By Jonah Finkelstein, EIT Spack Consulting
The 85th percentile speed can be a confusing statistic for some to understand. The Minnesota Department of Transportation defines the 85th percentile speed as, “the speed at or below which 85 percent of all vehicles are observed to travel under free-flowing conditions past a monitored point.” Traffic and Transportation Engineers use the 85th percentile speed as a guide to set the speed limit at a safe speed, minimizing crashes and promoting uniform traffic flow along a corridor.
With this definition, it would seem that the signed speed limit of a road would be highly influential in determining the 85th percentile speed, however the exact opposite is the case. Let’s take a deeper dive into 85th percentile speed, and why it is a major consideration in determining a roadway’s speed limit.
As mentioned earlier, the 85th percentile speed defines the speed that 85 percent of drivers will drive at or below under free-flowing conditions. Most people don’t drive according to the posted speed limit, but account for the visual aspects of the road and a ‘feel’ for the road. The visual factors that influence speeds can include:
- Lane and shoulder configurations and widths
- Presence of vertical and horizontal curves
- Sight distance and obstructions
- Presence of surrounding developments to the roadway
The ‘feel’ for the road can be as simple as being the regular route that someone drives for years. For example, on a recent trip to California I was driving along Highway 1, an extremely scenic and windy, 2-lane un-divided roadway along the ocean. I felt like I was driving a fast pace, however the local Fort Bragg in California, traffic thought otherwise, zipping past me at a speed I would definitely have been uncomfortable at traveling along the specific roadway.
With so many factors impacting the speeds on a roadway, the 85th percentile speed becomes a good metric that can quantify these variables and put them into one useful number. There are two main techniques for collecting this data, directional tube counts and handheld radar.
So how is the 85th percentile speed determined? By the completion of a spot speed study test. For a Spack Consulting project located south of Hibbing, Minnesota on Trunk Highway 73, roughly 100 free flowing vehicle speeds were collected over a two-hour period using handheld radar. These speeds were then plotted to create the following figure.
As shown in Figure 1, the 85th percentile speed of this roadway segment is roughly 62 MPH (blue line). Using this as the base point, the percent of vehicles traveling up to 5 MPH over and under this speed limit were also checked (yellow lines). As the figure shows, only 23% of the total vehicles on the road were traveling less than 5 MPH below the 85th percentile speed, while less than 2% of vehicles were traveling more than 5 MPH over the 85th percentile speed limit. This means that roughly 75% of vehicles on the road were traveling within 5 MPH of the 85th Percentile Speed.
This is why the 85th percentile speed is such a large consideration when determining speed limits. If the speed limit is set to the 85th percentile speed, we know that 85 percent of drivers will be driving at or below the speed limit, and that a majority of vehicles will be driving within 5 MPH of the speed limit (75% in our example).
This uniformity of vehicle speeds increases safety and reduces the risks for vehicle collisions. When vehicles deviate from a standard speed, either faster or slower, the potential for accidents increase, whether caused by a slow car in a rear end collision or a fast car completing lane changes to maneuver through slower traffic. By setting the speed limit to the 85th percentile speed this uniformity is achieved and safety is increased.
In our previous example, a 60 MPH speed limit would be recommended because speed limits are set in 5 MPH increments. 60 MPH was chosen over 65 MPH due to the current roadway speed limit of 55 MPH, the 2-lane layout of the road, and lack of separation between opposing traffic flows. The 85th percentile speed justifies this change and helps assure that it is a safe and strategic speed limit change.