Calculating the 85th Percentile Speed

By Jonah Finkelstein, EIT

We recently completed a Traffic Corner Tuesday where we discussed the importance of the 85th percentile speed and what it is used for in the world of traffic engineering.

Once the webinar was finished we received multiple messages about how people enjoyed the information on the importance of the 85th percentile speed, however, we did not discuss how the 85th percentile speed is determined on a roadway. Well… ladies and gentlemen, the wait is finally over. Here you can find a quick summary of the uses of the 85th percentile speeds, why it is important, and most importantly of all how to calculate the 85th percentile speed on a roadway using collected speed data.

Want more information on 85^th percentile speeds? Check out these resources:

• 85^th Percentile Speeds. Traffic Corner Tuesday video identifying the three key parts of setting speed limits – posted, design, and operating, and the different ways states or countries handle their speed limits
• Traffic Corner Tuesday. Free 30-minute webinar of traffic engineering best practices
• 85^th Percentile Speed Explained – A Deeper Look at Calculating the 85^th Percentile Speed
• Strick Speed Limits – Ignoring the 85^th Percentile