Where The ’85th Percentile Rule’ For Speed Limits Falls Short

The 85th percentile rule, a method used to determine speed limits, has been widely used since its introduction by Wilbur Smith in 1937. The rule suggests that the safest speed is near the upper end of the 10-mph band where most drivers travel.

However, researchers have found that this rule is not based on scientific evidence and can be flawed. The National Highway Transportation Safety Board (NTSB) reported in 2017 that there is no strong evidence that the 85th percentile speed equates to the speed with the lowest crash involvement rate.

The Federal Highway Administration (FHWA) has also acknowledged that relying solely on the 85th percentile rule is no longer a reliable method for setting speed limits. Instead, traffic safety engineers are encouraged to consider factors such as traffic flow, road condition, and crash and injury rates when determining speed limits.

The Insurance Institute for Highway Safety (IIHS) reports that 29% of all driving deaths in 2023 were attributable to excessive speed, and that raising the speed limit can create a feedback loop where drivers increase their speed. The 85th percentile rule has also been criticized for ignoring the safety of vulnerable road users, such as pedestrians and bicyclists.

According to the Centers for Disease Control and Prevention (CDC), the United States ranks last for pedestrian safety among 28 advanced economies, with pedestrian death rates increasing by 50% from 2013-2022. The Governors’ Highway Safety Association (GHSA) reports that in 2024, U.S. drivers struck and killed 7,148 pedestrians, a decrease of 4.3% from the previous year, but still a significant increase from 2009 to 2023.

The IIHS also notes that the risk of severe injury to pedestrians increases significantly with speed, with a pedestrian struck by a car traveling 20 mph having an 18% risk of severe injury, and a pedestrian struck by a car traveling 30 mph having a 50% risk of severe injury. Overall, the 85th percentile rule has been found to be an outdated and flawed method for determining speed limits, and alternative approaches that consider the safety of all road users are being encouraged.