Slower roads, cleaner skies? causal effects of speed limit reduction on urban air quality using propensity score matching and spatial difference-in-differences regression

Vehicle speed limit reduction policies have been widely studied for their impact on traffic safety outcomes, such as reducing crash frequency and severity. However, little attention has been paid to the effects on air quality that may have resulted from traffic volume changes induced by speed reductions. This study aims to assess the causal impact of the speed limit reduction policy in the Republic of Korea, “5030 Speed Limit Reduction"- urban roads from 60 to 80 km/h to 50–60 km/h and on local roads from 40 to 60 km/h to 30 km/h, on air quality, explicitly targeting particulate matter (PM10), carbon monoxide (CO), carbon dioxide (CO2), nitrogen oxides (NOx), and volatile organic compounds (VOCs). By employing a quasi-experimental study design that combines propensity score matching (PSM) and spatial difference-in-differences (SDID) regression, this research addresses gaps in existing studies by investigating the causal impact of speed limit reductions on air quality and spatial dependence, while controlling for confounding variables, such as built environment and socioeconomic status. The PSM result indicates that after matching, the mean differences between the treatment and control groups were substantially reduced, indicating that matching effectively balanced covariates. The results of three different difference-in-difference models (linear regression DID, spatial error DID, and spatial lag DID) were applied to five air pollutants: CO2, CO, NOx, VOCs, and PM10. The treatment effect is positive and statistically significant across all pollutants, except NOx. This confirms that the speed limit reduction policy increased traffic volume and, consequently, increased emissions, thereby worsening air quality. Although the policy successfully contributes to the United Nations' Sustainable Development Goals (SDGs) by significantly reducing crashes and enhancing road safety, it falls short of achieving another key aspect of sustainability, improving air quality. Therefore, comprehensive strategies are required to achieve sustainable urban environments.