Richard Florida has discovered that as cities get bigger, airport traffic goes up even faster than population. Here's his interpretation:
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.”
OK, but how much of this is what transport planners call the network effect? Just as transit planners try to locate connection points at already-active locations, airlines are motivated to locate their hubs in major cities, so that the city and the hub support each other in creating the largest possible market for flights in and out.
For example, ask: what's the biggest US metro area that is NOT an airline hub? Using this table (and setting aside greater Riverside because its airport is run by Los Angeles) I get all the way down to San Diego, the 17th largest, before I encounter one where hub effects have little role in the airport's function. So it doesn't surprise me that the scaling of big city airports is "superlinear" (a fancy word for "curving upward") compared to population, for the same reason that boardings at major transfer points in a transit network are superlinear compared to their immediate surroundings.
I could interpret this as a nice example of the very common tendency of people with social science training to overread demographics and economic indicators, while underreading the effects of network structure and how it determines the actual utility of transport service. This issue comes up in transit all the time.
But another interpretation, more to Mr. Florida's liking, would be that the connection in a transport network is not really all that different from the social connections that occur in urban life. Florida is famous for documenting how all these connections rise in non-linear ways as there are more people around to connect, thus leading to hotspots of creativity and innovation.
When I suggest that transit agencies use the word connection, instead of the dreary word transfer, it is to invoke exactly this association. A transport connection isn't just an analogy with the social, cultural, and intellectual connections that drive innovation. In its multiplying power it's almost the same thing.