Exploring the heterogeneity in impacts of 30 km/h speed interventions: A subgroup meta-analysis based on a systematic...

The 30 km/h speed zones or limits have been implemented worldwide. However, the knowledge of their effectiveness is limited and often based only on European studies. While impacts on speed and safety were generally known, additional impacts (e.g., on traffic volume, noise, emissions) have been studied less frequently, and with inconclusive results. In addition, the impacts were strongly varied, often based on less rigorous forms of literature review, with moderators behind the variations largely unexplored. To address these gaps, the present paper used systematic and unrestricted study selection with random-effects meta-analysis, including the subgroup analysis of moderating variables. This approach provided more robust effect sizes for crash reduction (17%), injury reduction (18%), and speed reduction (-4.68 km/h), also showing that effects on traffic volume were not statistically significant. Subgroup analysis highlighted statistically significant moderators of success for 30 km/h speed limit applications. For example, larger reductions of crashes were associated with 30 km/h traffic calming zones (instead of sign-only schemes), large area interventions, and higher baseline speeds. Some results were consistent with existing meta-analyses, but some were incomparable, likely because previous reviews may have overestimated the real impacts. This could be the case for traffic noise, where this meta-analysis estimate was approximately -1 dB, which is generally not noticeable. The paper provides important updates on the road safety and speed impacts of 30 km/h speed interventions, along with important moderators (as well as contraindicators) of success, which will be useful in future planning and evaluation of such speed limit applications.