Stewart Brand writes about the importance of cities and densities in the developing world. He goes on to stress the importance of density in rich nations, as well. There are lots of great quotes and points made, but this is my favorite bit:

Alleyways in squatter cities, for example, are a dense interplay of retail and services—one-chair barbershops and three-seat bars interspersed with the clothes racks and fruit tables. One proposal is to use these as a model for shopping areas. “Allow the informal sector to take over downtown areas after 6pm,” suggests Jaime Lerner, the former mayor of Curitiba, Brazil. “That will inject life into the city.”