Car-free days as sustainable urban transport emission reduction strategies: evidence from Kigali city, Rwanda

Introduction: Car-Free Days (CFDs) in Kigali, Rwanda, are held twice monthly on Sundays, when motorized vehicle traffic is restricted on major roads from 7:00 AM–11:00 AM to promote physical activity and reduce air pollution. Previous studies assessing the effectiveness of CFDs in reducing air pollution in Kigali have relied solely on short-term, low-cost fine particulate matter (PM2.5) measurements during the intervention period at a single site and have provided no post-intervention rebound assessment. The present study provides the first record of variations in PM2.5 and ozone (O3) to evaluate the effectiveness of CFDs as a sustainable transport emission-reduction strategy. We examine whether reductions in air pollution persist beyond implementation hours and contribute to overall daily improvements in air quality in Rwanda.

Methods: Continuous air quality data collected between 2022 and 2024 from two sites were analyzed. Pollutant concentrations during CFD Sundays were compared to normal Sundays across morning intervention (7:00 AM–11:00 AM) and post-intervention (11:00 AM–9:00 PM) periods.

Results and discussion: Results show that PM2.5 concentrations significantly decreased during CFD hours (−14.8%, P < 0.0001) and remained lower in the post-intervention period (−34.7%; P < 0.0001), with no evidence of rebound. Diurnal profiles showed continued declines after 11:00 a.m. and a suppressed evening peak, indicating a sustained reduction in daily exposure. In contrast, O3 showed no significant differences during CFD hours and increased in the afternoon, consistent with photochemical formation rather than direct traffic emissions. This study highlights that CFDs may serve as an effective, low-cost, and sustainable strategy to reduce transport emissions in Africa, where expensive interventions to reduce air pollution are lacking. These findings also highlight the potential of community-based interventions to enhance urban environmental sustainability and resilience, especially in rapidly urbanizing cities.