Developing a Safe System Approach to Setting Speed Limits

Griswold, Julia B; Lutzker, Liza; Fournier, Nicholas; Grembek, Offer; Fox, Jenn; Shahum, Leah · 2023 · ROSA P / California. Department of Transportation

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Summary

This report addresses the critical need to reform speed limit setting in California to align with the Safe System Approach, a framework aimed at eliminating traffic fatalities and serious injuries. The motivation stems from a surge in traffic deaths over the last decade, particularly pedestrian fatalities, which increased by 70% between 2012 and 2021. Currently, California sets speed limits based on driver behavior, specifically using the 85th percentile rule, which researchers argue is misaligned with safety objectives and fails to account for the human body’s tolerance to crash impacts. The report seeks to provide a framework for a new, context-sensitive approach that prioritizes the safety of all road users rather than accommodating prevailing driver speeds. The study was conducted by the Safe Transportation Research and Education Center (SafeTREC) at UC Berkeley and the Vision Zero Network for the California Department of Transportation (Caltrans). The methodology involved a comprehensive review of current regulations, barriers to safe speed limit setting, and alternative models. Researchers analyzed the limitations of California’s existing practices, including cost-prohibitive engineering requirements and institutional resistance to moving away from the 85th percentile. The team also examined case studies from within the United States and internationally, with a specific focus on New Zealand’s “Movement and Place” framework. Additionally, the researchers gathered practitioner perspectives through interviews with nine California-based traffic engineers and surveys of non-California practitioners to understand implementation challenges and opportunities. The findings highlight that the current 85th percentile approach contributes to “speed creep” and fails to protect vulnerable road users. While recent legislation (AB 43 and AB 1938) has granted local jurisdictions some flexibility to lower speed limits in specific contexts, these measures remain tied to driver behavior and do not fully embody the Safe System Approach. The report identifies three primary barriers to change: cost-prohibitive study requirements, limited local authority, and institutional resistance among traffic engineers. Drawing from New Zealand’s model, the authors propose a context-sensitive framework that categorizes roadways based on their function for movement and their role as a place for community activity. This approach suggests using California’s existing functional classification for movement and developing a spatial metric for “place” to determine safe, appropriate speed limits. The significance of this work lies in its provision of a actionable framework for California to transition toward a Safe System Approach for speed management. The report recommends technical, engagement, and legislative changes to support this shift, including the adoption of default speed limits, slow zones, and corridor-based safe speed studies. By moving away from percentile-based limits and toward limits designed around human vulnerability and context, California can better achieve its vision of zero fatalities by 2050. The authors conclude that while legislative and implementation challenges exist, leveraging successful international models tailored to California’s context is essential for prioritizing the well-being of all road users and fulfilling the ethical imperative of preventing serious injuries.

Key finding

The report concludes that California should replace its current driver-behavior-based speed-limit setting with a context-sensitive Safe System Approach that prioritizes the safety of all road users, drawing on models like New Zealand’s to set limits based on roadway function and land use.

Methodology

mixed_methods

Sample size: 9

Provenance

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Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.

Topics

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• speed management
• traffic safety culture
• speed choice
• perceptual countermeasures

Information type

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• Empirical Findings: crash risk outcomes