Thirty km/h speed limits for better public health: Study protocol of a natural experiment in Rotterdam, the Netherlands

Background: 

Thirty kilometre per hour (km/h) interventions effectively reduce road-traffic injuries, yet its broader health and social impacts remain unclear. The Municipality of Rotterdam, the Netherlands will implement 30 km/h interventions, offering opportunities for prospective natural experimental evaluation. This study aims, firstly, to assess impacts of 30 km/h interventions on self-reported changes in health outcomes, including physical activity (PA), active travel, noise exposure, and sleep quality and social outcomes, including neighbourhood cohesion and safety perceptions. Secondly, we aim to investigate impacts of 30 km/h interventions on changes in objective PA. Thirdly, we explore to what extent these impacts are moderated by age, individual and neighbourhood socio-economic position (SEP), ethnicity, and health status. 

Methods: 

We designed a prospective natural experiment by setting up a longitudinal open cohort study with pre-intervention (June–October 2023) and one-year follow-up (June–October 2024) measurements. Intervention sites are 15 streets in Rotterdam with speed limit changes from 50 km/h to 30 km/h, accompanied with physical calming measures. Control streets remained 50 km/h. To investigate unintended outcomes at adjacent streets, streets remaining 50 km/h and within <250 m from the nearest intervention street were categorized as spillover streets. At baseline, 10,860 households are invited for a household survey about socio-demographic characteristics and health and social outcomes. Survey participants could additionally wear an accelerometer (Actigraph GT3X) for 8 consecutive days. To assess impacts of 30 km/h interventions on changes in self-reported health and social outcomes and objective PA, we will use difference-in-difference analyses. We will also evaluate unintended effects on adjacent streets (i.e., spill-over effects). Stratified analyses will be conducted to assess moderation. 

Expected results and discussion: 

This study improves the understanding on health and social impacts of 30 km/h interventions. Furthermore, this study sets a real-world example of how to investigate outcomes through a natural experimental evaluation that are not routinely collected. Ultimately, this can inform policies towards healthy, liveable, and inclusive cities.