The right kind of sprawl

EVERY year the world’s urban population swells by about 75m people. That extraordinary growth—equivalent to adding eight Londons—is a wonderful thing. Cities throw people together, encouraging the exchange of ideas. The people who move there tend to grow richer, freer and more tolerant. What is rather less wonderful is the way in which many of the world’s fastest-growing cities are expanding.The trouble is not, as is often claimed, that cities in poor and middle-income countries are spreading like oil slicks. Most of them need to expand. Western cities can often accommodate their growing populations by squeezing more people in. But many poor cities are incredibly dense already: Dhaka, the capital of Bangladesh, is nine times as tightly packed as Paris, if you include their suburbs. And no Western city has ever added inhabitants as quickly as the poor and emerging-world champions are doing. African and Asian metropolises are bound to sprawl even if sensible pro-density reforms are passed, such as scrapping height restrictions on buildings.