SUSTAINABLE PLANNING AND DESIGN HAS A POSITIVE COST BENEFIT - Non-sustainable development assumes that it is too expensive to factor in the impacts of development on the natural environment and the livability of the overall community. Experience in the development of planned communities such as Reston, Virginia and the sustainable re-development of downtowns such as Portland, Oregon shows, however, that property values go up with sustainable development. People and businesses eventually flee unsustainable communities. Also, experience shows that ultimately unsustainable development costs more. Unsustainable buildings become out-dated as energy costs rise while residents in cities that have allowed unlimited sprawl pay more for traffic congestion and the costs of commuting. Ultimately, air pollution regulations might prevent all new development in cities that have air pollution levels that exceed standards, preventing the economic growth of such cities.