BalkansCat/Shutterstock How do traffic engineers come up with speed limits? Believe it or not, historically, it was dang near random. Okay, let's call it a matter of custom, to be kinder. But really, it's based on something called the 85th percentile rule, which is similar to an argument by Harvard PhD student Wilbur Smith in 1937. He hypothesized that the safest speed is near the upper end of the 10‑mph band where most drivers travel, and that if more than 15% of drivers are going faster than that band, a higher maximum speed is probably still safe. Would going even faster cause more death? That was the question. Smith's peers said you'd have to keep tabs on deaths related to higher speeds. However, that's not what happened. Researchers at the Institute for Transportation Studies at the University of California instead found that even though there was a continued understanding that drivers wouldn't always be "prudent" about the safest speeds, by the 1950s and 1960s, the 85th percentile rule had in fact become regulatory gospel. Yet, in 2017, the National Highway Transportation Safety Board (NTSB) would finally pierce the bubble that this assumed metric had any scientific basis in fact, stating that there was "not strong evidence that the 85th percentile speed within a given traffic flow equates to the speed with the lowest crash involvement rate." Why 85th Percentile Isn't Enough Anymore Kent Weakley/Shutterstock The NTSB report said that regulators ought to use "alternative approaches and expert systems" to arrive at speed limits, which account for elements like crash history and vulnerable road users. Pedestrians and bicyclists are prime examples of this vulnerable demographic. By the time we reach today, nearly a decade beyond that NTSB report, we get to the Federal Highway Administration (FHWA) finally acknowledging that leaning solely on the 85th percentile rule for its Speed Limit Setting Handbook was no longer a reliable yardstick. FHWA's most-updated, June guidelines suggest traffic safety engineers look at 85th percentile operating speeds, but also go way beyond that metric. They emphasize studying factors that include, but aren't limited to, traffic flow, road condition, and how developed or rural an area is, as well as crash and injury rates. In other words, just relying on how fast drivers want to go isn't going to cut it, and for good reason. According to the Insurance Institute for Highway Safety (IIHS), 29% of all driving deaths were attributable to excessive speed in 2023 (11,775 deaths). IIHS also cites the feedback loop problem of speeding – where raising the speed limit only increases that 85th percentile issue, where now everyone is driving just a bit faster. Researchers at the University of California suggest another problem: Since speed limits and enforcement are typically set in 5 mph increments rather than 1 mph, using the 85th‑percentile rule means a measured 85th‑percentile speed of 45 mph can be rounded up to a 50 mph limit, which can drive average speeds well above 50 mph. The 85th Percentile Rule and America's Pedestrian Safety Crisis Leopatrizi/Getty Images According to the Centers for Disease Control and Prevention (CDC), the United States ranks dead last for pedestrian safety among 28 advanced economies, with U.S. pedestrian death rates increasing by 50% from 2013-22. That increase is very tragically inclusive of teenagers, too, because it includes teens 15 and up, as well as adults from 18-24, and from 25–64. Meanwhile, the global fatality rate fell 19.4% in 27 other high-income countries. The Governors' Highway Safety Association (GHSA) says that in 2024, U.S. drivers struck and killed 7,148 pedestrians. That was actually a decrease of 4.3% from the year before, but in the period from 2009 to 2023, the death rate of pedestrians in traffic fatalities is up a miserable 80%. Even more grimly: In the past five years, more than a quarter of pedestrians were killed by drivers who fled the scene of the crime. Flock cameras to the rescue, ahem? What's all that got to do with speed limits and the 85th percentile rule? It ignores pedestrians, quite simply. For example, according to research by the IIHS, a pedestrian struck by a car traveling 20 mph has an 18% risk of severe injury, but at 30 mph, that risk jumps to 50%. With pedestrian death rates still far higher in the United States than they are globally in peer high-income countries, it's hardly surprising that traffic safety professionals are emphasizing alternative approaches to speed limits that study all road users – not just the fastest pace drivers can manage without killing each other, or pedestrians — too often. It only took looking past a nearly 100-year-old assumption that was never based on science.