Trends in Aggregate Vehicle Emissions: Do We Need To Emissions Test?

Vehicle emissions are falling. As the oldest vehicles in the fleet are scrapped and are replaced by cleaner vehicles, aggregate emissions decline. Given this trend, must costly used car regulation continue? The Clean Air Act of 1990 requires more stringent used car testing without considering the counter-factual of how aggregate emissions would evolve in the absence of more regulation. This paper use data on vehicle scrappage rates, vehicle emissions by model year, and county air quality levels in counties that do and do not emissions test to explore the past benefits of vehicle emissions testing and to extrapolate future emissions if regulation were not increased.