Twenty-five years ago, physicist Cesare Marchetti argued that people, on average, tend to keep their commutes to about an hour a day, round-trip. For Citylab, Jonathan English looks at how this inclination has interacted with advances in transportation to affect how cities grow and evolve. For instance, walking and travel by horse kept cities to an effective diameter of a few miles, allowing their density to grow over many centuries.

Sure enough, most cities from the ancients to the Industrial Revolution did not grow much bigger than a two-mile diameter. Their core areas were often even smaller, though some of the poor lived in settlements outside the city gates. Ancient Rome packed as many as a million people into an area a little more than two miles in diameter. Medieval Paris stretched about two miles from the Bastille to the Louvre, Vienna’s Innere Stadt measures only one mile in diameter, and the historic City of London is nicknamed the “Square Mile” for a reason. Beijing’s walls enclosed an inner city about three miles in diameter; into the 20th century, that still made up most of the developed area.

Rail, streetcars, bicycles, subways, and cars followed, each increasing the amount of distance from a city’s center that could be reached within Marchetti’s time limit.

The car on the expressway enabled large numbers of people to travel long distances on a day-to-day basis. Instead of small railroad suburbs, where housing was restricted to a short radius around stations, drivers spread out across suburbs could now commute 20 miles in 30 minutes. If the streetcar city covered 50 square miles, the 40-mile-diameter expressway city could cover over 1,250 square miles.*

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