Feature

These researchers have a novel way to classify every city

Their abstract model says cities are either like Buenos Aires, Athens, New Orleans, or Mogadishu

Cities touch you, each in their own special way. Walk its streets, and Seattle feels different than Berlin or Johannesburg or Tokyo. Each has its own fingerprint.

Still, those fingerprints have just four types, exemplified by Buenos Aires, Athens, New Orleans, and Mogadishu, argue researchers Rémi Louf and Marc Barthelemy.

Louf and Barthelemy trained in physics but have an ongoing interest in how one part of a city's core infrastructure — its streets — evolves and how that evolution relates to, say, where people live in relation to work. It's part of an emerging science of cities aimed at understanding how an urban environment's physical, social, and economic networks evolve over time, though much of the current research is somewhat abstract. In a typical model, street networks are just that — abstract representations that could be Paris or a map of the brain. But real streets are grounded in real cities, which take on attributes like size and shape. How should those factors be taken into account? How could different street plans change the way cities work?

No one's quite prepared to answer those questions yet, but Louf and Barthelemy decided to take a first step by at least categorizing what was out there in the maps of the 131 cities on six continents. Rather than study the street network itself, they considered the shapes and sizes of the blocks that streets form. Following academic geographers' lead, they computed shape factors: the ratio of a block's area to that of the smallest circle that could fit around it. Then, Louf and Barthelemy categorized cities based on their blocks' distribution of size and shape.

The analysis revealed four main city fingerprints. The largest group, comprising 102 cities, were New Orleans-like cities with the largest city blocks in a wide range of shapes. Athens-like cities made up another 27 cities and contained generally smaller blocks — less than about 10,000 square meters, or, for a square block, about 300 feet on a side — but a wide variety of shapes. Only two cities remained: Buenos Aires, with medium-sized square and rectangular blocks, and Mogadishu, which features almost entirely small, square blocks.

Interestingly, every major city the team looked at in the United States and Europe fell into the NOLA category except Vancouver and Athens. Europe and the U.S. have their own particular subtypes, but a few U.S. cities, including Portland, Oregon, and Washington, D.C., fall more into the European mold.

Those sorts of differences, the authors suggest, could be used to better understand how cities are born and evolve. Uniform block sizes, such as those found in New York, "could be the result of planning" while a city like Paris reflects a continual process of building and rebuilding that produces a range of block shapes and sizes.

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