The wrong way to set speed limits [st06]

Jason Slaughter dismantles a decades-old traffic engineering dogma that prioritizes driver comfort over human safety, arguing that the very signs meant to protect us are often part of the problem. The piece's most striking claim is that North American cities are failing not because drivers are reckless, but because our roads are physically designed to encourage speed, rendering speed limit signs psychologically irrelevant.

The Flawed Metric

Slaughter opens by exposing the "85th percentile rule," the standard method used across the continent to set speed limits. He describes a process where engineers measure how fast drivers are going, discard the fastest 15 percent, and set the limit at the highest remaining speed. "This method means that 15 of all drivers are breaking the speed limit," Slaughter writes, highlighting the inherent contradiction of a system designed to legitimize lawbreaking. He argues this metric is a relic of the 1960s, originally intended for rural highways where consistency of speed prevents collisions, but now dangerously misapplied to complex urban environments.

The author draws a sharp distinction between a "road" and a "street," a conceptual framework borrowed from Strong Towns. A road is a high-speed connector, while a street is a destination for community life. "Unlike a road a street is not a through fair it is a destination a place for productive city life," he explains. The core of his argument is that applying a highway metric to a street is a category error that ignores the presence of pedestrians and cyclists. Critics might note that changing established engineering standards requires massive capital investment and political will that many municipalities currently lack, but Slaughter insists the cost of inaction is measured in human lives.

"The old approach was to design the street check the 85th percentile speed and set the speed limit accordingly but the modern safer approach is to choose the speed limit that's appropriate for the context of the street and if drivers are consistently going faster than that then the street design should change until drivers are going the desired speed."

The Psychology of Speed

The commentary shifts to the human element, explaining why simply posting a lower number on a sign fails. Slaughter points out that driving is largely a subconscious activity. "The vast majority of drivers will drive at whatever speed feels comfortable for the road they're driving on," he observes. When the physical geometry of a road—wide lanes, straight paths, clear sightlines—suggests high speed, a driver's brain ignores a posted 30 km/h limit. This creates a dangerous disconnect where the "design speed" of the infrastructure conflicts with the legal limit.

He illustrates this with examples of "strodes," a hybrid of street and road that creates a high-risk environment. In these settings, engineers often resort to flashing signs to snap drivers out of their autopilot mode. "Driving is a subconscious activity as much as we'd like drivers to pay attention all the time we would get exhausted if we had to always consciously focus on driving," Slaughter notes. This framing is effective because it shifts the blame from the individual driver to the system that fails to cue the correct behavior. The evidence suggests that as vehicles become safer and more powerful, the gap between driver confidence and actual safety widens, making reliance on signs even more futile.

Designing for Safety

Slaughter contrasts the North American approach with practices in countries like the Netherlands and Sweden, where the physical environment dictates speed. He describes road designs that use visual narrowing, trees planted close to the pavement, and speed bumps to trigger an immediate, subconscious response to slow down. "To be safe the street must communicate the real level of risk to the driver," he writes, quoting from Chuck Marone's Confessions of a Recovering Engineer. In this model, the driver feels discomfort when driving too fast, which is a feature, not a bug.

The argument here is that we have the data and the tools to engineer safety without relying on human vigilance. "If you make it wide simple and straight most people will drive quickly if you make it narrow complex and twisty most people will drive slowly," Slaughter concludes. This is not rocket science, yet the majority of traffic engineers in the US and Canada remain "stubbornly stuck in the world of 1960s traffic planning." A counterargument worth considering is that such designs can sometimes increase congestion or frustrate commuters, but Slaughter counters that "traffic congestion actually improves when speed limits are lowered" due to smoother flow, a point he promises to explore further.

"It's not the 1960s anymore we have decades of data that can tell us with fairly good precision what speed a typical person will drive on a given road or street... we can't keep doing this it's time to stop blindly following the guidelines of the previous century."

Bottom Line

Slaughter's strongest asset is his ability to reframe speeding not as a moral failing of drivers, but as a predictable outcome of bad design. His biggest vulnerability is the sheer inertia of North American infrastructure, which makes the transition to "self-explaining roads" a slow and expensive process. The reader should watch for how local municipalities begin to adopt these design-first principles, as the era of relying on signs to save lives is clearly ending.