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Commuter data helps scientists define metropolitan boundaries

Bay area morning traffic backs up before the 880/580 split near Ashby Avenue in Berkeley, California in 2007. File Photo by Aaron Kehoe/UPI
Bay area morning traffic backs up before the 880/580 split near Ashby Avenue in Berkeley, California in 2007. File Photo by Aaron Kehoe/UPI | License Photo

May 1 (UPI) -- Researchers have developed a new method for identifying the confines of metropolitan regions using commuter data.

Cities have jurisdictional boundaries -- undisputed demarcation lines. But while police and policy makers may pay attention to these lines, often they demand little attention from residents.

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The true boundaries of most cities are more nebulous, and they often extend tens of miles beyond the city center. These boundaries aren't meaningless. They can influence the allocation of housing subsidies or funds for infrastructure projects.

Demographers and geographers sometimes use shifts in population density or the boundaries of media markets to define metropolitan regions, but these techniques can offer questionable results.

"Judgments about boundaries are difficult to account for in analytic projects that seek to explain economic growth patterns, productivity gains and city size distribution laws," scientists wrote in a new paper on the subject, published this week in the journal PLOS One. "They are also particularly problematic when governmental assistance programs rely on statistical information conditioned by the boundaries."

To better estimate the confines of a metropolitan region, researchers looked to census commuter data. Using methodologies from network science, researchers measured the connections between the 3,091 counties in the contiguous United States by tallying the number of commuters who cross county lines. Scientists also measured the levels of commuting within each county.

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The study showed 90 percent of commuters are concentrated in 182 different county clusters. Only 14 of the clusters featured a central node counter to which a large percentage of commuters commuted. Some 78 clusters lacked a strong central node.

These maps show a comparison of the metropolitan areas of the New York City region, major Texas cities and Minneapolis, on the left, with their associated commuter communities, on the right. Photo by Mark He et al, 2020/PLOS One

Not surprisingly, Los Angeles County was one of 90 counties in the United States that features especially large volumes of inner-county commuting. Twenty of the clusters identified by the new analysis consisted of more than 50 counties. Several of these clusters, mostly surrounding large cities, spanned across state lines.

Most of the 182 clusters identified by scientists featured more expansive boundaries than the boundaries of corresponding metropolitan regions established by traditional methods.

"The results suggest that traditional regional delineations that rely on ad hoc thresholds do not account for important and pervasive connections that extend far beyond expected metropolitan boundaries or megaregions," scientists wrote in their paper.

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