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Jonathan English explains the Marchetti Constant:
In 1994, Cesare Marchetti, an Italian physicist, described an idea that has come to be known as the Marchetti Constant [pdf]. In general, he declared, people have always been willing to commute for about a half-hour, one way, from their homes each day.
This principle has profound implications for urban life. The value of land is governed by its accessibility—which is to say, by the reasonable speed of transport to reach it.
English also takes a tour of urban history for examples of how the Marchetti Constant has played out, and how cities have changed as transportation technology has changed and enabled longer commutes. The pattern has repeated, writes English, over and over again, from Ancient Rome to Paris to London to Chicago and, finally, Atlanta.