In New York City, we like to think of our subway system as fairly unique. We have no true circle line, and our Outer Borough tendrils snake throughout the city. The amalgamation of old rights-of-ways and three independent builders, the current subway system arose through fate, fortune and good luck. Or so the story goes.
Two pieces making the rounds covering the same academic journal article beg to differ. As both Scientific American and Wired noted today, two scientists using two-dimensional spatial network analysis have determined general rules for any subway system. It’s interesting research, but it’s hardly groundbreaking to those who know urban planning.
Sarah Fecht of Scientific American summarized the “rules”:
First, subway networks can be divided into a core and branches, like a spider with many legs. The “core” typically sits beneath the city’s center, and its stations usually form a ring shape. The branches, which are more linear, extend outward from the core in many directions.
Second, the branches tend to be about twice as long as the width of the core. The wider the core, the longer the branches. And subway systems with more stations tend to have more branches. The number of branches corresponds roughly with the square root of the number of stations.
Last, an average of 20 percent of the stations in the core link two or more subway lines, allowing people to make transfers. [Physicist Marc] Barthelemy says his team does not know which factors are guiding subway networks to follow these general rules; perhaps the rules maximize efficiency. For example, too many branches or connections would be redundant and unnecessarily costly. In contrast, having too few branches would reduce the range of areas that the network services, and having too few connecting points would reduce travel efficiency.
Fecht notes, as an example, that New York City doesn’t have a clearly defined ring core, but even on that point, I beg to differ. It’s a four-train loop that isn’t particularly efficient, but the area constrained by the two IRT lines on the East and West Sides, the L along 14th St. and any of the 53rd, 59th or 42nd St. lines forms a loop around the city’s major work center. Lower Manhattan throws a wrench into this ring, but that’s what makes the city’s subway system works.
That’s neither here nor there though. New York’s subway system still fits under these rules as we have a clear core with branches feeding into that core. The branches are significantly longer than the core itself, and depending upon how you define the core, many of them offer multiple transfer points. Check, check, and check.
Of course, as interesting as this summary is, we’re not really learning anything new here. Subway systems exist to offer a cost-effective and relatively quick way to bring urban dwellers from mostly residential areas into and through the commercial centers. As the commercial centers generally form a core surrounded by other neighborhoods, a subway system will always follow this shape. That’s the whole point.
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