by Lori Miles at All Traffic Solutions
15% of drivers cause most of the accidents on the road
The 85th Percentile is the maximum speed that 85% of drivers will not exceed on a given road if there were no stop signs, speed limits or traffic signals. It’s seen as the safest speed on the roadway because 85% of the vehicles are naturally at that speed or below it. The majority of drivers drive safely because they it is in their own self-interest to remain safe.
The concept is important in traffic management as it is critical data in setting speed limits. Most drivers behave in a safe and reasonable manner, do not drive at excessive speeds and do not want to get into crashes. Research shows that vehicles traveling between the 50th and 85th percentile of speed have the lowest risk of crashing due to speed and drivers who exceed the 90th percentile have a significantly higher risk of crashing. Therefore many of vehicles exceeding the 85th Percentile are the ones causing the majority of the accidents.
Knowing this, the 85th Percentile is a good starting point for traffic engineers to determine where the speed limit should be set. Generally, other safety factors of the roadway must also be considered before the actual speed limit is determined. The end result is typically a speed limit that is between 10 mph lower than and the 85th percentile.
The MUTCD states that…….The most common factor considered in setting speed zone limits is the 85th percentile traffic speed.
Drivers don’t slow themselves down
Once this concept is learned and understood, it can be helpful in analyzing traffic data. For example, let’s say a new day care is built and despite a posted speed limit of 25 mph, drivers are going much faster than that past the school.
One way to understand this is by analyzing the 85th Percentile. Perhaps, the numbers come back and show the 85th Percentile is 42 mph. In that case, it’s understandable that cars are exceeding the speed limit because the road is designed for a higher speed, and this is also accommodated by the road’s environment.
To solve this issue, something is needed to slow down the vehicles such as a road diet (narrowing the lanes) or radar speed feedback sign. Drivers often need a reminder to slow them down.
How to quickly collect data on the 85th Percentile
Radar signs that collect data are a good tool for helping determine the 85th Percentile on a problematic road. Users just need to set up the sign and allow it to start collecting data. Today’s signs even automatically upload data to the cloud and run reports on the 85th Percentile without driving out to the sign. Analysis is just a couple clicks away instead of hours collecting the data and poring over an excel sheet.
There is much more to learn about analyzing traffic data but the 85th Percentile is one of the first and most important concepts needed to make our roads safe.