For the first time in human civilization, more people now live in urban areas than in the countryside. This shift creates a number of dilemmas, not least of which is how to move people within the world’s rapidly growing metropolises. Pollution and traffic point away from car-based options, while light-rail systems are slow to construct and prohibitively expensive. One disarmingly simple—and cheap—possibility is Bus Rapid Transit, which is engineered to operate like a subway on wheels. In these systems, concrete dividers on existing roads separate high-capacity buses from the rest of traffic. Riders pay before boarding, then wait in enclosed stations. When a bus arrives, sliding partitions open to allow riders to board from a platform that is level with the bus floor. The traffic-free thoroughfares, quick boarding times, and modern, comfortable stations resemble light-rail systems more than the chaos of typical bus travel. In Bogotá, Colombia, which has had seven Bus Rapid Transit lines in operation since 2001 (right), the buses handle 1.6 million trips a day. Its success has allowed the city to remove 7,000 private buses from the city, reducing consumption of bus fuel and its associated pollution by more than 59 percent.