In the pursuit of sustainable mobility and the decarbonization of transport systems, public authorities are increasingly scrutinizing the impact of travel speed on emissions within both low-speed and high-speed environments. This study critically examines the evidence concerning emission impacts associated with speed limit changes in different traffic environments by conducting a systematic review of the literature in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. A total of 25 studies that met the eligibility criteria were assessed. The results reveal mixed evidence for reducing emissions through speed limit reductions in low-speed areas. However, emerging evidence suggests that reduced urban speeds may abate emissions through enhanced traffic flow and a shift in modal preferences away from personal vehicle use. Additionally, in urban areas, minor observed emission reduction per vehicle can add up to large overall reductions due to the high number of vehicles. In high-speed contexts, the evidence is much clearer, showing that reduced speed limits correlate with significant reductions in NOx, CO₂, and particulate matter emissions. The extent of these reductions is highly variable and contingent upon the specific speed limits or limit reductions, the local context, the vehicle type, and the baseline types and levels of pollutants. Notably, there is a lack of research on the effects of speed on emissions, especially in low- and middle-income countries (LMICs), highlighting a critical area for future investigation. The findings of this study underscore the potential environmental benefits of speed management policies and advocate for the promotion of smoother and less aggressive driving behavior to mitigate emissions and enhance sustainable mobility in both low-speed and high-speed settings.

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.