The 85th percentile speed is the speed at or below which 85 percent of the drivers will operate with open roads and favorable conditions. The assumption underlying the 85th percentile speed is that most drivers will operate their vehicle at speeds they perceive to be safe. Speed limits set above or below the 85th percentile speed will create unsafe conditions due to speed differential as some driver adhere strictly to the law while others drive the naturally-induced speed.

This observation of human behavior has long been a source of frustration for safety advocates. While the process starts with an assumption that most people will drive a speed that is safe and reasonable, we can all point to the examples where this isn’t true. This causes a disconnect between perception and reality.

One hundred cars can drive by your house. If ten of them drive over the speed limit and one or two of those are driving way too fast, the perception will be that there is a speeding problem. When the engineer does the study and contradicts that perception, the entire concept of the 85th percentile speed — and any scientific approach to establishing speeds — is called into question.

Rather than throwing out this useful insight, we need to instead understand the limitations. The process for establishing speed limits using the 85th percentile speed works really well on roads. A road is a high-speed connection between productive places. Roads are simple, lacking the complexity of movement that comes with intersections, parking, and humans outside of a vehicle. For roads to function properly, we want people to be able to drive them without needing to be hyper-vigilant. The 85th percentile speed tells us what is safe in that environment.

A road is different than a street, which is a platform for building wealth within a community. On streets, where there is vastly more complexity, the 85th percentile speed is almost always misapplied. Especially where a forgiving design approach is use for street design, drivers will not perceive the complexity of the environment. When engineers widen lanes, create recovery areas, make curves more gentle, and use other design features developed for highways, they give the driver the incorrect signal that the environment is simple, that they can relax and not be hyper-vigilant. Speeds go up and tragedy is the inevitable result.

The answer for streets is fairly simple: Instead of throwing out the 85th percentile speed, we need to recognize the insight it is providing. It is telling us the speed that drivers perceive to be safe. If that is higher than the speed we know to be safe — if more than 15% of drivers travel within a neighborhood at speeds over 15 to 20 mph — then the design is giving drivers a false sense of security. If we want drivers to slow down, we don’t ignore human behavior and the 85th percentile speed. We change the street design to make it feel less safe for drivers. In other words: we change the street design to reflect reality.

We’ve been steadily working through the Strong Towns Academy course Aligning Transportation with a Strong Towns Approach. It’s been full of insights and we’re only about halfway done with the course. For those of you that have not tried the free Strong Towns 101 course, you should give that a try (4.25 CE credits and it’s great for sharing with your friends, colleagues, and local leaders). For those of you ready to create a transportation plan for your community, the transportation course can be had on its own or as part of an 8-course bundle.

Here’s how we tackle the 85th percentile speed, a good example of the kind of analysis you will get through this and all of the courses in the Strong Towns Academy.

With the pandemic continuing to impact the world, we know that our cities, towns, and neighborhoods are stressed. Local leaders inside and outside of government are on the front lines, responding to the crisis, often in new and innovative ways. Due to travel restrictions, we can’t be there with you to help you work through this, but we can share our insights and walk you through creating a plan for how to make your city strong and resilient. That’s the Strong Towns Academy.

If you have any questions or need more information, get a hold of my colleague Alexa Mendieta (alexa@strongtowns.org | 844-218-1681). Alexa is our Development Associate and she will track down the answer or connect you to someone who can provide it.

Thank you to everyone out there working to make their place better. You’re an inspiration. Keep doing what you can to build a Strong Town.

Top image from media.defense.gov.