Evaluating the impact of a 30 km/h speed limit reduction on NO₂ levels: A quasi-experimental controlled interrupted time series analysis

Introduction:

Road traffic is a major source of nitrogen dioxide (NO₂) emissions. While evaluations of speed limit reductions in high-speed contexts have shown reductions in NO₂, results in urban, low-speed environments remain inconclusive due to factors such as changes in acceleration and braking patterns. This study assessed whether the implementation of a speed limit reduction in Amsterdam, the Netherlands, from 50 to 30 km/h impacts street-level nitrogen dioxide (NO₂) concentrations. 

Methods:

We used a controlled interrupted time series analysis (January 2021-March 2025) to assess changes in four-weekly street-level NO₂ concentrations, assessed by Palmes diffusion tubes. We compared intervention streets (40 measurement stations), where the speed limit was reduced to 30 km/h, to control streets (19 measurement stations), which maintained the original 50 km/h speed limit. We controlled for background NO₂ concentrations, seasonality, and weather conditions, as well as accounted for autocorrelation at street level. 

Results:

On intervention streets, median NO₂ levels were 24.4 (IQR 8.3) µg/m³ pre-intervention and 22.1 (6.5) µg/m³ post-intervention. On control streets, these levels were 24.4 (9.9) µg/m³ and 22.2 (6.4) µg/m³, respectively. We observed no difference in NO₂ (0.09 µg/m³ (95% CI: −0.46; 0.65)) on the intervention streets compared to the control streets following the intervention. 

Discussion:

The speed limit reduction from 50 to 30 km/h in Amsterdam did not lead to a measurable reduction in street-level NO₂ concentrations. As adherence to the new speed limit was not optimal, further long-term monitoring is recommended to assess the potential air quality benefits of this policy.