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.
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7 Comments
Add CommentI knew there was something odd about Toronto's network...
Reply | Report Abuse | Link to thisI was very leery of this report until the authors provided a way to falsify their conclusions. I hope a followup article here reports their hypothesis' survival.
Reply | Report Abuse | Link to thisI suspect that the topological similarities they find are due to inevitable similarities in the way cities (that need subways) evolve into similar topologies based on the way humans use cities, modulo local geography.
To me, this is pseudoscience...sort of like observing that most flying birds have wings. It's an emergent characteristic of birds.
Reply | Report Abuse | Link to this"No two subway systems have the same design."
Reply | Report Abuse | Link to thisThey're SNOWFLAKES!!!
"Understanding how subway networks grow and evolve might one day help to design better systems....."
Reply | Report Abuse | Link to thisYeah ..... and it might not...
Anyway it looks like all the action is now going on above ground with light railways -- take a look at the London Docklands--
I don't find the conclusions and their explanations convincing. A single sample from a random set can appear to be deterministic. There is no mention of the time at which any subway began to take shape or the economic circumstance at the time, etc. . The author (B.) jumped the gun in submitting.
Reply | Report Abuse | Link to this"And all the world's megacities—those with populations of 10 million or more—have subway systems."
Reply | Report Abuse | Link to thisFalse.
Dhaka does not yet have one: http://en.wikipedia.org/wiki/Dhaka#Transport
Jakarta does not yet have one: http://en.wikipedia.org/wiki/Jakarta_Mass_Rapid_Transit
Karachi does not yet have one: http://en.wikipedia.org/wiki/Karachi#Rail
Lagos does not yet have one: http://en.wikipedia.org/wiki/Lagos#Rail