Are All of the World’s Subway Systems Starting to Look the Same?

A statistical analysis of the world's biggest subway networks shows they've all evolved in surprisingly similar ways.

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What do the world’s largest subway systems have in common? And if you answer “guys drumming on buckets for money,” you’re wrong.

According to a recent Journal of the Royal Society Interface paper, a study of the world’s 12 largest subway networks shows that over time, each appear to be converging on one basic shape, despite large economic and geographic differences.

As the report states, this fact suggests “the existence of dominant, universal mechanisms governing the evolution of these structures.”

Statistical Physicist Marc Barthelemy of France’s Center for Scientific Research told that this convergence is a sign that there are some basic, profound similarities in how urban systems evolve.

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In the report, Barthelemy and NCSR complex systems analyst Camille Roth focused on the subways of Barcelona, Beijing, Chicago, London, Madrid, Mexico, Moscow, New York City, Osaka, Paris, Seoul and Tokyo.

The team utilized equations used to study two-dimensional spatial networks (which apparently subway maps fall under) and repeated the analyses with data from each decade of the subways systems’ existence. They discovered several major patterns that repeated across all networks:

  • Core-and-branch topology, which essentially shows a central network of subway lines (core) with branching tracks that extend further outside that center.
  • Roughly half of a given system’s subway stations will be found on its outer branches rather than in the core.
  • The distance from a city’s center to its farthest terminus station is  equal to the diameter of the subway system’s core.

Subway systems seem to gravitate towards this arrangement organically, Barthelemy told Wired, through a combination of planning, expedience, circumstance and socioeconomic fluctuation.

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The researchers’ ultimate goal is to build a model of subway systems’ evolution based on real-world observations. Such a model would help for optimal creation and modifications of future transportation systems.

“We don’t have big ideas,” Barthelemy said to “We’re just in the process of trying to understand centuries of development.”

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