![]() In the present analysis, “congestion-related” impacts incorporate multiple interactions that occur with congestion. However, traffic volume is treated as a continuous variable, and strict definitions of congestion are not needed. (Other definitions use a speed threshold, a percentage of free-flow speed of a roadway, or other indicator.) The present study focuses on what might be called “recurring congestion,” specifically, congestion caused by high traffic volumes during weekday peak “rush hour” periods. While there are many definitions, congestion is often defined as periods when traffic volume exceeds road capacity. This is not necessarily true during congestion, the second category considered. As a result, the marginal impact of an additional vehicle is equal to the average impact of the vehicle fleet. In this case, each additional vehicle added to the road does not substantially alter traffic patterns, e.g., the speed and travel time of other vehicles are unaffected, and thus vehicle emission factors do not depend on traffic volume. First, “congestion-free” impacts refer to impacts of traffic at volumes below the level that produces significant congestion. It is useful to separate traffic-associated pollutant impacts and risks into two categories. These emissions contribute to risks of morbidity and mortality for drivers, commuters and individuals living near roadways, as shown by epidemiological studies, evaluations of proposed vehicle emission standards, and environmental impact assessments for specific road projects ( World Health Organization (WHO), 2005 Health Effects Institute (HEI), 2010). The increasing severity and duration of traffic congestion have the potential to greatly increase pollutant emissions and to degrade air quality, particularly near large roadways. In many areas, vehicle emissions have become the dominant source of air pollutants, including carbon monoxide (CO), carbon dioxide (CO 2), volatile organic compounds (VOCs) or hydrocarbons (HCs), nitrogen oxides (NO x), and particulate matter (PM) ( Transportation Research Board (TRB), 2002). and elsewhere over the past 20 years ( Schrank and Lomax, 2007). Traffic on roads has significantly increased in the U.S. Further, evaluations of risk associated with congestion must consider travel time, the duration of rush-hour, congestion-specific emission estimates, and uncertainties. This study suggests that health risks from congestion are potentially significant, and that additional traffic can significantly increase risks, depending on the type of road and other factors. These patterns result from changes in emission factors, the NO 2–NO x relationship, the travel delay for the on-road population, and the extended duration of rush hour for the near-road population. For an arterial road, incremental risks increased sharply for both on- and near-road populations as traffic increased. For a freeway, “U” shaped trends of incremental risks were predicted for on-road populations, and incremental risks are flat at low traffic volumes for near-road populations. ![]() ![]() An incremental analysis, which expresses the change in health risks for small increases in traffic volume, showed non-linear effects. ![]() The modeling used emission factors from two different models (Comprehensive Modal Emissions Model and Motor Vehicle Emissions Factor Model version 6.2), an empirical traffic speed–volume relationship, the California Line Source Dispersion Model, an empirical NO 2–NO x relationship, estimated travel time changes during congestion, and concentration–response relationships from the literature, which give emergency doctor visits, hospital admissions and mortality attributed to NO 2 exposure. Simulation modeling was used to estimate on- and near-road NO 2 concentrations and health risks for freeway and arterial scenarios attributable to traffic for different traffic volumes during rush hour periods. ![]() This study demonstrates an approach to characterize risks of traffic for on- and near-road populations. Presently, our understanding of the air pollution impacts from congestion on roads is very limited. Traffic congestion increases vehicle emissions and degrades ambient air quality, and recent studies have shown excess morbidity and mortality for drivers, commuters and individuals living near major roadways. ![]()
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