Missing the Warning Signs? The Case of “Yellow Air Day” Advisories in Northern Utah

Using a dataset consisting of daily vehicle trips, $$PM_{2.5}$$ P M 2.5 concentrations, and a host of climactic control variables, we test the hypothesis that “yellow air day advisories” issued by the Utah Division of Air Quality resulted in subsequent reductions in vehicle trips taken during northern Utah’s winter-inversion seasons in the early 2000 s. Winter inversions occur in northern Utah when $$PM_{2.5}$$ P M 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 rise above 15 $$\upmu \textrm{g}/\textrm{m}^3$$ μ g / m 3 toward the National Ambient Air Quality Standard average daily threshold of 35 $$\upmu \textrm{g}/\textrm{m}^3$$ μ g / m 3 , residents are informed via different media sources and road signage that the region is experiencing a yellow air day, and are urged to reduce their vehicle usage during the day. Our results suggest that the advisories have 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.