The Shifting Gears of Speed Limits: Rethinking the 85% Rule in an Evolving Landscape

How a century-old standard for setting speed limits is being challenged by modern traffic realities and a growing focus on road safety for all users.

For decades, the seemingly simple principle of the 85% rule has governed how speed limits are set across much of the United States. This methodology, rooted in mid-20th-century research, posits that the safest speed for a road is the speed at which 85% of drivers naturally travel in free-flowing traffic. The idea is that drivers, by “voting with their feet,” will self-regulate to a comfortable and therefore safe pace. However, a confluence of rising traffic fatalities, evolving urban landscapes, and a renewed emphasis on the safety of all road users—not just motorists—is prompting states and cities to re-examine this long-standing doctrine. The result is a complex debate involving public pressure, engineering principles, and advocacy for a more holistic approach to traffic management.

Context & Background: The Genesis of the 85% Rule

The practice of setting speed limits in the U.S. has a long and evolving history, with the 85% rule emerging as a dominant methodology. Its origins can be traced back to studies conducted on rural roads in the 1930s and 1940s. These studies, often cited by organizations like the Institute of Transportation Engineers (ITE), suggested that if a speed limit was set too far below the speed at which most drivers felt comfortable, it would lead to widespread non-compliance and a greater variance in speeds on the roadway. The theory held that this variance, rather than absolute speed, was a primary contributor to accidents. The 85% rule, therefore, aimed to set a limit that most drivers would naturally adhere to, minimizing the difference between the fastest and slowest vehicles and theoretically leading to safer conditions.

This approach essentially posited that drivers, as a collective, possess an innate understanding of a road’s safe operating speed. If a majority of drivers chose to exceed the posted limit, it was often interpreted as a signal that the limit itself was set too low and needed adjustment. This created a feedback loop where driver behavior directly influenced regulatory decisions. As the National Association of City Transportation Officials (NACTO), represented by Jenny O’Connell, noted, this could lead to “people speed, and then the speed limits will be ratcheted up to match that speed.” This mechanism, while seemingly democratic and responsive to driver behavior, has come under increasing scrutiny in recent years.

The context of the 1970s also played a role in shaping speed limit policy. The oil crises of that era led to the imposition of a national maximum speed limit of 55 mph, an effort to conserve fuel. While this was a federal mandate, the authority to set speed limits on non-interstate roads was eventually returned to the states in 1995. Since then, many states have gradually increased their speed limits, with some now permitting speeds of 80 mph or even higher on certain highway stretches, as exemplified by North Dakota and Texas, which features an 85 mph limit on a segment between Austin and San Antonio.

However, this trend towards higher speed limits, coupled with a recent nationwide spike in road fatalities, has spurred a critical re-evaluation of the foundational principles guiding speed limit setting. The narrative of “drivers set the speed” is being challenged by a growing awareness that the “safest speed” might not be the fastest speed, especially in environments with diverse road users and varying levels of vulnerability.

In-Depth Analysis: Critiquing the 85% Rule and Emerging Alternatives

The core of the debate surrounding the 85% rule lies in its fundamental assumptions, which are increasingly being seen as outdated and potentially counterproductive in contemporary urban and suburban environments. The original research was largely based on rural roads, which differ significantly from the complex, multi-use streets common in cities today. These modern streets are often characterized by a mix of vehicles, pedestrians, cyclists, and public transit, creating a more dynamic and potentially hazardous environment than the open roads of the past.

Jenny O’Connell of NACTO highlights a critical flaw: the feedback loop inherent in the 85% rule can inadvertently encourage higher speeds. If drivers consistently exceed a posted limit, that behavior can lead to the limit being raised, perpetuating a cycle of ever-increasing speeds. This approach, critics argue, prioritizes the convenience and perceived efficiency of motor vehicle travel over the safety of more vulnerable road users like pedestrians and cyclists. As Kendra Ramsey of the California Bicycle Coalition puts it, there remains “a very heavy mindset that automobiles are the primary method of travel and they should be given priority and reverence.”

The Federal Highway Administration (FHWA) has acknowledged these concerns. In a significant update to its “Manual on Uniform Traffic Control Devices” (MUTCD) in 2009 and subsequent communications, the FHWA clarified that the 85% rule is not a rigid mandate but rather a guideline. The agency now encourages states and local authorities to consider a broader range of factors when setting speed limits, including the road’s context, intended use, the presence and vulnerability of pedestrians and cyclists, and crash history. This shift represents a subtle but crucial federal nudge away from an over-reliance on driver behavior as the sole determinant of safe speeds.

In response to these critiques and changing federal guidance, new approaches are gaining traction. NACTO, for instance, developed its “City Limits” program, which advocates for setting speed limits based on a formula that factors in a street’s activity level, the likelihood of conflicts between users, and the potential severity of injuries. This method aims to minimize risk for all road users by proactively setting limits that reflect the inherent dangers of a particular street environment. The emphasis shifts from what drivers *do* to what is *needed* for safety.

Cities are beginning to implement these alternative strategies. Madison, Wisconsin, with its “20 Is Plenty” campaign, has proactively lowered speed limits on residential streets from 25 mph to 20 mph. A similar pilot program in Seattle reportedly saw a decrease in serious injury crashes and a notable drop in the 85th percentile speed, suggesting that reducing the posted limit can influence driver behavior and create safer conditions.

However, the transition is not uniform. States like California, while having loosened restrictions to allow local departures from strict 85% adherence when safety needs can be cited, still have legal frameworks that heavily favor the established rule. Advocates argue that these changes are a step in the right direction but do not go far enough in prioritizing non-motorist safety.

On the other side of the debate, organizations like the National Motorists Association, represented by Jay Beeber, argue that adhering to the 85% rule can actually reduce speed variance. Beeber posits that if a road is designed to encourage higher speeds, a lower posted limit might be incongruent with the road’s physical characteristics, leading to enforcement issues and driver frustration. This perspective emphasizes the importance of aligning posted limits with the physical design and perceived “travel speed” of a roadway.

The Insurance Institute for Highway Safety (IIHS), a research arm funded by auto insurers, has provided data underscoring the safety implications of speed. A 2019 study indicated that every 5 mph increase in a state’s maximum speed limit correlates with an 8.5% increase in fatalities on interstate highways and a 2.8% increase on other roads. Chuck Farmer, the study’s conductor, notes that modern vehicles can create a sense of detachment from speed, making drivers less aware of their actual velocity compared to older vehicles. This further questions the reliance on driver perception as the sole metric for safety.

Pros and Cons: Evaluating the 85% Rule and Its Alternatives

The 85% Rule

• Pros:
  • Simplicity and Familiarity: It is a widely understood and long-established methodology, making it familiar to engineers, policymakers, and the public.
  • Driver Acceptance: By aligning speed limits with prevailing driver behavior, it can potentially lead to higher voluntary compliance and reduce the perception of unfair enforcement.
  • Minimizing Speed Variance (Theoretical): The theory suggests it can reduce the difference between the fastest and slowest drivers, which is often cited as a factor in crashes.
  • Responsiveness to Road Design: It can implicitly reflect the design characteristics of a road that may encourage higher speeds.
• Cons:
  • Outdated Basis: Rooted in 1930s/40s research, it may not adequately account for modern traffic volumes, vehicle types, or the presence of vulnerable road users.
  • Prioritizes Motorists: Critics argue it prioritizes driver convenience and speed over the safety of pedestrians, cyclists, and residents.
  • Feedback Loop for Higher Speeds: Can lead to a continuous ratcheting up of speed limits in response to driver behavior, potentially increasing overall speeds.
  • Ignores Road Context: May not sufficiently account for specific local conditions like proximity to schools, parks, or residential areas where lower speeds are critical.
  • Potential for Increased Fatalities: Studies suggest higher speed limits are correlated with increased fatality rates, a consequence the 85% rule may overlook if it leads to higher limits.

Alternative Approaches (e.g., NACTO’s “City Limits,” Vision Zero principles)

• Pros:
  • Holistic Safety Focus: Explicitly prioritizes the safety of all road users, including the most vulnerable.
  • Context-Sensitive: Incorporates factors like road design, land use, pedestrian and cyclist activity, and crash data into speed limit setting.
  • Proactive Risk Mitigation: Aims to set speeds that proactively reduce the likelihood and severity of crashes, rather than reacting to existing driver behavior.
  • Encourages Safer Urban Design: Can support the development of “complete streets” that accommodate all modes of transportation safely.
  • Evidence-Based Adjustments: Utilizes a broader range of data and analysis to inform speed limit decisions.
• Cons:
  • Increased Complexity: The analysis and modeling required can be more complex than simply observing driver speeds.
  • Potential for Public Pushback: Lowering established speed limits can sometimes face resistance from drivers accustomed to higher speeds or those who feel the new limits are overly restrictive.
  • Enforcement Challenges: Lowering speed limits may require increased enforcement efforts to achieve compliance and realize safety benefits.
  • Less Familiarity: These newer approaches may be less familiar to practitioners and the public, requiring more education and outreach.

Key Takeaways

• The 85% rule, a long-standing method for setting speed limits, suggests setting the limit at the speed 85% of drivers travel.
• This rule, originating from mid-20th-century rural road studies, is being re-evaluated due to its potential to create a feedback loop that raises speed limits.
• Critics argue the 85% rule often prioritizes motorist convenience over the safety of pedestrians and cyclists.
• A recent nationwide increase in traffic fatalities has prompted a broader discussion about road safety and speed management.
• The Federal Highway Administration (FHWA) has clarified that the 85% rule is a guideline, not a mandate, and encourages consideration of road context, user vulnerability, and crash data.
• Alternative methods, such as NACTO’s “City Limits” approach, focus on a formulaic assessment of street activity and conflict potential to set safer speeds.
• Cities like Madison and Seattle have seen positive results from proactively lowering speed limits on residential streets.
• Organizations representing drivers, like the National Motorists Association, caution against setting limits that are inconsistent with road design and driver expectations.
• Research from the Insurance Institute for Highway Safety indicates a correlation between higher speed limits and increased fatality rates.
• The trend towards higher speed limits continues in some areas, with several states allowing 80 mph or more on certain highways.
• Ohio’s evolving guidelines now allow for greater consideration of roadway context and the use of the 50th percentile speed in pedestrian-heavy areas.
• The debate reflects a fundamental tension between facilitating efficient vehicle movement and ensuring the safety of all road users in diverse environments.

Future Outlook: Towards Safer, More Contextual Speed Management

The ongoing re-evaluation of the 85% rule signals a potential paradigm shift in how speed limits are conceived and implemented across the United States. As more data emerges about the relationship between speed and injury severity, and as cities continue to champion “Vision Zero” principles—the goal of eliminating all traffic fatalities and severe injuries—the pressure to move away from a one-size-fits-all approach will likely intensify. The FHWA’s updated guidance provides a crucial opening for states and localities to explore more nuanced and safety-focused speed management strategies. This could involve greater use of the methods advocated by NACTO, where speed limits are tailored to the specific characteristics and risks of each roadway, rather than being primarily dictated by prevailing driver behavior.

The example of Sylvania Township, Ohio, where an 85% rule analysis suggested raising a limit on a busy residential road that serves schools and a park, starkly illustrates the limitations of the traditional method. While county engineers adhered to the data from the rule, township officials sought to lower the speed. Ohio’s evolving approach, allowing consideration of the 50th percentile speed and roadway context in areas with significant pedestrian and cyclist presence, offers a glimpse into a more flexible and safety-conscious future. Many states are expected to follow suit, adopting policies that enable local authorities to set lower speed limits based on demonstrable safety needs, even if it deviates from the 85% standard.

However, the transition will not be instantaneous or without challenges. Resistance from motorists accustomed to higher speeds, the need for robust data collection and analysis, and potential enforcement hurdles will need to be addressed. Educational campaigns will be vital to foster public understanding and acceptance of the rationale behind these changes, emphasizing that lower speeds are not about punitive enforcement but about creating safer, more livable communities for everyone. The success of these initiatives will hinge on a commitment from transportation agencies to prioritize safety over expediency and to embrace a more comprehensive understanding of what constitutes a “safe” speed in a complex, multi-modal transportation system.

Call to Action

Residents concerned about speed limits in their communities are encouraged to engage with their local and state transportation authorities. Understanding the principles behind speed limit setting and advocating for data-driven, safety-focused approaches can influence policy. Supporting initiatives that prioritize pedestrian and cyclist safety, such as “Vision Zero” campaigns and efforts to implement context-sensitive speed management, is crucial. By fostering informed dialogue and demanding a transition from outdated methodologies to more contemporary, safety-conscious practices, communities can work towards creating roadways that are safer for all users.