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.

The city on foot: 800 BC-1700 AD

Until the Industrial Revolution, there was pretty much only one way for most people on dry land to get around: on foot. With services concentrated in the center of cities, the radius of development from the heart of the city was limited to not much more than one mile—about the distance a person can walk in 30 minutes.

In 1830, British civil engineer George Stephenson opened the world’s first steam-powered public railway between Manchester and Liverpool. Soon, railways focused as much on proto-commuters as they did on intercity travelers. The London and Greenwich railway opened in 1836, and new lines quickly began radiating out of central London. Cities across Europe and the United States soon followed.

fter World War II, two dominant urban planning ideas sought to solve what was seen as the great urban problem: overcrowding.