The Wrong Way to Set Speed Limits [ST06]

Speed limits based on the “85th percentile rule” are a poor fit for city streets because they were built for rural roads—and the mismatch helps explain why dangerous, high-speed downtown driving persists. The rule typically has an intern collect driver speeds during good weather, discards the top 15%, then sets the posted limit to the highest remaining speed (rounded down). That design quietly guarantees that about 15% of drivers will exceed the limit, and it was never meant to govern the complex, human-filled environments of streets where pedestrians and cyclists are present.

The core problem isn’t just the math; it’s the way North American traffic engineering treats streets like rural through-roads. Rural roads are relatively simple: fewer crossings, fewer driveways, and fewer people outside vehicles. In that setting, keeping vehicles at a consistent speed can reduce passing and conflict. But streets are destinations—lined with homes and businesses, with frequent interactions and vulnerable road users. When the rural logic gets applied to streets, the result is often a “strode,” a road-street hybrid that’s inefficient, expensive, and dangerous. Cities then wonder why pedestrian fatalities stay high and downtown retail struggles, even as they keep designing for vehicle throughput rather than safe, comfortable street life.

Speed is also treated as a sign-and-enforcement issue instead of a physical safety issue. Higher speed increases the likelihood that crashes become fatal, makes it harder for drivers to react in time, and lengthens stopping distance—especially for cyclists and pedestrians. Meanwhile, modern vehicles feel safer to their occupants, which can increase driver confidence and worsen speeding. The most effective way to reduce fatalities is therefore straightforward: lower the speeds vehicles actually travel.

Yet many places try to lower speeds by changing only the posted number. A sign can nudge average behavior, but it’s not a durable solution because most driving is subconscious. People generally don’t constantly check their speedometers or read every sign; they drive at the “design speed” the road invites. If the posted limit doesn’t match that design speed, drivers will keep going fast until something in the environment forces them to slow down.

That’s why flashing signs often fail. They act as a last-minute reminder on streets that visually and geometrically signal speed—wide lanes, straight alignments, and minimal friction—so drivers ignore the message unless they happen to notice it. The transcript contrasts this with approaches used in countries like the Netherlands and Sweden, where street design communicates risk directly. Narrowing roads visually with trees and lane treatments, adding speed bumps, and creating tight, one-car-at-a-time passages make low speeds feel natural without relying on constant signage. In that model, the speed limit follows the context, and if drivers still go too fast, the street geometry changes until actual speeds match the intended safe range.

The takeaway is a shift in responsibility: stop setting street speeds by what drivers already do under a 1960s metric, and instead design streets so drivers feel discomfort at unsafe speeds—so the desired speed emerges from the road itself, not from a flashing sign.