The University of Denver used its remote sensor for motor vehicle CO and HC emissions to measure the tailpipe concentrations of over 90,000 California Vehicles in a 30-day period in 1991. The study consisted of three phases; a series of controlled tests, a pullover study of high-emitters, and a series of measurements at a variety of sites around the South Coast Air Basin and northern California. The highest CO emissions occurred under hard accelerations, while the highest HC emissions occurred during decelarations. In the pullover study, over 92% of the vehicles identified as high emitters failed the roadside inspection, equivalent to a California Smog Check. More than 60% of the vehicles stopped had defective emission control equipment (over 40% were tampered). The highest emitting vehicles showed the most variability in their emissions. This variability carries implications for the design of inspection and maintenance programs.