Context & Background: The 85% Rule and Its Origins

The current framework for setting speed limits in many parts of the United States is heavily influenced by a concept developed in the mid-20th century: the 85% rule. This principle, rooted in studies conducted on rural roads during the 1930s and 1940s, posits that the safest speed for a road is the speed at which 85% of drivers travel when traffic is flowing freely. The idea is that drivers, in their collective wisdom, will gravitate towards a speed that feels safe and manageable for the road’s conditions. The posted speed limit, therefore, should ideally align with this observed 85th percentile speed, rounded to the nearest 5 mph increment.

This approach was born from an era where the automobile was becoming increasingly dominant, and the focus was largely on facilitating efficient vehicle movement. The rationale was that if a majority of drivers were traveling at a certain speed, that speed was likely perceived as safe and appropriate for the road’s design. The Institute of Transportation Engineers, in an old brochure, even described this phenomenon as drivers “voting with their feet,” implying that their driving habits offered a democratic consensus on the optimal speed.

However, critics argue that this method creates a self-perpetuating cycle. As Jenny O’Connell, director of member programs for the National Association of City Transportation Officials (NACTO), explains, “People speed, and then the speed limits will be ratcheted up to match that speed.” This can lead to a gradual escalation of speeds on roadways, potentially increasing the severity of crashes when they do occur. NACTO, in response to the limitations of the 85% rule, has developed an alternative called “City Limits.” This framework aims to minimize injury risk by incorporating factors such as a street’s activity level and the potential for conflicts between different road users, moving beyond a purely vehicle-speed-centric model.

The foundational research for the 85% rule was conducted in a very different era. The report from NACTO points out that “these historic roads are a far cry from the vibrant streets and arterials that typify city streets today.” Modern urban and suburban environments often feature a much higher density of mixed-use development, increased pedestrian and cyclist activity, and a greater presence of schools, parks, and residential communities. Applying a rule developed for predominantly rural, low-conflict environments to these complex, multi-modal settings is increasingly being seen as problematic.

The Federal Highway Administration (FHWA), in its most recent update to its national guidelines manual (first updated since 2009), has subtly but significantly nudged states away from an over-reliance on the 85% rule. The agency clarified that while the 85th percentile speed can be a consideration, communities should also take into account other critical factors. These include how a road is actually used, the inherent risks to pedestrians and cyclists, and the frequency of crashes. This guidance acknowledges that the 85% rule, while seemingly objective, may not always align with the broader goals of traffic safety and community well-being.

Leah Shahum, director of the Vision Zero Network, an organization advocating for street safety, expressed a desire for the manual to have more strongly de-emphasized the 85% rule. Nevertheless, she recognizes that the FHWA’s clarification is a step in the right direction and is already influencing how some states approach speed limit setting. However, she notes that “The 85th percentile should not be the Holy Grail or the Bible, and yet over and over again, it is accepted as that.” This suggests that inertia and the familiarity of the old system are still significant hurdles to overcome.

In-Depth Analysis: The Case for Change

The impetus for reconsidering the 85% rule stems from several interconnected factors, most notably a recent nationwide increase in traffic fatalities and a growing awareness of the diverse needs of all road users. The story of Rose Hammond, an 85-year-old resident in northwest Ohio, exemplifies the real-world frustrations that arise when speed limits fail to account for local context. Hammond has been advocating for years to lower the 55 mph speed limit on Mitchaw Road, a two-lane thoroughfare that passes her assisted living community, a church, two schools, and a busy park. She voiced her concerns, asking officials, “What are you waiting for, somebody to get killed?” Her complaints about speeding motorcycles were met with a study that, based on the 85% rule, technically suggested the speed limit should be *raised* to 60 mph, a finding that surprised local officials but not the engineers who conducted the analysis according to the established methodology.

This anecdote highlights a core criticism of the 85% rule: it can prioritize vehicular speed over community safety, especially in areas with significant pedestrian and non-motorized traffic. The “drivers set the speed” philosophy, while appealing in its simplicity, can inadvertently endorse higher speeds in environments where lower speeds are crucial for preventing injuries and fatalities. The feedback loop described by O’Connell is particularly concerning when applied to streets that are not simply conduits for traffic, but rather integral parts of community life.

Cities are beginning to experiment with alternative approaches. In Wisconsin, the capital city of Madison, under its “20 Is Plenty” campaign, has been actively reducing speed limits on residential streets from 25 mph to 20 mph. Similarly, a pilot program in Seattle several years prior that lowered speed limits saw a reduction in serious injury crashes and a decrease in the observed 85th percentile speed. These initiatives suggest that actively setting lower, more appropriate speed limits can influence driver behavior and improve safety outcomes, rather than simply reflecting existing driving habits.

California serves as an interesting case study, as it has historically relied heavily on the 85% rule for setting speed limits. However, recent legislative changes have provided local governments with slightly more latitude to deviate from these guidelines, provided they can demonstrate a clear safety need. Kendra Ramsey, executive director of the California Bicycle Coalition, acknowledges this progress but argues it is insufficient, stating, “We still have a long way to go in California in terms of putting value on all road users. There’s still a very heavy mindset that automobiles are the primary method of travel and they should be given priority and reverence.” This sentiment points to a deeply ingrained cultural prioritization of the automobile in some regions, which the 85% rule can inadvertently reinforce.

Conversely, proponents of the 85% rule, such as Jay Beeber, executive director for policy at the National Motorists Association, argue that adhering to it is the most effective way to minimize speed differentials among drivers. He contends that a road’s design often implicitly encourages a certain speed, and penalizing drivers for exceeding a lower, arbitrarily set limit would be unfair. “It doesn’t really matter what number you put on a sign,” Beeber states. “The average driver drives the nature of the roadway. It would be patently unfair for a government to build a road to encourage people to drive 45 mph, put a 30 mph speed limit on it, and then ticket everyone for doing what they built the road to do.” This perspective emphasizes the importance of road design in influencing speed and the potential for mismatched speed limits to lead to enforcement issues and driver frustration.

The trend toward higher speed limits on highways is also a significant factor. The repeal of the national maximum speed limit in 1995 and the subsequent handover of authority to states have led to a gradual increase in posted limits. North Dakota recently became the ninth state to allow 80 mph on some highways, and Texas has an 85 mph segment. While these higher limits are often applied to major interstates outside of urban centers, research from the Insurance Institute for Highway Safety (IIHS) underscores the risks associated with increased speed. A 2019 study found that every 5 mph increase in a state’s maximum speed limit correlates with an 8.5% increase in the likelihood of fatalities on interstates and a 2.8% increase on other roads. Chuck Farmer, the IIHS’s vice president for research, who conducted the study, noted the disconnect between modern vehicle insulation and driver perception of speed: “Maybe back when you were driving a Model T, you had a real feel for how fast you were going. But in modern vehicles, you don’t have a sense of what 80 mph is. You’re in a cocoon.” This suggests that the increased speeds are not necessarily driven by a better understanding of the road or increased safety, but rather by the vehicle’s capability and a diminished perception of risk.

In-Depth Analysis: The Case for Change (Continued)

The situation in Sylvania Township, Ohio, where residents like Rose Hammond are advocating for a reduction in speed on Mitchaw Road, encapsulates the tension between the traditional 85% rule and the desire for safer, more livable streets. The county engineers’ finding that the 55 mph limit was technically too low according to the 85% rule, suggesting it should be 60 mph, created a dilemma for township officials who sought to lower it to 40 mph or less. Lucas County Engineer Mike Pniewski emphasized the importance of data-driven decision-making, stating, “If we don’t make decisions based on data, it’s very difficult to make good decisions.” However, the definition of “good decisions” in this context is what is being debated – decisions that prioritize traffic flow or those that prioritize community safety and well-being?

Fortunately, Ohio’s approach to speed limits is evolving. While state law previously set maximum speeds for various roadway types irrespective of local context, the state now incorporates more consideration for roadway characteristics. Critically, Ohio law allows cities to reduce speed limits based on a lower standard, the 50th percentile speed, when there is a significant presence of pedestrians and cyclists. This legislative flexibility acknowledges that different road environments require different speed management strategies. Furthermore, authorities in Ohio have hired consultants to explore additional modifications, drawing inspiration from best practices in other states.

Michelle May, who manages Ohio’s highway safety program, noted this gradual shift: “States have very slowly started to move away from the 85th percentile as being kind of the gold standard for decision-making.” She articulates the core reason for this movement: “People are traveling and living differently than they did 40 years ago, and we want to put safety more at the focus.” This statement underscores the fundamental reorientation occurring in transportation planning – a move from a car-centric paradigm to one that embraces a more holistic view of road safety and urban design.

Despite these evolving policies and the growing body of research highlighting the limitations of the 85% rule, the implementation of meaningful change on specific roads like Mitchaw Road can still be a slow and arduous process. Rose Hammond’s ongoing efforts and her sentiment of being “so discouraged” reflect the challenges faced by community advocates who are trying to effect change in a system that is often resistant to altering long-standing practices. The question of whether the changes in Ohio’s guidelines will ultimately impact the posted speed on Mitchaw Road remains uncertain, illustrating that policy shifts at the state level do not always translate immediately into tangible improvements on the ground.

The federal government’s updated guidance from the FHWA is a crucial catalyst. By clarifying that the 85% rule is not a strict mandate but rather one factor among many, the agency empowers local authorities and state departments of transportation to adopt more nuanced approaches. This includes prioritizing data beyond just prevailing speeds, such as crash statistics, pedestrian volumes, and the presence of sensitive land uses like schools and parks. This broadened perspective allows for speed limits to be set as a proactive safety measure, rather than a reactive reflection of observed behavior.

The “City Limits” initiative by NACTO, for instance, offers a framework that considers what they term “road diets” and the potential for “road user conflicts.” This approach encourages engineers to think about streets as shared public spaces rather than mere conduits for vehicular traffic. The formula they propose aims to create speed limits that reflect the street’s functional classification, its design characteristics, and the specific safety needs of the users within that context. For example, a street identified as a neighborhood connector with a high incidence of children walking to school might warrant a significantly lower speed limit than a highway designed for high-speed, long-distance travel.

The debate is not simply about lowering speeds for the sake of it; it is about creating streets that are safe and accessible for everyone. Advocates for pedestrians and cyclists often point to the disproportionate impact of higher speeds on their safety. At higher speeds, the kinetic energy of a vehicle increases exponentially, making crashes more likely and far more severe. For a pedestrian struck by a vehicle traveling at 20 mph, the survival rate is significantly higher than if struck by a vehicle traveling at 40 mph or 60 mph. This fundamental physics of impact is a strong argument for setting speed limits that reflect the reality of human vulnerability.

The insurance industry’s perspective, as represented by the IIHS study, also lends weight to the argument for more conservative speed limits, particularly on higher-speed roads. While the focus of much of the current re-evaluation is on urban and suburban streets, the findings on highway fatalities underscore a broader concern about the relationship between speed and safety. The “cocoon” effect of modern vehicles, where drivers may not accurately perceive their speed, further complicates the notion that drivers can be trusted to self-regulate their speed based on the road’s perceived conditions.

Pros and Cons

Pros of Revising the 85% Rule and Adopting New Speed Setting Methodologies:

• Enhanced Safety for Vulnerable Road Users: Prioritizes the safety of pedestrians, cyclists, and children by setting lower, more appropriate speeds in areas with mixed traffic and high activity levels. This aligns with the principles of Vision Zero, aiming to eliminate traffic fatalities and serious injuries.
• Reflects Modern Roadway Context: Acknowledges that urban and suburban environments have evolved significantly since the 1930s/1940s research that informed the 85% rule, with increased density, mixed-use development, and greater pedestrian and cyclist presence.
• Reduces Crash Severity: Lower speeds directly correlate with reduced impact forces in the event of a collision, significantly increasing the chances of survival and decreasing the severity of injuries for all involved.
• Encourages Multi-Modal Transportation: Creating safer streets can encourage more people to walk, bike, and use public transit, contributing to healthier communities and reduced traffic congestion.
• Addresses the “Feedback Loop” Problem: Moves away from a system where speeding automatically leads to higher speed limits, thus breaking a cycle that can lead to progressively faster and more dangerous traffic conditions.
• More Holistic Data Integration: Allows for the incorporation of a wider range of data beyond just prevailing speeds, including crash history, pedestrian volumes, street design, and land use, leading to more context-sensitive speed limit decisions.
• Federal Support: The FHWA’s updated guidance signals a national shift and provides a foundation for states and local jurisdictions to implement more safety-focused speed management strategies.

Cons of Revising the 85% Rule and Adopting New Speed Setting Methodologies:

• Potential for Increased Driver Frustration: Drivers accustomed to higher speeds may perceive lower limits as overly restrictive or arbitrary, potentially leading to reduced compliance and increased frustration.
• Enforcement Challenges: Lowering speed limits without concurrent infrastructure changes or increased enforcement may lead to a higher proportion of drivers exceeding the posted limit, creating enforcement difficulties.
• Resistance to Change: The 85% rule is a long-standing and familiar methodology, making it difficult to implement new approaches due to institutional inertia and established practices.
• Perceived Impact on Travel Times: Drivers may worry that reduced speed limits will significantly increase their commute times, particularly on arterial roads or highways.
• Cost of Implementation: Redesigning roads to support lower speeds or implementing new data collection and analysis processes for setting speed limits can involve upfront costs.
• Conflicting Interests: Balancing the needs of drivers who prioritize rapid vehicle movement with the safety needs of pedestrians and cyclists can lead to conflicting demands and public debate.
• “Road Diet” Misconceptions: Some may view efforts to reduce speeds as an attack on drivers’ freedom or an impediment to efficient traffic flow, rather than a necessary measure for overall safety and community livability.

Key Takeaways

• The 85% rule, a decades-old method for setting speed limits, bases limits on the speed at which 85% of drivers travel, assuming this reflects a safe speed.
• Critics argue the 85% rule creates a feedback loop where speeding leads to higher speed limits, and it fails to adequately account for pedestrians, cyclists, and urban context.
• Recent trends show a nationwide increase in traffic fatalities, prompting a re-evaluation of speed setting practices.
• The Federal Highway Administration (FHWA) has updated its guidance, encouraging states to consider factors beyond the 85th percentile speed, such as road use and pedestrian risk.
• Cities like Madison, Wisconsin, are proactively lowering speed limits on residential streets, and pilot programs in Seattle have shown positive safety results.
• Advocacy groups, such as the Vision Zero Network and the California Bicycle Coalition, are pushing for greater prioritization of all road users’ safety over vehicular speed.
• Organizations like the National Motorists Association defend the 85% rule, arguing it minimizes speed variance among drivers and that road design influences observed speeds.
• Studies, like one from the Insurance Institute for Highway Safety, link increases in maximum speed limits to higher fatality rates.
• States like Ohio are beginning to incorporate more contextual factors and allow for lower speed limits in areas with significant pedestrian activity.
• The challenge lies in shifting from a car-centric approach to one that balances vehicular flow with the safety and accessibility of streets for all members of the community.

Future Outlook

The momentum to move away from an exclusive reliance on the 85% rule for setting speed limits appears to be growing, driven by increasing traffic fatalities, advocacy for vulnerable road users, and updated federal guidance. We can anticipate a continued trend of states and local jurisdictions experimenting with and adopting more data-driven, context-sensitive approaches to speed management. This may involve greater use of “Vision Zero” principles, which advocate for a systemic approach to eliminating traffic deaths and serious injuries.

Expect to see more cities implementing lower speed limits on residential streets and in areas with high pedestrian and cyclist activity, supported by infrastructure changes such as traffic calming measures (e.g., speed humps, curb extensions, narrower lanes) that naturally encourage slower speeds. The concept of “complete streets,” which prioritizes all modes of transportation, will likely gain further traction, influencing speed limit decisions to better serve pedestrians, cyclists, and transit users.

However, the transition will likely not be without its challenges. Public perception, political will, and the practicalities of enforcement will continue to play significant roles. It will be crucial for transportation agencies to effectively communicate the rationale behind these changes and to demonstrate their positive impact on safety and community livability. Furthermore, the evolving nature of urban development and the increasing adoption of new mobility technologies will necessitate ongoing adaptation and refinement of speed management strategies.

Ultimately, the future of speed limit setting in the U.S. is likely to be characterized by a more nuanced understanding of how speed interacts with safety, land use, and the diverse needs of a modern populace. The old adage of “drivers set the speed” may gradually give way to a philosophy where responsible governance and thoughtful design set the pace for safer, more equitable streets.

Call to Action

Residents concerned about speed limits in their communities are encouraged to engage with their local and state transportation officials. Understanding the principles behind speed limit setting and advocating for data-driven, safety-focused approaches can make a significant difference. Supporting initiatives that prioritize pedestrian and cyclist safety, and encouraging the adoption of holistic street design principles, are crucial steps towards creating safer and more livable environments for everyone.

Learn More and Get Involved:

• National Association of City Transportation Officials (NACTO): Explore their “City Limits” initiative and resources on street design. Visit NACTO
• Vision Zero Network: Discover how communities are working towards eliminating traffic fatalities and injuries. Visit Vision Zero Network
• Federal Highway Administration (FHWA): Review their updated guidance on speed management and traffic safety. Visit FHWA Safety
• Insurance Institute for Highway Safety (IIHS): Access research and studies on traffic safety and speed. Visit IIHS