Quick Summary
- The 85% rule sets speed limits based on how fast 85% of drivers go.
- Once considered the safest method, it’s now criticized for fueling higher speeds and accidents.
- States like California, Wisconsin, and Ohio are experimenting with safety-focused alternatives.
- Lower urban speed limits show measurable reductions in injuries and fatalities.
- The debate highlights a clash between driver convenience and community safety.
For decades, most US states have relied on the 85% rule to set speed limits: the idea that the safest speed on a road is what most drivers already choose. But as fatalities rise and city streets grow busier, officials are beginning to question whether this decades-old formula is putting people at risk.
What is the 85% rule?
The 85% rule traces back to research in the 1930s and 1940s on rural roads. It states that a road’s posted limit should be based on the speed at or below which 85% of drivers travel under free-flowing conditions.
- Example: If 85 out of 100 drivers on a road are driving at 60 mph or below, engineers would set the speed limit at 60 mph.
- Rationale: Advocates argue that minimizing the speed difference between the fastest and slowest drivers reduces crashes.
For decades, traffic engineers and legislators embraced this approach, believing drivers naturally “vote with their feet.” But critics argue that this method is outdated, especially in busy urban corridors with pedestrians, cyclists, and schools.
Why are states rethinking the rule?
The US is facing a troubling rise in traffic deaths. According to the National Highway Traffic Safety Administration, nearly 43,000 people died on American roads in 2022, one of the deadliest years in two decades.
Experts say speed is a major factor. Every 5 mph increase in a maximum speed limit raises fatal crash risk by:
- 8.5% on interstate highways
- 2.8% on other roads
(Source: Insurance Institute for Highway Safety)
Critics argue the 85% rule creates a “feedback loop”: as more people speed, limits get raised to match, normalizing unsafe speeds.
What are the alternatives?
City Limits model
The National Association of City Transportation Officials (NACTO) created “City Limits,” which prioritizes safety over driver behavior. Instead of measuring speed, it uses:
- Street activity (e.g., residential vs. commercial zones)
- Pedestrian and bicycle traffic
- Historical crash data
Cities like Seattle and Madison have used this model to lower speed limits on residential streets. Seattle saw a 7% reduction in average driving speeds and fewer serious crashes after lowering limits to 20 mph.
Context-sensitive speed setting
Some states, like Ohio, now allow engineers to weigh factors such as:
- Pedestrian density
- Bicycle activity
- Roadway context (schools, parks, assisted living facilities)
This shift acknowledges that urban roads are fundamentally different from highways.
Where do states stand?
- California: Still heavily tied to the 85% rule but recently gave local governments more flexibility to cite safety concerns when lowering limits. Advocates say progress is slow.
- Wisconsin (Madison): Rolled out a “20 is Plenty” campaign, reducing local residential limits from 25 mph to 20 mph.
- Ohio: Exploring alternatives, even allowing speed limits based on the 50th percentile of driver speeds in high-pedestrian areas.
- Highways nationwide: Maximum speed limits continue to climb. North Dakota recently joined eight other states allowing 80 mph, while Texas boasts a 40-mile stretch permitting 85 mph.
Why does this matter?
Speed determines crash survival. At 20 mph, most pedestrians survive a collision. At 40 mph, the chance of survival plummets. Lowering speed limits, especially in cities, can save lives without significantly increasing travel times.
Yet the debate remains contentious:
- Safety advocates argue outdated formulas prioritize cars over people.
- Driver groups counter that arbitrary reductions punish motorists and ignore natural traffic flow.
As Michelle May, Ohio’s highway safety program manager, notes:
“People are traveling and living differently than they did 40 years ago, and we want to put safety more at the forefront.”
The road ahead
The federal government has signaled that the 85% rule is a guideline, not a mandate, opening the door for states to rethink their approach. But shifting decades of policy and driver expectation won’t happen overnight.
For communities like Sylvania Township, Ohio—where residents have begged officials to lower a 55 mph limit near schools and parks—the outcome remains uncertain. Even as state laws evolve, locals fear action may come too late.