3D: Air quality advisories may not have the desired impact on reducing vehicle emissions

Using data on daily vehicle trips, PM_2.5 concentrations, and a host of climactic control variables, USU research tested the hypothesis that “yellow air day advisories” issued by the Utah Division of Air Quality resulted in drivers reducing the number of vehicle trips taken during northern Utah’s winter-inversion seasons in the early 2000s. Winter inversions occur in northern Utah when PM_2.5 concentrations (derived mainly from vehicle emissions) become trapped in the lower atmosphere, leading to unhealthy air quality over a span of time known colloquially as “red air day episodes.” When concentrations rose above 15 µg/m³ toward the National Ambient Air Quality Standard (NAAQS) average daily threshold of 35 µg/m³, residents were informed via different media sources and road signage that the region was experiencing a yellow air day. Residents were urged to reduce their vehicle usage during the day.

Results from this research suggest that the advisories provided at best weak, at worst perverse, incentives for reducing vehicle usage on yellow air days, and ultimately for mitigating the occurrence of red air day episodes during northern Utah’s winter inversion seasons. A perverse incentive occurs when individuals react to an advisory by increasing their vehicle trips (and thus reducing their outdoor exposure to particulate pollution) in order to better protect themselves from the elevated PM_2.5 concentrations. Thus, “soft policies” such as issuing advisories are not, in and of themselves, sufficient to control vehicle emissions’ impacts on air quality during inversions in northern Utah.