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Tag Archive for: speeding

What is the 85th Percentile Rule?

June 23, 2025/by Jared Sanchez

And why it belongs in the dustbin of transportation history.

In 2021, CalBike helped pass the Slower Speeds Save Lives Bill, AB 43, authored by former California Assemblymember (and current U.S. Representative) Laura Friedman. The bill gives communities limited relief from the 85th percentile rule, allowing them more leeway to lower speed limits. This year, CalBike supports AB 1014 (Rogers), which will extend the ability to set lower speed limits on state-controlled roadways. CalBike’s Executive Director, Kendra Ramsey, is testifying in support of AB 1014 at the Senate Transportation Committee.

The history of the 85th percentile rule

When a local government wants to lower the speed limit on a particular street, it has to do a speed study to determine how fast drivers are going on that street. Under the 85th percentile rule, the new speed limit must be set within 5 mph of the speed that 85% of drivers travel at or below. 

As with most things that are no longer working well, the 85th percentile rule for setting speed limits arose to serve a genuine need. In rural areas, towns would sometimes set speed traps, lowering the speed limits within town boundaries much lower than those of the surrounding area. Towns raised revenue by handing out speeding tickets to unsuspecting motorists.

The 85th percentile rule was never designed to determine safe speeds in urban areas, yet it is now widely applied for that purpose.

There has been a concerted effort across the country to change the way speed limits are set.

The National Transportation Safety Board (NTSB), the National Association of City Transportation Officials (NACTO), and more recently, the California State Transportation Agency (CalSTA) have all called for moving away from using the 85th percentile as the basis for setting speed limits.

In 2018, former Assemblymember Friedman initiated a bill to revise the ingrained 85th percentile rule, which was eventually transformed into the Zero Traffic Fatalities Task Force, in which the body firmly focused on the rule. This led to the passage of AB 43 in 2021. 

How does the 85th percentile rule work?

The 85th percentile rule is easiest to explain with an example. If the current speed limit on a street is 35 mph and a speed study shows that 85% of drivers are driving at or below 37 mph, the 85th percentile rule would dictate that the speed limit remain unchanged because it fits driver behavior. Even if the community had determined that 35 mph was too fast for safety, the local government’s hands would be tied.

In some cases, the rule leads to higher speed limits. For example, if the speed study showed that 85% of drivers in that 35 mph zone were going 41 mph or slower, the local government would have to raise the speed limit to 40 mph.

What AB 43 did to speed limits

Under AB 43, communities can reduce speeds in business zones without a speed study. In cases where a speed study is still required, California jurisdictions have more flexibility to lower speed limits. San Francisco has helpful resources on this topic, and UC Berkeley’s SafeTREC, along with ARTC Logistics, developed a California Safe Speeds Toolkit to help local governments set safer speed limits.

For example, if a speed study showed that 85% of drivers traveled up to 34 mph in a 35 mph zone, under the old rule, the city would need to keep the posted speed at 35 mph. Under AB 43, the city has the option to round down and reduce the speed limit to 30 mph.

In addition, communities can reduce speeds by another 5 mph after an engineering study in areas with high volumes of bike or pedestrian traffic or in areas that are designated as safety corridors.

Why California needs lower speed limits on state routes

Speed is a factor in at least 25% of traffic fatalities in California. Even at the relatively low speeds in urban areas, collisions can be fatal, particularly for seniors and children. As the cars, trucks, and SUVs on the road have gotten heavier and their grilles higher, injuries to vulnerable road users hit by cars, even at slow speeds, are more likely to be life-threatening.

Some of the most dangerous streets in many California cities and towns are state routes. These local highways turn into city streets while still funneling fast-moving traffic past homes and schools. AB 1014 would extend the same speed limit setting parameters to these state routes. Caltrans would be empowered to set prima facie speed limits of 25 or 20 mph in certain areas. It would also have the option to round down instead of up from the 85th percentile speed and to reduce speeds by another 5 mph in safety corridors and near facilities used by people biking and walking.

Of course, speed limits can’t prevent all reckless and dangerous driving. We need improved infrastructure for that. But measures like AB 43 and AB 1014 are an excellent start and may provide an impetus for better infrastructure if speed safety targets aren’t met.

https://www.calbike.org/wp-content/uploads/2024/03/speed-cars-traffic-blur-scaled.jpg 1707 2560 Jared Sanchez https://www.calbike.org/wp-content/uploads/2018/07/calbike-logo.png Jared Sanchez2025-06-23 16:43:522025-06-30 11:59:47What is the 85th Percentile Rule?

Speed Kills: Addressing the Real Road Hazard

September 18, 2023/by Kendra Ramsey

Cities throughout California have taken steps to regulate e-bikes based on the belief that increased e-bike use is leading to collisions and dangerous conditions. These efforts ignore the real road hazard: speeding motor vehicle drivers. 

Reducing speeds on city streets is the best way to protect people biking and walking, especially those most vulnerable. A Streetsblog article by Angie Schmidt shows that the chances of a pedestrian dying in a 20 mph crash are three times as high for a 70-year-old as for a 30-year-old. Children are particularly vulnerable as well because they’re shorter, less visible, and more likely to be struck on the upper body or head.

We accept thousands of fatalities (4,407 in California in 2022, around 25% of those vulnerable road users) and many more injuries and lives upended due to traffic violence as a fact of modern life. But we don’t have to. In this post, we examine the factors that contribute to the culture of speeding and what we can do to change it.

The tyranny of the 85th percentile rule

According to the Insurance Institute for Highway Safety (IIHS), 29% of traffic fatalities in 2021 were caused by speeding. As speeds increase, the energy of the impact goes up exponentially, radically increasing the risk of serious injury and death. For example, in the Streetsblog article referenced above, Schmidt shows that the fatality rate nearly triples if a vehicle hits a pedestrian at 30 mph rather than 20 mph. 

A survey of pedestrian fatality data found that the chance of a pedestrian being killed when hit by a car doubled from 5% to 10% when speed increased from 18 to 23 mph (30 to 37km/h). The authors recommended maximum speed limits of 18 to 25 mph (30 to 40 km/h) in pedestrian zones.

Yet these recommendations can be hard to implement, even when California communities want to. The state’s primary method for determining allowed speed limits is the 85th percentile speed. This requires an engineering study to determine speed distribution on a street, and the speed limit is pegged to the speed 85% of drivers are driving at or below. AB 43, passed in 2021, gives communities increased flexibility to round speed limits down rather than up, but it doesn’t do away with the 85th percentile rule altogether.

Until California communities can set speed limits based on safety rather than car driver behavior, we’re missing a critical tool to protect vulnerable road users.

Invisible pedestrians

One factor that has contributed to an uptick in pedestrian deaths over the past few years is changes in vehicle design. Newer SUVs and pickup trucks often have much higher front grills than earlier models. This creates a large blind spot in front of the vehicle, which particularly endangers children. 

A study of crashes involving SUVs found that children were killed disproportionately by SUVs. In addition, trucks with front grills that top out at around five feet off the ground are more likely to strike adults in the head and neck and more likely to drag a pedestrian under the vehicle rather than over the hood. 

NACTO has called on the federal government to change the way it rates the safety of new cars to include danger to people outside the vehicle as well as inside. But the current generation of killer trucks and SUVs is likely to be on the roads for many years to come.

The car as a weapon

The vast majority of drivers don’t wish to harm anyone. But there’s been a growing trend of people (usually men) using cars as weapons. One of the more recent incidents happened in Huntington Beach, where a teenager deliberately hit three people riding bikes, killing one of them. This violent spree happened a few days after the Huntington Beach City Council considered a proposal to regulate e-bike riders because “[E]-bikes have not only become a nuisance to drivers but those driving the e-bikes have become a danger to vehicles and a danger to themselves.”

While there has been a lot of discussion of the dangers of teen e-bike riders, teen car drivers pose a much graver risk to our communities. Efforts to regulate e-bikes will do little or nothing to improve safety. But other measures can.

One study of speed interventions found that outliers (people driving far above the speed limit) had an outsized impact on pedestrian injuries. That points to the role of infrastructure changes, which can physically prevent drivers from speeding.

Built for speed: Dangerous street design contributes to fatalities

One might ask: Why would the 85% speeds be higher than the posted speed limit on a roadway? And further, how can drivers feel comfortable going so fast? The answer to both is in the design of our roads. Many of our roadways were designed by traffic engineers to provide unobstructed throughput for as many vehicles as possible. Travel lanes are often set wide enough for heavy trucks even when few (or none) use the road, and curbs are sloped to allow vehicles to turn without much slowing. 

Historically, engineers have added vehicle lanes to decrease delay for drivers at peak commute time, creating wide roadways with capacity far exceeding what’s needed for most of the day, all in the name of free flow of automotive traffic. These additional lanes provide a “cushion” for car drivers that helps them feel comfortable driving faster, but they actually increase congestion and delay on the road over time. 

These design choices, allowed within the prevailing guidance documents for engineers, combine to create an environment where car drivers feel comfortable — and have no physical restraints to prevent — consistently driving far above the posted speed limit. 

Fortunately, there are design tools that create safer facilities for people using all modes. The National Association of City Transportation Officials (NACTO), for example, has multiple guides that provide evidence-based ways to increase safety for people biking and walking. The Federal Highway Administration (FHWA) provides a list of proven safety countermeasures, many of which are aimed at preventing serious injuries and fatalities for vulnerable road users. In addition, the prevailing street design regulation and guidance documents (such as the MUTCD, Highway Design Manual, and AASHTO “Green Book”) provide for the use of “engineering judgment” to design facilities that may diverge slightly from the standard (car-centric) treatment.

CalBike has fought hard to create an environment where communities have the option to build less lethal streets. We helped pass legislation to legalize protected bikeways and to spread the word about Class IV protected bikeways. Protected bikeways have been shown to reduce fatalities not just for people on bikes but for drivers and pedestrians as well.

We continue to work to change attitudes about infrastructure. We created a Quick-Build Guide with Alta Planning + Design to help communities rapidly add elements to protect people biking and walking. And we’re surveying the condition of state highways that double as local streets to see where Complete Streets upgrades are needed. 

Slow Streets toolbox

The good news is that we know how to make our streets safer. The bad news is, we aren’t always using those tools. Pandemic Slow Streets spawned a movement to make those changes permanent, and some California cities have kept car-free or car-light spaces, while more have plans to do so. 

Here are some other measures that can help us rein in speeding motor vehicles:

  • AB 645 will allow six cities to pilot speed cameras, which have been proven to deter speeding and reduce injuries. It’s on the governor’s desk now.
  • AB 251 will study a tax on heavier vehicles, which could create an incentive for car manufacturers to make smaller, safer cars and trucks. It’s also waiting for the governor’s signature.
  • AB 413 prohibits parking within 20 feet of a crosswalk approach, which will improve visibility. This bill is also with the governor.
  • The NACTO Urban Street Design Guide includes a number of mechanisms to reduce vehicle speeds, including pinch points, chicanes, and speed humps. Infrastructure changes are the slowest and most costly way to prevent speeding, but they are the most effective.
  • The League of American Bicyclists is hosting a webinar, “Slow Roads Save Lives,” on September 21. You can register here.
  • CalBike is collecting data on local streets controlled by Caltrans to find where Complete Streets elements are needed to enhance the safety of people biking and walking. Take our survey by October 10, 2023, and tell us about your experience on your local streets.
https://www.calbike.org/wp-content/uploads/2018/08/speeding_cam.jpg 626 1200 Kendra Ramsey https://www.calbike.org/wp-content/uploads/2018/07/calbike-logo.png Kendra Ramsey2023-09-18 18:15:042023-09-18 18:15:06Speed Kills: Addressing the Real Road Hazard

Zero Fatalities Task Force Tackles Dangerous Street Design, Speeding

November 6, 2019/by Kevin Claxton

In 2018, Governor Brown signed AB 2363 to establish the Zero Traffic Fatalities Task Force to make recommendations for eliminating traffic fatalities in California. CalBike’s Executive Director, Dave Snyder, is a member of the task force.

 The bill, authored by Assembly Member Laura Friedman, originally removed the 85th percentile rule in current law about setting speed limits. The rule requires government agencies to set the speed limit at or near the 85th percentile speed of existing traffic on the street or road. In other words, before setting enforceable speed limits, a city has to measure existing speeds, and use the speed of the third fastest car out of 20 to set the speed limit. This requirement has forced cities to actually increase speed limits on many streets, resulting in higher speeds, more crashes, more killings, and more devastating injuries. 

Unfortunately, the bill’s provision to rescind this requirement was opposed by California’s AAA affiliates and other motorist advocacy organizations. In the end, the bill was amended to require a task force to study the issue. 

That Task Force has held three of four scheduled meetings before the Secretary of Transportation David Kim must submit a report of the Zero Traffic Fatalities Task Force’s findings to the Legislature. Its members have studied existing law and debated alternatives to the 85th percentile rule, the efficacy and feasibility of enforcement including automated enforcement (speed cameras), and the potential of engineering to reduce speeds and fatalities. 

A key change to existing law that CalBike has advocated in these meetings is the ability to set speeds lower than 25 mph on neighborhood greenways and other streets. These are places where we want to encourage calm, livable streets safe for walking and biking. On streets with higher speed limits, equitable and automatic enforcement can reduce crashes and save lives of people walking and biking.

Another recommendation is an expansion of the kinds of streets and areas where local agencies may reduce the speed limit below what current law allows. The most important expansion would be on streets identified as “high injury corridors,” where speeding motor vehicles demonstrably cause deaths and serious injuries. Another expansion would broaden the definition of a business district or a school zone. 

When the Task Force completes its work, CalBike, California Walks, and other allies will make sure the insights and recommendations from the Task Force are distributed broadly. More local partners and community members should be engaged in the discussion so that many more voices than were included in the Task Force can amplify the most important recommendations. Together, we can empower local residents and elected officials to set speed limits where they should be to support our goals of creating safe and healthy neighborhoods, and not crossed by streets designed for people to drive through quickly.

https://www.calbike.org/wp-content/uploads/2019/09/37988989534_05d23e0000_k.jpg 1365 2047 Kevin Claxton https://www.calbike.org/wp-content/uploads/2018/07/calbike-logo.png Kevin Claxton2019-11-06 14:51:282019-11-08 13:39:17Zero Fatalities Task Force Tackles Dangerous Street Design, Speeding

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