On June 23, 2022, the Board of County Commissioners voted to increase speed limits on 13 segments of roads in unincorporated Johnson County, upon the recommendation of the Johnson County Sheriff’s Office and Public Works Department.

An engineering study was conducted to determine the 85th percentile speeds as well as a review of safety and sight distance issues at intersections and entrances.

The 85th percentile is simply the speed that 85% of the drivers will operate and is an important component considered when posting speed limits. The 13 segments that met the criteria for safe increases in speed limits to either 40 or 45 MPH are shown in the table below.

All the segments, with approved speed limit increases, have significant evidence of non-compliance of the existing 35 MPH speed limits by having 85th percentile speeds in the 43 to 54 MPH range. Drivers need to perceive the speed limits as credible if adequate levels of voluntary compliance are to be achieved.

Routine violation of speed limits by most drivers may breed contempt not only for speed limits but also for other traffic regulations. As a general guideline, speed limits should be set at levels that are largely self-enforcing (i.e. near the 85th percentile speed) so that law enforcement officials can concentrate their efforts on the worst offenders. Public support (i.e., willingness to obey) is closely linked with reasonableness of speed limits. 

Additionally, this is a bit counter intuitive, but studies have shown that posting the speed limit at or near the 85% percentile, groups the traffic speeds more closely together and reduces the conflicts between vehicles driving at different speeds. Issues such as the need to pass vehicles or miss-calculating the speeds of other vehicles is reduced.

Less conflict with vehicles of different speeds and better opportunity for speed enforcement should help make the segments listed below safer for drivers.