The impact of 30 km/h urban speed limits on physical activity-related outcomes: A repeated cross-sectional, controlled natural experiment in two Dutch cities with up to 15 months follow-up.

Speed limit reductions for motorised transport may promote physical activity (PA), yet evidence remains limited. This natural experiment assessed the effects of 30 km/h policies on PA-related outcomes in two Dutch cities. Amsterdam and Rotterdam implemented 30 km/h policies (2023-2024), via city-wide speed limits and street-specific 30 km/h zones, respectively. Cross-sectional surveys were distributed to 9,680 Amsterdam (sample: baseline n = 382; 12-month follow-up n = 480) and 10,860 Rotterdam addresses (sample: baseline n = 548; 15-month follow-up n = 598), with residents in similar 50 km/h streets as controls. The international Physical Activity Questionnaire (IPAQ) and travel diaries assessed transport-related and leisure PA and sedentary behaviour. Accelerometers assessed PA without domain differentiation (subsample Rotterdam). Changes were estimated using multi-level linear difference-in-differences models. In Amsterdam, IPAQ analyses indicated consistent albeit non-significant increases in leisure-time moderate-to-vigorous PA (MVPA) (β = 58.89 min/wk; 95%-CI:-23.33, 141.11); transport walking (β = 68.63 min/wk; 95%-CI:-42.64, 179.91); and transport cycling (β = 82.48 min/wk; 95%-CI:-30.91, 195.86). Travel diaries showed similar patterns for cycling (β = 31.62 min/day; 95%-CI:-30.93, 94.18) and walking (β = 24.07 min/day; 95%-CI:-33.81, 81.95). In Rotterdam, 30 km/h zones increased the probability of daily public transport use by 15 percentage points (95%-CI:0.05, 0.24), while other IPAQ, travel diary and accelerometer estimates were mixed and non-significant (e.g., transport cycling (β =  - 48.41 min/wk; 95%-CI:-143.34, 46.51) and transport walking (β = 5.08 min/wk; 95%-CI:-96.77, 106.92)). Effects in Amsterdam were in the hypothesised direction, while Rotterdam yielded mixed results. The study provides an important first indication, but its small sample sizes underscore the importance of future studies with larger samples, longer follow-up periods, using similarly robust methods to evaluate city-wide 30 km/h policies.