A New Law of Intercity Mobility

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.