From ancient Rome to modern Atlanta, the shape of cities has been defined by the technologies that allow commuters to get to work in about 30 minutes.
In 1994, Cesare Marchetti, an Italian physicist, described an idea that has come to be known as the Marchetti Constant. 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.