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Rethinking the 85th percentile rule: Prioritizing safety over speed
5 Min Read
One of the longstanding principles in road safety is the 85th percentile rule, a traffic engineering standard that sets speed limits based on the speed at or below which 85% of vehicles are traveling under free-flowing conditions.
While the 85th percentile rule is based on the premise that most drivers naturally will travel at a speed they perceive to be safe, and setting speed limits accordingly would result in better compliance and safer roads, it is becoming increasingly clear that this axiom is no longer viable.
Today, speeding accounts for one in every three traffic-related fatalities in the United States. That amounts to an average of more than 33 deaths each day. The number of people struck and killed while walking on America’s streets, meanwhile, has jumped 75% since 2010. And if those numbers aren’t staggering enough, data indicate that numerous drivers in speeding-related crashes are engaged in risky driving behaviors, from not wearing a seat belt to cell phone use and drinking while driving.
In an era in which the safety of all road users—drivers, pedestrians and cyclists—must be prioritized, it is becoming abundantly clear that the time is now for state and local government officials to rethink the 85th percentile rule and adopt new approaches to create safer, more inclusive streets.
Limitations of the 85th percentile rule
While the 85th percentile rule proved to be effective when the primary goal was to ensure the smooth and efficient movement of vehicular traffic, it has always had significant limitations, particularly in urban environments where roads are shared by a wide range of users. These limitations include:
The safety of non-motorized users The 85th percentile rule primarily considers the behavior of drivers, often at the expense of pedestrians and cyclists. As a result, the higher speed limits resulting from the rule can create dangerous environments for vulnerable road users. Because urban streets are shared spaces, speed limits need to be set in a way that prioritizes safety for everyone, not convenience for drivers.
Urban complexity In densely populated urban areas, the presence of crosswalks, intersections, bike lanes and public transport facilities requires lower speeds to ensure safety. Unfortunately, the 85th percentile rule does not adequately account for such complexities, often resulting in speed limits that are much too high for such compromising conditions.
Encouraging higher speeds Because the 85th percentile speed is influenced by a small percentage of drivers who exceed safe speeds, speed limits are gradually pushed higher over time. As a result, basing speed limits on the 85th percentile rule tends to perpetuate a cycle in which each iteration of speed surveys leads to incrementally higher speed limits and riskier driver behavior.
Public health and environmental concerns Because the 85th percentile rule ultimately results in higher speeds, it also contributes to the increased emissions and noise pollution which accompany those speeds. Conversely, lowering speed limits not only slows traffic but leads to better air quality and a more pleasant urban environment that encourages more walking and cycling—all of which have significant public health benefits.
A new approach to speed management
With those inherent limitations in mind, government officials and traffic engineers must push for a new approach to traffic management that bypasses the 85th percentile rule in favor of measures that prioritize safer, more inclusive streets. Such an approach would likely encompass four key steps to better control traffic flow and manage speed:
Vision zero This strategy, which aims to eliminate all traffic fatalities and severe injuries, would redesign current (and design future) road systems in a way that considers the inherent vulnerability of the human body and the need for safer speeds to protect all road users. Speed limits would be set based on what is survivable in a collision, typically 20-30 mph in urban areas.
Contextual speed limits Speed limits would be based on the specific context of the road environment, with consideration given to factors such as pedestrian density, proximity to schools, and the presence of bike lanes. This more nuanced approach ensures speed limits that reflect the actual conditions and risks which are present in different areas, as opposed to a “one rule for all” process.
Traffic calming measures To create environments in which drivers are encouraged to drive at safer speeds, regardless of posted speed limits, this measure would use physical design features such as road narrowing, speed humps, raised crosswalks and roundabouts that naturally slow down traffic.
Automated enforcement Enforcement tools that consistently discourage speeding and reinforce the importance of adhering to speed limits for safety—including speed cameras and other automated enforcement tools—would be employed to ensure compliance with lower speed limits.
It is important to note, however, that these steps will be ineffective without a sufficient level of community engagement. Involving local communities in the decision-making process for setting speed limits ensures the needs and concerns of all road users are addressed. Public input can also provide valuable insights into areas where lower speed limits are essential for safety.
How do we know such an approach to speed management will work? Quite simply, the proof is readily found in those cities which have already moved away from the 85th percentile rule. New York, for example, lowered the default speed limit to 25 mph in 2014. This change has led to a significant reduction in traffic fatalities and injuries, particularly among pedestrians. Similarly, London’s introduction of 20 mph speed limits in many areas has resulted in a noticeable decrease in traffic-related injuries and deaths. Oslo, Norway’s reduction of speed limits to 19 mph in residential areas and implementation of comprehensive traffic calming measures has achieved zero pedestrian and cyclist fatalities.
In short, the 85th percentile rule, once the cornerstone for setting speed limits, is no longer adequate for the complex, multimodal environments of today’s urban areas. To create safe streets for everyone, we must adopt a new approach that prioritizes the safety and well-being of all road users.
