When Driving Data Clashes with Community Safety: States Rethink the 85% Rule for Speed Limits
Decades-old traffic engineering principles, designed for a different era, are being challenged as a surge in road fatalities prompts a reevaluation of how speed limits are set.
For years, residents of Sylvania Township, Ohio, have voiced concerns about the speed limit on Mitchaw Road. Rose Hammond, an 85-year-old resident, has been a vocal advocate, highlighting the presence of an assisted living community, a church, two schools, and a popular park frequented by children for youth sports leagues. Her pleas to lower the 55 mph speed limit, often punctuated by the roar of motorcycles racing by, centered on a simple question: “What are you waiting for, somebody to get killed?”
In response to this mounting public pressure, Sylvania Township requested an analysis of Mitchaw Road’s posted speed limit. The findings, however, presented a surprising paradox: according to the prevailing methodology, the road’s speed limit was technically too low, suggesting an increase to 60 mph. This outcome is rooted in a long-standing principle in traffic engineering known as the 85% rule, a concept that has guided speed limit setting across the United States for decades, even in contexts vastly different from those for which it was originally developed.
This discrepancy between community safety concerns and established traffic engineering practices is at the heart of a national conversation. As road fatalities climb, states are beginning to re-examine their reliance on the 85% rule, a guideline born from studies conducted in the 1930s and 1940s. These foundational principles, while once considered the gold standard, are increasingly being scrutinized for their applicability to today’s complex and evolving road networks, particularly in urban and suburban areas where mixed-use and pedestrian activity are common.
Context & Background: The Enduring Influence of the 85% Rule
The 85% rule, a cornerstone of speed limit determination for much of the 20th century, operates on a seemingly intuitive premise: the safest speed for a road is the speed at which most drivers naturally travel. Specifically, it suggests that the posted speed limit should be set at the speed at or below which 85% of drivers are traveling, rounded to the nearest 5 mph increment, under free-flowing traffic conditions. The underlying philosophy is that drivers, by and large, possess an innate understanding of a road’s safe operating speed. If the posted limit is too low, drivers will simply “vote with their feet” and exceed it. Conversely, if the limit is too high, they will naturally slow down.
This approach, deeply ingrained in traffic engineering manuals and state statutes, has provided a seemingly objective and data-driven method for setting speed limits. Its proponents argue that adhering to the 85th percentile speed helps minimize the speed differential between drivers, thereby reducing the likelihood of erratic behavior and potential conflicts. The Institute of Transportation Engineers (ITE), an influential professional organization, has historically supported this methodology. In a past brochure, the ITE described this driver behavior as a form of democratic consensus on the road.
However, this “driver-led” approach has drawn significant criticism in recent years, particularly in light of rising traffic fatalities and a growing understanding of the impact of speed on crash severity. Critics, such as Jenny O’Connell, director of member programs for the National Association of City Transportation Officials (NACTO), point to a potential feedback loop inherent in the 85% rule. “People speed, and then the speed limits will be ratcheted up to match that speed,” she explained. This cycle, they argue, can lead to a continuous escalation of speed limits that do not necessarily reflect the safety needs of all road users, especially vulnerable populations like pedestrians and cyclists.
The research underpinning the 85% rule originated in an era with far less dense road networks, lower vehicle speeds, and a different pattern of land use. Modern urban and suburban streets, often characterized by mixed-use development, increased pedestrian activity, and the presence of schools, parks, and residential areas, present a stark contrast to the rural roads of the mid-20th century. The NACTO, recognizing this evolution, developed an alternative framework called “City Limits.” This approach prioritizes minimizing injury risk by incorporating factors beyond simply observing free-flowing traffic speeds. It emphasizes a street’s activity level, the potential for conflicts between different modes of transportation, and the overall risk to all road users.
The federal government, through the Federal Highway Administration (FHWA), has also begun to signal a shift away from an over-reliance on the 85% rule. In its first update to national guidelines for traffic signs since 2009, the FHWA clarified that the 85% rule is not an absolute mandate. The updated manual encourages communities to consider a broader range of factors, including road usage patterns, the safety of pedestrians and cyclists, and crash frequency, when setting speed limits. While some advocates, like Leah Shahum, director of the Vision Zero Network, an organization dedicated to eliminating traffic fatalities, acknowledge this as a positive step, they also note that the change may not be transformative enough and that many jurisdictions continue to prioritize the simplicity and familiarity of the 85% rule.
“The 85th percentile should not be the Holy Grail or the Bible, and yet over and over again, it is accepted as that,” Shahum stated, underscoring the deep-seated adherence to the traditional methodology.
In-Depth Analysis: The Limitations and Criticisms of the 85% Rule
The foundational studies that led to the 85% rule were conducted in a bygone era. Research from the 1930s and 1940s, often on rural roadways with limited development and fewer non-motorized users, informed the concept that driver behavior dictates safe speeds. This research assumed a relatively homogenous traffic stream and a road environment where pedestrian and cyclist interaction was minimal. However, the modern transportation landscape is vastly different. Urban and suburban arteries are now complex environments where high-speed vehicular traffic frequently intersects with pedestrian crossings, bicycle lanes, schools, parks, and residential neighborhoods.
Jenny O’Connell of NACTO articulates a primary criticism: the “feedback loop.” When drivers perceive a speed limit as too low for the conditions they encounter – for instance, on a wide, straight road that encourages higher speeds – they may exceed the posted limit. If this behavior is prevalent, the 85% rule dictates that the speed limit should be raised to match this observed behavior. This creates a cycle where speed limits are adjusted to conform to prevailing speeds, rather than proactively setting limits that prioritize the safety of all road users. This can inadvertently legitimize and encourage faster driving, potentially at the expense of those less protected, such as pedestrians and cyclists.
The “City Limits” initiative by NACTO offers an alternative by proposing a more holistic approach. This framework emphasizes setting speed limits based on a formula that considers the specific context of a street. Key elements include the road’s design, the level of activity along the corridor (e.g., businesses, residences, schools), the presence and volume of vulnerable road users, and the likelihood of potential conflicts or crashes. The core idea is to move away from a purely observational, driver-dominant method to one that proactively designs for safety and considers the needs of the entire community. The report accompanying this initiative highlights that “these historic roads are a far cry from the vibrant streets and arterials that typify city streets today,” underscoring the obsolescence of the original research’s context.
The Federal Highway Administration’s (FHWA) guidance, while not mandating a complete abandonment of the 85% rule, encourages a broader perspective. By clarifying that the rule is not a strict regulation and by encouraging consideration of factors like road use, pedestrian risk, and crash frequency in its updated manual for traffic signs, the FHWA has subtly nudged states toward more nuanced speed limit setting. This change aims to empower local authorities to set limits that better reflect local conditions and safety objectives.
However, the persistence of the 85% rule as the primary determinant of speed limits in many jurisdictions is a significant challenge. Leah Shahum of the Vision Zero Network notes that despite these federal nudges, the 85th percentile “is not the Holy Grail or the Bible, and yet over and over again, it is accepted as that.” This suggests a cultural inertia within traffic engineering and a preference for the quantifiable simplicity of the 85% rule, even when it may not align with contemporary safety goals.
Cities like Madison, Wisconsin, are actively working against this inertia. Their “20 Is Plenty” campaign involves lowering speed limits on residential streets from 25 mph to 20 mph. Similarly, a pilot program in Seattle that reduced speed limits on certain streets not only saw a decrease in serious injury crashes but also a 7% drop in the 85th percentile speed. This indicates that intentional efforts to lower speed limits can be effective and can, in turn, influence observed driving speeds.
California serves as an interesting case study. The state traditionally leans heavily on the 85% rule. However, recent legislative changes have provided local governments with more flexibility to deviate from these guidelines, provided they can demonstrate a safety need. While proponents of pedestrian and bicycle safety, such as Kendra Ramsey of the California Bicycle Coalition, view these changes as beneficial, they also argue that they do not go far enough. Ramsey points to a prevailing mindset that prioritizes automobiles, suggesting that “there’s still a very heavy mindset that automobiles are the primary method of travel and they should be given priority and reverence.”
Conversely, organizations like the National Motorists Association, represented by Jay Beeber, advocate for the continued use of the 85% rule. Beeber argues that it is often the most effective method for minimizing speed variations among drivers. He contends that the road’s physical characteristics often dictate how drivers will use it, and that setting a speed limit significantly lower than what the road design seems to encourage can lead to unfair enforcement. “It doesn’t really matter what number you put on a sign,” Beeber stated. “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 highlights the tension between engineering for perceived efficiency and engineering for mandated safety outcomes.
The debate over the 85% rule, therefore, is not simply about numbers; it is about fundamental philosophies of road design, the role of technology in driving, and the prioritization of different users within the transportation system. As states grapple with increasing road safety challenges, the relevance and application of this long-standing methodology are under increasing scrutiny.
Pros and Cons of the 85% Rule
The 85% rule, while facing increasing criticism, does have certain advantages that have contributed to its widespread adoption and continued use.
Pros:
- Simplicity and Objectivity: The rule provides a straightforward, data-driven method for setting speed limits. It relies on observable driver behavior, making the process appear objective and less susceptible to subjective political pressures.
- Driver Acceptance and Compliance: Proponents argue that when speed limits are set closer to the speed most drivers are already traveling, it leads to greater voluntary compliance and reduces instances of speeding. This can, in theory, lead to more uniform traffic flow.
- Alignment with Road Design (Historically): In an era when roads were often designed with less consideration for mixed-use or pedestrian activity, the 85% rule could be seen as aligning with the intended operational speeds of those roadways.
- Reduced Speed Variation: By setting limits closer to the 85th percentile speed, the goal is to minimize the gap between the fastest and slowest drivers, potentially leading to smoother traffic flow and fewer overt conflicts caused by extreme speed differences.
- Ease of Implementation: Collecting speed data and applying a simple calculation is generally less complex and resource-intensive than implementing more sophisticated safety-focused methodologies.
Cons:
- “Feedback Loop” Phenomenon: As highlighted by critics, the rule can create a cycle where prevailing speeds are simply codified into higher speed limits, rather than proactively setting limits for safety. This can lead to a continuous escalation of speeds.
- Disregard for Vulnerable Road Users: The rule primarily focuses on vehicular traffic and driver behavior, often neglecting the safety needs of pedestrians, cyclists, children, and the elderly, especially in areas with high mixed-use activity.
- Outdated Research Basis: The studies that established the 85% rule were conducted decades ago on different types of roads with different traffic characteristics and land use patterns than modern urban and suburban environments.
- Ignores Crash Severity: The rule does not inherently account for the fact that higher speeds drastically increase the likelihood and severity of injuries and fatalities in the event of a crash. A 5 mph increase can have significant consequences.
- Can Encourage Speeding: By setting limits that reflect existing high speeds, the rule can implicitly condone or legitimize behavior that is demonstrably less safe.
- Assumes Homogeneous Driver Behavior: The rule assumes a uniform understanding of safe speeds among drivers, which may not hold true across diverse driving populations and skill levels.
- Inflexibility in Context: Applying the same observational methodology to vastly different road contexts (e.g., a quiet residential street versus a multi-lane arterial) can lead to inappropriate speed limits.
Key Takeaways
- The 85% rule, a long-standing method for setting speed limits based on observed prevailing speeds, is being re-evaluated across the U.S. due to a rise in traffic fatalities.
- Critics argue the 85% rule can create a feedback loop, where higher speeds lead to higher speed limits, and that it often prioritizes vehicular speed over the safety of pedestrians and cyclists.
- The rule’s origins are in mid-20th century studies of rural roads, which are contextually different from modern urban and suburban environments.
- Alternative approaches, like NACTO’s “City Limits,” focus on street context, activity levels, and the risk to all road users to set safer speed limits.
- The Federal Highway Administration (FHWA) has encouraged states to consider a broader range of factors beyond the 85% rule when setting speed limits.
- Cities like Madison, Wisconsin, are proactively lowering speed limits on residential streets, and pilot programs in Seattle have shown positive safety outcomes.
- States like California are gradually allowing more local flexibility, but advocates argue that a deeper shift in mindset is needed to prioritize all road users.
- Organizations representing drivers argue that the 85% rule helps maintain consistent traffic flow and that setting limits too far below observed speeds can be unfair.
- The debate involves balancing driver convenience and perceived efficiency with the critical need to reduce road deaths and injuries, particularly for vulnerable populations.
Future Outlook: Shifting Towards a Safety-First Paradigm
The current momentum suggests a slow but steady shift away from an exclusive reliance on the 85% rule in speed limit setting. The Federal Highway Administration’s updated guidance, while subtle, provides a crucial governmental endorsement for a more nuanced approach. This endorsement empowers states and local jurisdictions to incorporate a wider array of safety considerations into their decision-making processes. States like Ohio are already exploring these avenues, with updated guidelines that allow for greater consideration of roadway context and the potential to use lower percentiles (like the 50th percentile) in areas with significant pedestrian and cyclist activity.
The success of initiatives in cities like Madison and pilot programs in Seattle offers compelling evidence that lowering speed limits, even when they deviate from the 85th percentile, can yield positive safety outcomes. These examples may encourage more jurisdictions to follow suit, especially as the human and economic costs of traffic crashes become increasingly evident.
However, overcoming the entrenched reliance on the 85% rule will require sustained effort. Advocacy groups will likely continue to push for legislative changes that mandate or strongly encourage safety-focused methodologies. Traffic engineering practices may evolve to incorporate more advanced tools and data analytics that can better model the risks associated with different speed levels in complex environments. Furthermore, public awareness campaigns that educate communities about the relationship between speed and safety could foster greater support for lower speed limits, as seen with Rose Hammond’s persistent advocacy in Sylvania Township.
The challenge for states will be to find a balance between adopting data-driven safety principles and addressing concerns about driver convenience and the practicalities of enforcement. The future likely holds a more diversified approach to speed limit setting, where the 85% rule might still be a factor, but no longer the sole determinant. Instead, it will likely be one of many inputs, weighed against a stronger emphasis on minimizing risk for all road users, particularly in areas where people live, work, and play.
The increasing adoption of Vision Zero principles—the commitment to eliminating all traffic fatalities and severe injuries—by cities and even some states will undoubtedly drive this evolution. As more communities adopt a “people-first” approach to transportation, the methods used to set speed limits will naturally need to adapt to reflect these safety priorities.
Call to Action
For residents concerned about speed limits in their communities, understanding the principles behind speed limit setting is crucial. Engaging with local officials, participating in public comment periods for transportation projects, and supporting advocacy groups like the Vision Zero Network or local pedestrian and bicycle coalitions can all contribute to shaping safer streets.
For policymakers and transportation engineers, the call to action involves a critical re-evaluation of current practices. Embracing updated federal guidance, exploring alternative methodologies that prioritize safety, and fostering collaboration between different stakeholders – including community members, safety advocates, and drivers – are essential steps. Investing in data collection and analysis that goes beyond simple speed observations to include factors like pedestrian exposure and crash severity will be key to developing effective and equitable speed management strategies.
Ultimately, the conversation about speed limits is a conversation about the kind of communities we want to build and live in. It’s about ensuring that our streets are safe and accessible for everyone, regardless of how they choose to travel. By moving beyond outdated methodologies and embracing a safety-first paradigm, we can work towards a future with fewer traffic tragedies.
Fast Company: States rethink setting speed limits based on how fast drivers travel
Federal Highway Administration: Traffic Signs
National Association of City Transportation Officials (NACTO)
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