High-occupancy toll (HOT) lanes were recently proposed for I-85 in Atlanta, Georgia, as a way to relieve congestion and to provide a reliable commute time for single-occupant drivers who are willing to pay a toll. It is important to evaluate the air quality impacts of such a proposal in the context of environmental regulations, such as the National Environmental Policy Act (NEPA), and transportation conformity regulations. Several factors affect mass vehicle emissions, such as vehicle activity, speed, age distributions, and class distributions. These factors are incorporated into a base scenario that models the current conditions on I-85 with high-occupancy vehicle lanes by using data collected in the corridor during the summer of 2007, and a future scenario that models the implementation of HOT lanes on this corridor by using information from cities that already have HOT lanes. The MOBILE-Matrix modeling tool, recently developed by the Georgia Institute of Technology, is used for the emissions analysis by means of the input factors described above. It calculates mass emissions for five pollutants: hydrocarbons (HC), oxides of nitrogen (NOx), carbon monoxide (CO), particulate matter with an aerodynamic diameter of 2.5 ‚µm or smaller (PM2.5), and PM10 (with an aerodynamic diameter of 10 ‚µm or smaller) as a function of fleet composition and on-road operating conditions. The modeling work predicts extremely small increases in mass emissions for NOx, CO, PM2.5, and PM10, and an extremely small decrease in mass emissions for HC. In addition, the postimplementation emissions changes fall well within the motor vehicle emissions budgets for the facility that are used in air quality planning. Therefore, implementation of HOT lanes on I-85 in Atlanta should not violate the emissions budget requirements of the federal Transportation Conformity Rule. For NEPA purposes, this analysis could be used to make the case that air quality impacts are not significant, and therefore further detailed analyses are not required.