What if the equation that controls most speed limits in America was established for rural highways nearly a century ago? That is the uncomfortable truth facing transportation engineers and policymakers as they revisit the “85% rule,” a 1930s and 1940s traffic studies-based practice still determining speed limits on pedestrian-filled, cyclist-filled, and dense city activity streets.
The 85% rule is more straightforward than it appears: take free-flow speed measurements, find the speed at or below which 85 percent of drivers are operating, and round to the nearest 5 mph. The technique relies on the notion that the collective wisdom of drivers generates the safest speed a notion that perhaps was reasonable in lightly used rural roads but fails in heavy, multimodal conditions. As Jenny O’Connell of the National Association of City Transportation Officials cautions, “People speed, and then the speed limits will be ratcheted up to match that speed.” This feedback loop can incrementally increase operating speeds, eroding safety objectives.
The rule has been criticized for ignoring basic principles of human injury tolerance. The Safe System approach, adopted by the U.S. Department of Transportation in its National Roadway Safety Strategy, restates the issue: speed should be regulated on the basis of human bodies’ kinetic energy tolerance, not current driving practice. Scientific evidence demonstrates that decreasing average urban speed by 1 mph reduces injury by 5%. For pedestrians, chances of death increase from about 8 percent at 31 mph to 50 percent at 47 mph.
Such cities as Madison, Wisconsin, and Seattle, Washington, are doing this philosophy justice. Madison’s “20 is Plenty” initiative reduced residential speed limits from 25 to 20 mph. Seattle’s previous pilot not only lowered serious injury crashes but also reduced the 85th percentile speed by 7 percent. Such changes are in accordance with the Vision Zero approach, which emphasizes safe mobility for everyone over vehicular throughput. In Seattle, arterials speed limits decreased from 30 to 25 mph, accompanied by new signs and public education, resulting in a statistically significant decrease in injury crash probability 17 percent downtown and 20 percent on arterials.
The Federal Highway Administration’s recent revision of the Manual on Uniform Traffic Control Devices made it clear that the 85% rule is guidance, not requirement, and encouraged engineers to balance context: pedestrian use, crash history, and street function. Some states are taking action. Ohio has speed setting based on the 50th percentile permitted when vulnerable road users are common, and is considering further reforms. California, long a stronghold of the 85% rule, has begun granting localities limited authority to deviate for safety reasons.
Engineering measures are key to the effectiveness of lower limits. The Institute of Transportation Engineers calls for “self-enforcing” highway design narrow lanes, curb extensions, roundabouts, and medians that prompts motorists to slow down without ongoing enforcement. Salisbury, Maryland, is transforming overbuilt five-lane collectors into quieter streets with bike boulevards and lane reductions through a $12 million federal Safe Streets and Roads for All grant.
Technology is also coming into play. Speed cameras, when implemented with equity protections, have reduced road fatalities and injuries by 20 to 37 percent in American programs. New York City’s wide network lowered speeding at camera locations by 72 percent. Intelligent Speed Assistance (ISA), required in all new EU cars from 2024, employs GPS and sign recognition to avoid speeding; a New York City pilot determined ISA-equipped fleet cars met limits 99 percent of the time.
The stakes are high. A 2019 Insurance Institute for Highway Safety study found that each 5 mph increase in a state’s maximum speed limit raised fatality risk by 8.5 percent on interstates and 2.8 percent on other roads. Modern vehicles, with quiet cabins and advanced suspensions, can mask speed, dulling drivers’ perception of danger. As Chuck Farmer of IIHS observed, “You’re in a cocoon.”
For policymakers and engineers, abandoning the 85% rule is not merely a technical change it is a paradigm shift. It requires the blending of human factors science, crash energy physics, and fair design into speed policy. The difficulty is in substituting a well-known, simple metric with a more complex, context-based approach that accounts for the way humans actually live, travel, and survive on modern streets.
