New game-like computer software is empowering ordinary citizens to help design better cities. Can the professionals and the public learn to play well together? ___ FIFTEEN YEARS AGO, the future of urban planning arrived in the form of a wonkish but strangely addictive new computer game. In SimCity, a player assumed the twin roles of mayor and city planner, creating elaborate cityscapes, managing zoning, transportation, and growth, while fighting off poverty, crime, traffic, and pollution. SimCity went on to become the best-selling game title in history, but its reach has extended far beyond the realm of ordinary gameplay. As Princeton...