85th Percentile Speed Explained

A Deeper Look at Calculating the 85th Percentile Speed

Guest post 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 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.

85th-percentile-chart

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.

 

Please note: I reserve the right to delete comments that are offensive or off-topic.

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9 thoughts on “85th Percentile Speed Explained

  1. mike-
    I guess you are an engineer (so was I, graduating from Annapolis) but you have some difficulties in a few areas…

    Ft Bragg is in North Carolina,

    “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.” Totally absurd statement. IF you doubt me, go to the autobahns and note the speeds…and the road conditions are not that different from US. Of course the speed limit affects the speed that people drive…and to use that coerced speed as the determinant for limits is a impure as your spelling (or word choice…I offer “defiantly” as an example.

    Good idea on the dissertation, not so strong on the actual performance.

  2. The 85th vs. the design speed of a roadway comes up more often than I’d like in my job. The frustrating thing is having a road designed, constructed, and initally signed to a 45mph design speed, and with the safety factors (or based on drivers with a slower reaction time) that appear to be built into design speed particularly regarding horizontal alg and superelevation, motorists often drive at 55mph+. Subsequently the county (with state’s oversight) then updates the road for radar at 55mph and signs it as such. The problem then becomes as new developments come into the area, for a road with considerable H&V curvature, obtaining a location that meets ISD can become very difficult (and where crest curves were built to the min value for 45, you no longer have stopping sight distance). Additionally on one of our newer 4 lane roads with median, there is a desire to landscape the road, but now with a 55mph zone, you can’t do much in the median, and off to the side of the road you have to push trees further away due to clear zone distances increasing with the 55mph limit.

  3. Thank you Mike for your insight on the 85 percentile. Using the 85th percentile makes sense for roadways that do not have cross traffic, traffic entering lanes of traffic from driveways, line of sight problems that became evident because of the 85th% posting instead of prima fascia speed law. Which, I believe contiguous properties to the thoroughfare have an influence. In my research I believe the ingress and egress easements are extensions to the properties they serve, therefor those properties are contiguous to the thoroughfare and prima fascia speed law should apply, when you add line of sight issues, cross traffic, etc on old roadways that would need to be completely realigned to make the 85% a safe posting. does this make sense.

  4. Does the exisitng posted speed limit affect your speed test results? Drivers usually drive about 7 mph (10 km/h) higher than the posted speed limit. If the posted speed limit was 60 mph, you may get a 67 mph 85%ile speed.

  5. Good Article but the definition for pace is the 10 mph range that has the highest number of observations. Using the graph it looks like the pace would be 54 to 64 mph.

  6. The ten MPH pace is defined as the 10 MPH band of travel speeds containing the largest number of observed vehicles.

    ITE draft guidelines suggest that the speed limit within a speed zone shall be set at the nearest 5 MPH increment to the 85th percentile speed or the upper limit of the 10 MPH pace.

  7. Mike – Do you cover existing speed limit signs during the survey so drivers are not influenced by legal ramifications?

  8. You write, “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. That may be true for minor roads and urban streets having no or very little enforcement. But, on highways, a large number of people drive about 5 to 10 mph above the speed limit. People may desire to drive even faster but do not want speeding tickets and drive a speed where they think they will not be pulled over. So, when we rise the speed to the 85%-ile, which typically will be around 5 mph above the old speed limit, people will drive even faster. It is in my opinion a grave misconception that drivers know what is a safe speed, and drive at such speeds naturally. If we want reasonable safety, we should rather set speed limits that are reasonable and then enforce them with frequent speed cameras. The US is back to around 35,000 fatalities per year. If we had Scandinavian safety on our roads, the US would have much fewer. Sweden had 260 fatalities in 2015 and with the US having 30 times more people, the US should have around 7,800. Considering that Americans drive about 30% more miles per year than the average Swede, some people could argue that the US could be afforded around 10,000 fatalities per year but one should also consider that Americans walk and ride bicycles less so if we here switched to Scandinavian habits, we would probably have many more fatalities among those alternative user groups. One reason Sweden has fatality rates that are a third of the US ones is that speed limits are set by ‘experts,’ not by drivers, and that speed limits are enforced to a higher degree and with very low margins.