The 85% Rule and the Debate Over Speed Limits
For years, Rose Hammond, an 85-year-old resident of northwest Ohio, has been urging local authorities to lower the speed limit on Mitchaw Road, a two-lane road that passes her assisted living community, a church, two schools, and a busy park used for youth sports. She has grown increasingly frustrated with the lack of action, especially regarding the motorcycles that frequently race past the area.
Her concerns have not gone unnoticed. In response to public pressure, Sylvania Township recently asked county engineers to evaluate whether the current 55 mph speed limit on Mitchaw Road is appropriate. The surprising outcome was that the speed limit is actually 5 mph too low, according to the analysis.
This revelation highlights a broader issue in how speed limits are determined across the United States. The 85% rule, a long-standing principle in traffic engineering, suggests that the posted speed should be based on the speed of the 15th-fastest vehicle out of every 100 traveling on a road in free-flowing conditions. This approach assumes that the majority of drivers will naturally adjust their speed to match the safest pace for the road.
However, this method has come under scrutiny in recent years. Critics argue that it creates a feedback loop where speeding becomes normalized, leading to higher speed limits being set to accommodate faster drivers. This has sparked a movement among some states and federal agencies to reconsider the role of the 85% rule in setting speed limits.
A Shift in Thinking
The National Association of City Transportation Officials (NACTO) has developed an alternative approach called “City Limits,” which focuses on minimizing risks for all road users rather than simply following the 85% rule. This model takes into account factors such as pedestrian activity, the likelihood of collisions, and the overall context of the road.
Federal officials have also begun to question the dominance of the 85% rule. In its most recent update to national traffic guidelines, the Federal Highway Administration emphasized that communities should consider additional factors beyond just the speed of vehicles. These include how the road is used, the risk to pedestrians, and the frequency of crashes.
Leah Shahum, director of the Vision Zero Network, acknowledges that while the updated guidelines are a step in the right direction, more needs to be done to move away from the 85% rule as the primary determinant of speed limits.
Real-World Examples
Several cities have already started implementing changes based on these new ideas. Madison, Wisconsin, launched a campaign called “20 is Plenty,” lowering speed limits on residential streets from 25 mph to 20 mph. Seattle saw similar success when it reduced speed limits in a pilot program, resulting in fewer serious injury crashes and a decrease in the 85th percentile speed.
California, however, still heavily relies on the 85% rule. While the state has made some adjustments allowing local governments to set lower speed limits if there is a demonstrated safety need, advocates say more progress is needed. Kendra Ramsey of the California Bicycle Coalition argues that the state still places too much emphasis on cars over other modes of transportation.
On the other hand, Jay Beeber of the National Motorists Association believes that the 85% rule is often the safest way to ensure consistency in driving speeds. He argues that drivers tend to follow the nature of the road, and changing the speed limit without a clear reason could lead to confusion and unfair enforcement.
The Broader Implications
The debate over speed limits is not just about safety—it’s also about how roads are designed and used. As communities grow and change, so do the needs of their residents. The Insurance Institute for Highway Safety found that increasing speed limits by 5 mph can significantly raise the risk of fatalities on highways and other roads.
Despite these findings, many states continue to push for higher speed limits. North Dakota recently became the ninth state to allow 80 mph on certain highways, and Texas has a 40-mile stretch between Austin and San Antonio where 85 mph is permitted.
In Ohio, where the Mitchaw Road case has drawn attention, the state is slowly evolving its approach. While the 85% rule is still a factor, Ohio now considers the context of the road and allows cities to lower speed limits based on the 50th percentile speed when pedestrians and cyclists are present.
A Long Road Ahead
While changes are happening, they are often slow and incremental. For Rose Hammond, who has spent years advocating for a lower speed limit on Mitchaw Road, the process feels frustrating and unproductive. She remains skeptical about whether any of the proposed changes will ultimately make a difference on her community’s road.
“I just get so discouraged,” she said.
