This is a guest blog post by Ian Thomas. Ian is the State and Local Program Director at America Walks and a City Council Member in his hometown of Columbia, MO.

As an advocate for safe, walkable communities, I am encouraged by the National Transportation Safety Board’s recent study, Reducing Speeding-Related Crashes Involving Passenger Vehicles , casting doubt on the “85th Percentile Rule.”

Baked into transportation engineering ideology since the 1950s, this rule states that the legal speed limit on a road segment should be approximately equal to the speed at or below which 85% of vehicles are traveling. This means that the speed limit is determined by the actual speeds currently being driven, rather than by the character of the road, adjacent land uses, or prevalence of bicyclists and pedestrians.

The 85th Percentile Rule has baffled and infuriated active transportation advocates for decades, who feel that a logical process for ensuring traffic safety would be:

  1. Decide on a safe maximum speed, based on land use, access requirements, pedestrian/bicycle use, and other environmental factors;
  2. Design the road to achieve that maximum speed, by selecting the appropriate design parameters (lane width, curvature, sight lines, etc.);
  3. Set the legal speed limit equal to the “design speed.”

Instead, many engineers build new roads to be wide and straight with long, panoramic sight lines, measure vehicle speeds over a period of time, and then set the speed limit equal to the 85th percentile speed.

This arcane and archaic practice is based on research conducted by David Solomon more than 50 years ago. The so-called Solomon Curve purports to illustrate that vehicles traveling at the 85th percentile speed have the lowest probability of being involved in a collision with another vehicle. By assuming that “most drivers are reasonable and prudent, do not want to have a crash, and desire to reach their destination in the shortest possible time,” engineers make the leap to the flawed conclusion that setting the speed limit equal to the 85th percentile speed maximizes safety.

There are several serious shortcomings in this research and its application:

  • It was conducted only on rural highways;
  • It ignores the existence of pedestrians and cyclists, and their likelihood of being involved in a crash;
  • It overlooks the fact that high-speed collisions cause vastly more deaths and serious injuries than low-speed collisions;
  • It has nothing to say about the concept of “design speed;”
  • It guarantees that 15% of motorists break the law.

In my experience, most transportation engineers adhere rigidly to this rule while being unwilling (or unable) to provide a rational justification. At the same time, the few who oppose the rule regard it as an abdication of the engineer’s responsibility to address excessive speeds. And that’s why recent statements by the National Transportation Safety Board (NTSB, an independent federal agency tasked with investigating safety issues in aviation and surface transportation) are significant.

In the abstract to their new study, the NTSB included the following findings:

  • “… there is not strong evidence that, within a given traffic flow, the 85th percentile speed equates to the speed with the lowest crash involvement rate on all road types.”
  • “Unintended consequences of the reliance on using the 85th percentile speed for changing speed limits in speed zones include higher operating speeds … in the speed zones, and an increase in operating speeds outside the speed zones.”

NTSB’s acknowledgement of flaws in the way the 85th percentile rule is applied, at a time when Vision Zero policies are increasingly popular, opens a door to much better conversations about speed limits and safety. If you’re in the middle of one of these fights, call your local transportation engineer today!