Abstract:In most areas of California, motor vehicles continue to produce significant amounts of emissions that result in photochemical smog. The smog problem is exacerbated in the South Coast Air Basin (which includes Orange County and major portions of Los Angeles, Riverside, and San Bernardino Counties), a result of local topography and weather conditions. The ozone problem of the South Coast Air Basin is so severe that experts estimate emissions of hydrocarbons must be cut by as much as 82% to meet the national ambient ozone standards. Although air pollution levels are not as severe in other areas in Calfiornia, almost every urban area is currently violating, or close to violating, ambient air quality standards for ozone, nitrogen oxides, or particulate matter. Heavy-duty vehicles are significant contributors to the emission inventory in urban aeas. The history of truck-related transportation control measures, current transportation control measures under consideration in California, and uncertain effects of proposed measures are reviewed.