Traffic engineers have long determined speed limits through methods that once seemed nearly arbitrary or rooted in local tradition. The practice largely stems from the 85th percentile rule, an idea tracing back to a 1937 hypothesis by Harvard researcher Wilbur Smith. He proposed that the safest speed lies near the top of the 10 mph range where most motorists travel, and that exceeding this band by more than 15 percent of drivers might still allow a higher limit.

Early discussions questioned whether faster travel would raise fatalities, suggesting ongoing monitoring of speed related deaths. Instead, studies from the University of California Institute for Transportation Studies showed that by the 1950s and 1960s the 85th percentile concept had become standard practice despite awareness that drivers might not always choose prudent speeds.

In 2017 the National Transportation Safety Board challenged this foundation, noting insufficient proof that the 85th percentile speed in traffic consistently produces the lowest crash rate.

Why the 85th Percentile Metric Requires Updates

The NTSB called for alternative methods and expert tools when setting limits, incorporating crash histories and risks to vulnerable users such as pedestrians and cyclists.

Nearly a decade later the Federal Highway Administration updated its Speed Limit Setting Handbook in June, advising engineers to consider the 85th percentile speed alongside traffic volume, pavement condition, surrounding development, and injury statistics.

Depending only on observed driver speeds proves inadequate. The Insurance Institute for Highway Safety reports that speeding contributed to 29 percent of traffic fatalities in 2023, totaling 11,775 deaths. Raising limits can also create a feedback effect that elevates the 85th percentile further. University researchers note that rounding measured speeds in 5 mph steps rather than 1 mph increments often pushes average travel well above the posted limit.

How the 85th Percentile Rule Relates to the Pedestrian Safety Challenge

Centers for Disease Control data show the United States ranks lowest in pedestrian safety among 28 high income nations, with pedestrian fatalities rising 50 percent between 2013 and 2022. This trend affects teens aged 15 and older as well as adults 18 to 64, while comparable countries saw a 19.4 percent decline.

The Governors Highway Safety Association recorded 7,148 pedestrian deaths in 2024, a 4.3 percent drop from the prior year yet part of an 80 percent rise in the pedestrian share of traffic fatalities since 2009. More than one quarter of recent pedestrian deaths involved drivers who left the scene.

The 85th percentile method overlooks pedestrians. Institute for Highway Safety findings indicate an 18 percent risk of serious injury at 20 mph that climbs to 50 percent at 30 mph. With U.S. pedestrian death rates remaining far above those in peer nations, safety experts now favor comprehensive evaluations that account for all road users rather than driver speeds alone. This shift moves beyond a nearly century old assumption lacking solid scientific grounding.