Major efforts to control motor vehicle emissions have been made in recent years, both through improved emission control technologies and through gasoline reformulation. Our assessment of the impacts of these efforts was conducted in the San Francisco Bay Area, in lanes of a highway tunnel where heavy-duty vehicles are not allowed. This study focuses on the afternoon rush hour, during which over 4000 vehicles per hour travel uphill through the tunnel. Concentrations of CO, CO2, NOx, and total and speciated non-methane organic compounds (NMOC) have been measured during summers 1994-1997, 1999, and 2001. Emission factors for CO, NMOC, and NOx decreased by factors of 2-3 over the 7-year period between 1994 and 2001, with CO and NMOC showing greater percentage reductions than NOx. From our data, fleet turnover appears to have a greater overall impact on exhaust emissions than fuel changes for most pollutants. However, emissions of benzene have been greatly affected by changes in fuel composition.