All traffic is local, and nothing gets people riled up more than speed limits. The 85th percentile has been the standard for measuring speed limits for years. Due to pressure from groups such as Vision Zero and Complete Streets, this standard of measuring speed is losing ground in states such as California, Oregon, and Massachusetts, and even on a federal level. But it’s still the standard in many places around the country.
The 85th percentile is “the speed at or below which 85 percent of all vehicles are observed to travel under free-flowing conditions past a monitored point.”
In the past, traffic engineers employed the 85th percentile by observing traffic on the ground using a radar gun on the part of the road that needed a survey. More recently, traffic engineers have used pneumatic road tubes stretched across a roadway, which according to the US Office of Highway Policy Information, can measure the following:
- How many drivers use the road every day
- The direction the vehicle travels and during what time of day
- What class of vehicle (car, tractor-trailer) uses the road
- The speed at which drivers are traveling
Here is a short video from the Bloomington, Minnesota, streets department that describes how the tubes work.
The data collection process can be labor-intensive, expensive, and doesn’t always return the most accurate results. Using GPS solves this problem.
For the past ten years, TomTom, a consumer navigation company, has been mapping the world using GPS. We also recently learned that along with navigation, the company has been putting together speed profile calculations. Utilizing street data from a company like TomTom, traffic engineers can take the political rhetoric out of speed limit determinations.
What is GPS? First developed for the US military at the beginning of the space race, civilians have had access to GPS since 1989. Onboard vehicle navigation systems use 24 satellites that orbit the Earth twice daily to calculate routes, find local addresses, and monitor road conditions. This navigational technology offers users PNT (positioning, navigation, and timing) services.
The New Hampshire Union Leader recently interviewed state traffic engineer William Lambert to discuss a data system his department bought recently from TomTom. Within a month of purchase, Lambert already ran 30 traffic reports to determine the speeds vehicles traveled on targeted roads in the state.
In the past, the process would have taken much longer. Lambert says he can now analyze the TomTom speed profile calculations data on his computer to see how traffic behaves along a targeted route. He added, “I’m trying to use this data to make sure that speed limits are credible for the character and conditions of the roads across the state.”
For Lambert, reducing a speed limit due to a “wishful thinking value’ usually doesn’t work. He added, “We find that traffic is not going any slower because we want them to.”
Lambert says that target speed is the answer and the lowest crash rate occurs at the 85th percentile of measured traffic speed. He’s right when he said in the interview that ensuring speed limits on particular roadways are ‘credible’ will make all roads safer. Otherwise, drivers will ignore posted speed limits. “If some speed limits are credible and others aren’t, and people disrespect them in places where they’re not, then they can disrespect them in other places and go too fast.”
Using this kind of aggregated digital data will help traffic engineers do their job by setting safe speed limits at the 85th percentile and taking the arbitrariness of setting speeds out of the equation.
We all want safer roads, and this kind of technology can certainly help make that happen with real-time traffic data.
To learn more about the 85th Percentile, check out these previous NMA Newsletters: