Traffic Congestion and Air Pollution Exposure for Motorists: Comparing Exposure Duration and Intensity

Abstract

This article investigates the effects of congested freeway traffic conditions on motorists’ exposure to traffic-related air pollution using real-world traffic data and a framework of established emissions and dispersion models. The intent is to isolate and compare the influences of congested traffic characteristics on exposure duration and exposure intensity. Mass inhalation of carbon monoxide (CO) and nitrogen oxides (NOx) is estimated for 45,226 simulated trips through a 14-mile congested freeway corridor. Results show that congestion increases total trip pollutant inhalation primarily through motorist delay (exposure duration), and to a lesser extent through increased concentrations (exposure intensity). The effects of varying wind and background concentration can be sufficient to obscure the influence of congestion on exposure intensity. The variability of exposure intensity due to traffic is mitigated by offsetting impacts among traffic flow, emissions rates, and pollutant dispersion. Exposure intensity increases with higher traffic flow and with lower traffic speed, but the impact of lower traffic speeds (through increased emissions rates and decreased dispersion) is smaller. The importance of exposure “hot spots” at traffic bottlenecks also increases in congestion. These findings suggest that traffic-based motorist exposure mitigation should focus on reducing travel duration on high-volume corridors through reduced vehicle flows (i.e., demand-side congestion mitigation). This analysis does not include non–congestion-based mitigation strategies such as cleaner vehicle engine technology or improvements in vehicle cabins, which can also reduce exposure intensity. On an individual scale, motorists can greatly reduce their own exposure during travel by, among other strategies, adjusting their departure time to less congested, lower volume periods.