathematics of that relationship has wider implications.   

In general, the bigger the city the more airport activities there are, however they are measured. Metro size is strongly correlated with the total number of flights (.56) and the number of passengers (.59), but less so with total cargo (.24). This makes sense, as cargo hubs are often sited in locations that are equidistant from a number of different markets, rather than at the center of one major one.

The graph above, put together with data from José Lobo of Arizona State University, plots the relationship between passengers and metro population. The line slopes straight upwards, showing the close relationship between the two. Not surprisingly, larger urban areas witness a greater flow of passengers. But the slope of the line fitted to the scatter plot (in which the data is expressed as natural logs) does tell us something surprising: an increase in urban population of one percent is associated with an increase in airline passengers of nearly 1.5 percent. In plain language, for every person added to an urban area’s population, almost one and a half passengers are added to airport traffic. Increasing population size brings about a disproportional increase in the churning of people which airport traffic represents.       

As with other features of metropolitan life, such as productivity or invention, airport passenger traffic increases disproportionately with population size. This is yet another manifestation of the overall acceleration of social and economic life that occurs in urban population centers—a phenomenon that has been dubbed “superlinear scaling.” Cities differ from all other social and biological organisms, according to research by Geoffrey West and his collaborators at the Santa Fe Institute, in that their metabolisms speed up rather than slow down as their size increases, speeding the flow of goods, people and ideas and accelerating the rate of innovation and economic growth. The fact that they scale even faster than city and metro size suggests that airports play a key role in the speed and velocity of cities and metros areas. Airports, it appears, are inextricably connected to the faster pace of urban metabolism.

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