According to Redfin, several American cities – some the usual progressive suspects, but others quite surprising – are making moves to build more homes in walkable neighborhoods. Other, however, are stuck in the past, building more of the distant suburbs.Why do we need more walkable cities? Quite simply because walkable cities are, by definition, sustainable cities. Transportation remains a major source of greenhouse gas pollution, and, unlike electricity or agriculture, the United States remains firmly stuck on a fossil-fuel dependent transport infrastructure. When we live in spread out suburbs, far from work, shopping, schools, and cultural centers, we have to drive. Often, we drive inefficient, single-occupancy vehicles, burning more fossil fuels, and creating more traffic.