Impact of a 30 km/h speed limit on NO2 levels: A 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.

Key messages

• While adherence was not optimal, 30km/h speeds did not impact air quality in Amsterdam, advocating the use of lower speeds in European urban cities.

• Speed limit reductions need strong enforcement and complementary measures to deliver measurable benefits in air quality.