Evaluating the Impact of the Self Car-Free Day System on Driving Distance: Evidence from Busan, South Korea

Voluntary vehicle use restriction policies targeting a broad population of vehicles may be more effective in reducing driving distance, as participation is self-selected by drivers. However, because groups that benefit more from participation-related incentives are more likely to enroll in the program, it is necessary to address potential sample imbalance between treatment and control groups when comparing policy effects before and after implementation. In addition, analyses relying on indirect indicators, such as air quality, have limitations in accurately estimating the actual policy effect. To address these issues, this study applies propensity score weighting (PSW) to correct for imbalance between the treatment and control groups and directly estimates the policy effect using vehicle-level driving distance data. Since driving distance may inherently reflect unobserved heterogeneity across vehicles (e.g., driving habits) as well as temporal variation, a difference-in-difference (DID) approach with two-way fixed effects is employed to control for these factors and estimate the causal effect of the policy. Focusing on Busan, South Korea, the effects of the Self Car-Free Day System—a voluntary vehicle use restriction policy—are examined. The results show that vehicles registered in the program experienced an additional reduction of 3.563 km/day in average daily driving distance compared to unregistered vehicles. This reduction exceeds the expected decrease under the assumption of strict compliance with the designated no-driving day alone, suggesting that participants may have voluntarily refrained from vehicle use on days other than the designated restriction day.