By Eric Murphy
States are increasingly moving past the outdated “85th percentile rule” for setting speed limits, weighing factors other than the observed traffic flow in those calculations. The AP recently highlighted Ohio among a wave of efforts to consider roadway context and the presence of walkers and bikers in allowing lower speeds to ensure safer roads. For states looking to follow suit, a newly released report from the Center for Pedestrian and Bicyclist Safety offers states a data-driven, objective framework to set speed limits based on roadway context, not just vehicle movement.
A 2023 update to the Manual on Uniform Traffic Control Devices (MUTCD), which sets standards that all states must largely conform to, has let states move away from setting speeds based on the 85th percentile driver on a road, a benchmark that encourages unsafe driving speeds. Until recently, the MUTCD encouraged states to set speed limits that weren’t already set by law by conducting a speed study. The limit would be set at the speed 85% of cars were traveling slower than. This method leads to high speed limits, and can lead to a feedback loop where higher limits lead to higher 85th percentile speeds, which continue to push the limit higher as the fastest drivers set the new floor.
Engineers used this “85th percentile rule” because they thought doing so led to lower variation in speed and therefore fewer crashes, as professor Wes Marshall lays out in his 2024 book Killed By A Traffic Engineer. But a 2017 National Transportation Safety Board study found that “there is not strong evidence that the 85th percentile speed within a given traffic flow equates to the speed with the lowest crash involvement rate for all road types.”
Some states took note. With the MUTCD’s 2023 update making even clearer that, far from being a gold standard for decision-making, the 85th percentile rule is often not an appropriate way to set speeds, states are exploring new approaches. Now, the MUTCD encourages approaches like Ohio’s that consider things like pedestrian and bike activity, land use, crash history, and even 50th percentile speeds.
On some of the most dangerous roads, doing so is a must. The manual now reads, “On urban and suburban arterials, and on rural arterials that serve as main streets through developed areas of communities, the 85th-percentile speed should not be used to set speed limits” without also considering a long list of factors related to safety and roadway context.
The Center for Pedestrian and Bicyclist Safety’s new framework for context-sensitive roadway classification can offer states a way to consider context, not just vehicle movement, in setting speed limits. Doing so — and rethinking the 85th percentile rule — can help states reach their multimodal and safety goals. Setting speed limits even 5 mph lower than engineering recommendations has been shown to reduce crashes resulting in death or injury.
Photo credit: Joshua Hoehne on Unsplash. License.