Do air quality alerts reduce traffic? An analysis of traffic data from the Salt Lake City metropolitan area, Utah, USA

Air quality alert programs are a form of "soft" or persuasion-based policy that is common in metropolitan areas that do not meet federal air quality standards in the United States. These programs disseminate daily air quality conditions, with ordinal ratings describing the health implications and suggesting reductions in automobile use and other actions that contribute to the poor air quality. Evidence regarding the effectiveness of air quality alert programs on driving is less encouraging than other soft policies to discourage driving, with many studies reporting small or no reductions in traffic. This study examines evidence for the effectiveness of air quality alert systems in reducing traffic over a 10-year period in Salt Lake and Davis counties, Utah, USA, a metropolitan area that often does not meet US federal air quality standards for both ground-level ozone in the summer and PM 2.5 pollution in the winter. We find that while air quality alerts have some effectiveness for reducing traffic in the center city, these small reductions are exceeded by larger increases in traffic near the edge of the metropolitan area. These effects are stronger during the PM 2.5 alert season than during the ozone alert season. These increases can be explained as discretionary trips by individuals escaping poor air quality by driving to the mountains. A policy implication is that soft policies alone may not be effective at reducing driving behavior when the public health implication of the message conflicts with its public responsibility implication.