and

Growing cities, featuring more people with higher incomes who live and work in the suburbs and do not commute by public transit should be a recipe for increased air pollution. Instead, California’s major polluted urban areas have experienced sharp reductions in air pollution. Technological advance has helped to “green ” the average in fleet vehicle. Such quality effects have offset the rising quantity of miles driven. This paper uses several data sets to investigate how California’s major cities have enjoyed environmental gains over the last 20 years despite ongoing growth.