Jam today

CITIES are the world’s economic engines—and the bigger they are, the better. Middle-aged ones like London and New York are booming, and would be doing even better if they were not constrained by green belts, zoning and other NIMBYism. But the real giants are in the emerging world. Fast-growing metropolises like Lagos and Manila, with populations of more than 15m, perform an amazing alchemy by turning poor rural migrants into better-educated, wealthier urbanites. Unfortunately, these engines can barely run.