Assessing the effectiveness of speed limit reduction in Edmonton: A case study analysis

This project was a collaborative research project between University of Alberta and the City of Edmonton (CoE) on assessing the effectiveness of speed limit reductions in Edmonton

Background 

Traffic safety remains a critical concern in urban environments, with cities worldwide seeking to mitigate road-related injuries and fatalities. Speed limits play a significant role in this endeavor, as higher speeds have been shown to increase both the likelihood and severity of collisions. In response, many cities, including Edmonton, Canada, have implemented speed limit reductions to create safer and more livable streets and communities. This study focuses on evaluating the impact of Edmonton's decision to lower the default speed limit on residential roads and high pedestrian density arterials from 50 km/h to 40 km/h. The research aims to provide evidence-based insights into the effectiveness of this measure, contributing to the broader discourse on urban traffic safety and the Vision Zero initiative, which seeks to eliminate severe traffic injuries and fatalities. 

Methods 

An observational study of speed and collision data was utilized to evaluate the impact of a speed limit reduction from 50 km/h to 40 km/h. The approach used is based on a before-and-after study with a comparison group for collision evaluation. The change in the number of collisions occurring at the treatment group of locations (50 km/h roads converted to 40 km/h in August 2021) was assessed in conjunction with the comparison group, which, in this study, were roads that maintained a 50 km/h speed limit after August 2021. Collision data from the comparison group were used to estimate the collision trends that would have occurred at the treated sites had the treatment not been introduced. Speed surveys were conducted at 219 locations across the city before (May-July 2021) and after (June-August 2022) the implementation of the new speed limit, covering diverse road types and neighborhoods. The NC-200 traffic analysis device was used to collect speed data, which was then processed to examine changes in average speed, 85th percentile speed, and compliance rates. In parallel, collision data from three distinct periods—pre-COVID (2017-2019), COVID (2019-2021), and post-implementation (August 2021-July 2022)—were analyzed to evaluate the effect of the speed limit change on various safety indicators, including the number of collisions, injuries, and fatalities. The impact of speed reduction implementation on collision frequency across road segments was evaluated by calculating the treatment effect and Z score to determine the significance of the changes in the number of collisions, injuries, and fatalities as well as speeds. 

Results 

The results of the study showed a significant improvement in road safety following the reduction of the default speed limit to 40 km/h. The analysis revealed statistically significant reductions in the overall number of collisions and injuries and fatalities resulting from collisions, by 25% and 31%, respectively. Speed data showed statistically significant reductions in the average and 85th percentile speeds, with drivers lowering their speeds at 115 locations, accounting for 53% of the surveyed locations. The study also categorized neighborhoods and roads to demonstrate the areas that experienced the greatest benefits. 

Discussion and Conclusion 

The study's findings underscore the positive impact of speed limit reductions on urban road safety, supporting the case for such measures as part of broader traffic safety strategies like Vision Zero. The significant reductions in speeds as well as collisions, injuries, and fatalities indicate that lower speed limits can effectively enhance the safety of all road users, including drivers, pedestrians, and cyclists. The greater speed reductions on local and narrower roads suggest that these areas, in particular, benefit from the speed limit change, likely due to their higher pedestrian density and the increased risks associated with higher speeds. While the results are promising, the study also highlights the need for continuous monitoring and evaluation to sustain the observed safety benefits. Future research could explore the long-term effects of speed limit reductions and the potential for further lowering speed limits in other urban areas. Overall, this study provides valuable insights for policymakers and urban planners seeking to implement or refine speed limit reduction programs to improve road safety in their communities.