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Open Access Publications from the University of California

Fast, Furious & Fatal: An Assessment of Speed-Setting Methodology in California


Speed is a key factor in roadway safety. As vehicle speed increases, the probability of a crash and the severity of the resulting injury increase. Recognizing this relationship, the City of Los Angeles updated speed limits on over 800 miles of streets in 2017 as part of its Vision Zero program. California’s methodology for setting speed limits, known as the 85th percentile rule, caused Los Angeles to increase speed limits on over 90 miles of streets with a history of known collisions. This report focuses on California’s current methodology for speed limits and investigates the following questions: 1. What is the current methodology for setting speed limits in California? 2. What is the relationship between this methodology and roadway safety, particularly in urban areas?3. Are there other approaches to setting speed limits that would improve roadway safety in California, particularly in urban areas?

To answer these questions, I employed a mixed-methods approach that included: a literature review of past research into speed, speed limits, and roadway safety; an analysis of text pertaining to California speed limits in the California Vehicle Code (CVC) and the California Manual on Uniform Traffic Control Devices (CA MUTCD); an analysis of 2017 speed survey data from the City of Los Angeles; a review of case studies of alternative speed limit methodologies in Washington, Oregon, and Sweden; and a summary of legislative action in California since 1996 pertaining to the speed limit methodology.

I found that the current laws for setting speed limits incorporate safety by relying mostly on the 85th percentile speed and allowing for adjustments for context, though only to a limited degree. The case study of Los Angeles revealed the shortcomings of the current methodology for setting speed limits in California, particularly in urban areas, such as non-normal distribution of traffic flow, increasing operating speeds, the practice of using a single day of data and the overused discretion of the 5 mph reduction. The Washington and Oregon case studies illustrated that there are many ways to increase flexibility for local jurisdictions to set speed limits in urban areas, including setting a statewide maximum speed limit for urban areas, authorizing local jurisdictions to set speed limits on their streets, and employing pilot projects to test alternative methods for setting speed limits in cities. The Swedish case study provided insight into a different approach to setting speed limits that is not based on operating speed of vehicles but on the potential for fatalities and severe injuries on streets.

I conclude that California should end its practice of setting speed limits based on the 85th percentile speed and shift the authority for setting speed limits to local jurisdictions. Local jurisdictions should set speed limits using methods, such as the injury minimization method, that focus on safety outcomes, rather than solely on operating speed. Speed limits should be paired with the adoption of automated speed enforcement, which would increase compliance with speed limits.

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