There exists a category of American cities in which the line between suburban and urban is almost impossible to locate. These suburban cities arose in the last half of twentieth-century America, based largely on the success of the single-family home, shopping centers, and the automobile. The low-density, auto-centric development of suburban cities, which are largely in the arid West, presents challenges for urban sustainability as it is traditionally measured. Yet, some of these cities—Los Angeles, Las Vegas, Phoenix, Salt Lake, Dallas, Tucson, San Bernardino, and San Diego—continue to be among the fastest growing places in the United States.