Wednesday, September 18, 2019

Cruise control: Setting the speed limits in Kansas

By Ashley Tammen
Public Affairs Manager
Northcentral Kansas

When it comes to setting the speed limits in Kansas, “Increase the speed” is often a statement we hear the most. As you may already know, safety is a top priority for KDOT, and we look at many factors to determine a safe speed for a road or highway. KDOT determines the speed limit of a roadway through a speed study which may be conducted multiple times in a given year.

What is a speed study?

A speed study is done using a radar gun to collect speed data.  Normally, this data is collected during off-peak hours from about 50 random vehicles on a given roadway in each direction at free flow speeds. The vehicles are selected at random to avoid bias in the study results and the 85th percentile speed is the speed at which 85% of the drivers are traveling at or below. The 85th percentile speed reflects the safe speed as determined by a large majority of drivers.

All factors considered!

Many factors are considered in speed studies including crash history, roadway geometry, activity within the driving environment, and politics or state law. According to Brian Gower, Bureau Chief of KDOT’s Bureau of Transportation Safety and Technology, one of the most important factors considered in a speed study is engineering judgement from a traffic engineer. The engineering judgment of a traffic engineer may determine, if any, of the factors in a speed study warrant a downward adjustment of the 85th percentile speeds.

Establishing realistic speed limits

Often, speed studies may not result in a speed change as this may not be the best decision for everyone such as the speed study conducted in February 2016 of K-9 from Beloit to Clifton. Other times, only sections of a roadway may result in a speed zone change like the speed study conducted in May, 2015 of U.S. 81 and K-9 in Cloud County where it was recommended only to raise the speed limit at State Street. Once all variables are considered, a safe and reasonable speed limit is established for a given road section.

Now that you’ve learned all this you may be wondering how we determine when a speed study will be conducted or who initiates the request for a speed study. Anybody can initiate a traffic study or check when the last one was completed, simply contact your local KDOT office.

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