Can traffic management strategies improve urban air quality? A review of the evidence

This paper reviews the effectiveness of traffic management strategies (TMS) for mitigating emissions, ambient concentrations, human exposure, and health effects of traffic-related air pollution in urban areas. The objective is to summarize the evidence base for a range of moderate-scale strategies broadly relevant to municipal and regional government decision-making. A systematic literature search was carried out to identify empirical studies of TMS effects on emissions, air quality, exposure, or health. Identified studies were reviewed to assess the state of evidence that TMS can improve urban air quality and pollution-related health outcomes for exposed populations. Overall, the evidence base is weak for these effects. There is limited evidence of effects on emissions for 7 of the 22 studied strategies, and limited evidence of effects on air quality for 2 of the strategies: area road pricing and low emission zones. Insufficient evidence exists for all other TMS and effects. Existing evidence suggests that aggressive area-based TMS such as low emission zones are needed to generate substantial air quality benefits, and that TMS must be implemented with care to avoid unintended detrimental and rebound effects. The evidence base is limited by a lack of ex post evaluations of implemented strategies, lack of evaluation of exposure and health impacts, small intervention effects relative to the influences of other factors, and insufficient evaluation of spillover and indirect effects. Evolving vehicle fleets add further uncertainty to the long-range effects of TMS on air quality. Effects of TMS on measured population exposure and public health outcomes have not been well-studied. An evidence-based approach to transportation systems planning necessitates additional resource allocation to ex post evaluations and performance monitoring for air quality impacts of traffic management strategies.