Of course, local factors may cause some deviations from the rules. New York City’s subway system does not have a clearly defined ring-shaped core; Barthelemy thinks the reason may be that the core’s shape is constrained by the proximity of the East and Hudson rivers which surround the city center—the long, but narrow island of Manhattan.
The Moscow Metro is also something of an outlier, in that its branches are three times longer than the core, and 50 percent of its core stations connect multiple trains. Levinson chalks up these differences to strong Soviet-era central planning; in contrast, many of the other cities' networks were built by private companies in piecemeal fashion.
Nevertheless, Barthelemy contends that if local factors posed the most powerful influence on the shape of subway networks, then the team wouldn’t have discovered these universal rules. “To me, it means that there is some general mechanism that overcomes local details,” he says.
But, “the question is, how closely do they follow these rules?” Levinson asks. “If every city’s subway network had exactly the same features, it’d be obvious the rules were really strong.” Because they’re not all the same, it shows that local details also help to shape the network. Levinson says that it is also important to understand the source of those differences.
Barthelemy thinks the rules are universal, but are adapted to different environments. In future experiments, he plans to plug the newly discovered rules into simulations, applying them in different geographic contexts. If the rules are correct, he says, then simulating the growth of a network around a lake will produce a network similar to Chicago’s.
Understanding how subway networks grow and evolve might one day help to design better systems, Levinson says.