Here is an overview of two ways to model intercity mobility: the “gravity law” and the “radiation model” which was just recently proposed in Nature:

The reigning model of intercity mobility, used to predict patterns of movement from commuting to the spread of infectious disease, is called the “gravity law.” It was developed in the early 1940s by a Harvard lecturer named George Zipf and is, of course, based on Newton’s law, which says gravitational force increases when the mass of two objects is great and the distance between them is minimal.

In that same spirit, Zipf’s “gravity law” of mobility assumed that movement between two cities would be most frequent when their populations were large and their separation small. In reality, however, the “gravity law” doesn’t do a great job estimating the intercity movement it was intended to predict. While Zipf’s law frowns on the notion that people travel frequently between distant cities, recent research on so-called “super-commuters,” outlined by our own Richard Florida, shows that a considerable subset of urban populations is actually willing to commute quite far…

The “radiation model,” as the new idea is called, makes several assumptions the gravity model does not. For starters, it downplays the distance between two cities and emphasizes not only the cities themselves but the density of the areas surrounding them. That enables the model to estimate the number of jobs in a region more accurately. It also accounts a bit more for actual human behavior: while the radiation model presumes that people choose a job based on a balance of proximity and benefits, it recognizes that they’re willing to make long commutes if few jobs in their region satisfy their requirements.

As a result, the radiation model out-predicts the “gravity law” in direct competition. As an example, the researchers looked at mobility between two pairs of counties in Utah and Alabama. Both counties of origin had similar populations, as did both destination counties, and both pairs are more or less equidistant from one another. Actual Census data shows that 44 people make the commute in Utah, while six do in Alabama.

This sounds very interesting and required advances in data collection on this topic as well as modeling social networks and demographics. The main finding seems to be this: distance is not the only factor that matters in looking at trips between cities. As the case of the super-commuters suggests, people will live one place and work in another place far away in the right circumstances. Perhaps we should have already known this because of the relative importance of different cities: world-class cities or cultural centers or centers for certain industries (New York City, Los Angeles, and San Francisco, respectively) would draw people from longer distances compared to “average” big cities (St. Louis? Denver?). Or, if we put this in world systems theory terms, certain cities sit at the center of American urban life and businesses and industries tend to concentrate within them while other cities are in more peripheral positions.

I would be interested to know whether the “radiation model” can suggest whether the number of super commuters will increase in the long-term and how this is affected by the strength of the overall economy and housing market.