It’s commonly assumed that speed limits are set according to scientific criteria demonstrated to create safe streets. However, the 85th percentile rule shows us that’s not really the case.
Examples of engineering measures to slow vehicular speeds for safer streets.
As the economy slowly reopens, our streets are giving way to returning traffic and increased emissions. With the rise in car sales (especially larger vehicles like pick-ups and SUVs), and pedestrians twice as likely to die when struck by a large vehicle — it’s time to rethink the way we approach safety on our streets.
The reality is that speed limits on most U.S. streets are not chosen based on what speed is safest for everyone using the roadway—they are set by the speed at or below which 85 percent of the motorists drive on a given road, an outdated guideline promoted by the Federal Highway Administration (FHWA).
Today’s speed limits are not set for our safety. They are, in fact, making us less safe. From 1974 to 1995, the federal government set maximum speed limits for interstate highways under the National Maximum Speed Law. Research shows that roadway fatalities declined during this period. States began to increase speed limits on interstate highways when the law was repealed—in the ten years following over 12,000 deaths across the US were attributed to rising speed limits, and 37,000 over 25 years.
According to the National Highway Traffic Safety Administration, speeding has been a factor in nearly one-third of traffic fatalities over the past 20 years. Slower driving is demonstrably safer, and with all evidence pointing to the dangers of carelessly set speed limits, government agencies must revisit their speed limit-setting and road-building practices.
What’s so Wrong with the 85th Percentile?
When following the 85th percentile guideline, engineers recommend a speed limit that falls near the 85th percentile of the average speed naturally observed on a roadway. The method assumes two things: 1) that drivers pick a speed that is safe for the road conditions and 2) that it is safest to drive the speed desired by fellow drivers.
Say you live along a state route in a rural area. If 85% of drivers drive at or below 55 mph, 55 mph would become the starting speed limit for said road. Engineers can then make adjustments to the starting speed limits based on neighborhood factors such as physical design of the street or volume of pedestrian traffic. However, in some states like California, engineers can only adjust the speed by 7mph in either direction, giving the 85th percentile rule enormous weight influencing the outcome.
Researchers from University of California Institute of Transportation Studies point out that there is no empirical study demonstrating that the 85th percentile speed optimizes safety, but the guideline is difficult to get around because of its presence in the FHWA’s Manual on Uniform Traffic Control Devices (MUTCD) and promotion by the Institute of Transportation Engineers (ITE). For decades, transportation engineers routinely follow guidelines outlined by the MUTCD and ITE as standard practice.
In some states, such as California, the 85th percentile guideline is further reinforced by its presence in the official state Department of Transportation's Manual for Setting Speed Limits. As the L.A. Times editorial board recently wrote, this method of using common speeds to set speed limits is “outdated, absurd and downright dangerous for bicyclists and pedestrians.”
Additional examples of engineering measures to design more inclusive streets - for people of all ... [+]
Speed Limits Won’t Save Us; Design Needs to Step In
Simply lowering speed limits may not be enough, though. Drivers don’t sufficiently reduce their speed unless the roadway is designed and engineered to make them slow down. The 85th percentile guideline was actually born from this knowledge — if speed limits are reduced too far below the average speed, crashes can increase because drivers stop paying attention.
Policy makers and advocacy groups are proposing new paths forward. The National Association of City Transportation Officials (NACTO) just released a recent report, City Limits, which calls for designing slow zones, along with setting default speed limits on many streets at once, and using conflict density and activity level to set corridor speed.
In January 2020, California State Transportation Agency (CalSTA) released findings from the Zero Fatalities Task Force (disclosure: my company, Remix, was on the task force as an advisory member), which called for setting a different approach to setting speed limits to prioritize safety outcomes to meet the needs of all road users. Developing policies and encouraging measures like traffic calming, lane narrowing, and other interventions to reduce speed (explore more design examples here) will prioritize the safety of our most vulnerable street users.
The Time is Now
The critique of the 85th percentile is not new—advocates have long called for its removal. What is new is broad-based support and awareness as evidenced by NACTO’s City Limits report, and government initiatives such as CalSTA’s Zero Fatalities Task Force. These efforts are major steps forward to unwind this harmful practice of speed-setting, and reorient around neighborhood and design considerations.
We cannot usher in a multimodal future without lowering speeds and making streets safe for all users. We must support the work of those undoing harmful legacy methods, and encourage the adoption of policies that embrace and emphasize design. These methods are the future and they cannot come fast enough.
Tiffany Chu is an environment commissioner for the City of San Francisco and the chief executive officer of Remix. This piece was co-written by Rachel Zack and Janice Park.
