If you've ever stood in front of a map of the New York subway system trying to figure out how to get from point A to point B, you know it's no easy task. In fact, new research shows, it might be more than our brains can handle.
"The number of 'megacities'—urban areas in which the human population is larger than 10 million—has tripled since 1990," write complex systems researchers Riccardo Galloti, Mason Porter, and Marc Barthelemy in Science Advances. The growth of those cities usually involves expanding public transportation systems. "This leads to a natural question: Is navigating transportation systems in very large cities too difficult for humans?"
In other words, could subway systems like the New York Subway or the Paris Metro exceed our brains' cognitive limits? The existence of such limits is nothing new, of course—famously, we can keep about seven things in our heads at once, and we can maintain about 150 stable social relationships. What Galloti, Porter, and Barthelemy wanted to know, essentially, is whether the growth of urban environments is starting to bump up against similar barriers.
"Is navigating transportation systems in very large cities too difficult for humans?"
For answers, the researchers first turned to maps of the world's 15 largest transit systems, including those in New York, Paris, and Tokyo. Then, they asked how much information it would take to solve the following problem: Given a starting point on a particular subway line, find the fastest route to a given destination on a different line, with no more than two stops to switch trains in between—that is, the fastest route between two points that's still cognitively manageable. (That problem derives from the fact that we can keep only three or four points on a map in our heads at once; in this case, starting and ending points, plus one or two transfer points.)
Across all 15 transit systems, the worst-case-scenario answer to that problem (from the bizarrely convoluted New York subway, incidentally) is about eight bits, or one byte of information, corresponding to a search of about 250 connections linking A to B—and, notably, less than a billionth of the information stored on a typical smartphone. Beyond that, you'd probably need an actual smartphone to navigate efficiently.
Good thing for iPhones, then. Galloti, Porter, and Barthelemy also calculated how much information it would take to find the quickest routes between two places, regardless of the number of transfers. They found that, for most trips in the majority of the largest 15 transit systems, the information needed to compute the fastest route far exceeds the eight-bit limit, a problem compounded by the fact that many urban transit systems consist not just of subways but also buses, commuter trains, and other modes of public transportation.
"Human cognitive capacity is limited, and cities and their transportation networks have grown to ... a level of complexity that is beyond humans' processing capability to navigate in them," the team writes.
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