| Traffic congestion has increased significantly in urban areas over the past several decades and is associated with significant environmental and health impacts. This research characterizes air pollutant emissions, exposures and health risks due to traffic, particularly when congestion is present. It examines key factors affected by congestion, including time allocation patterns, vehicle emissions, and near-road exposures.;Congestion alters time allocation patterns of commuters since more time is spent in traffic, and thus less time must be spent elsewhere. Time allocation shifts between time spent in a vehicle and other microenvironments were derived using the National Human Activity Pattern Survey and robust regression techniques. Congestion primarily reduced the time spent at home, especially for children and retirees.;Vehicle emissions occurring during traffic congestion, especially in work zones, have received little attention. A field study was conducted to collect data on speed-acceleration profiles in work zone, rush hour and free-flow conditions, and a power demand-based emission model was used to simulate emissions. Acceleration and deceleration significantly increased emission rates. Emission rates differed from those based on average speed, and depended on vehicle type and congestion condition.;Statistical and process-based estimates of traffic impacts on near-road air quality were derived using generalized additive models, the Motor Vehicle Emissions Factor Model 6.2 (MOBILE6.2), and the California Line Source Dispersion Model. The simulation model performed reasonably well for carbon monoxide (CO), but significantly underestimated PM2.5 (particulate matter less than 2.5 mum in diameter) concentrations, a likely result of underestimating PM2.5 emission factors.;An approach was developed to identify pollutant exposures and health risks associated with traffic congestion. Scenarios for arterial roads and freeways suggest that air pollution and health impacts attributable to congestion are significant, although limitations in the information and models available lead to large uncertainties, particularly with respect to estimating the emissions that are attributable to congestion and the dose-response relationships.;This study highlights the importance of accounting for changes in time allocations, vehicle emissions, and exposures due to traffic congestion. The research results are applicable to air quality, exposure and health risk assessments, as well as transportation planning. |
