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KITTITAS COUNTY <br />DEPARTMENT OF PUBLIC WORKS <br />PUBLIC WORKS — BOARD OF COUNTY COMMISSIONERS <br />ENGINEER'S REPORT — SPEED REDUCTION REQUEST FOR THRALL ROAD <br />(MP 0.05 TO MP 1.10) <br />PUBLIC HEARING DATE: November 17, 2020 2:00 PM Local Time <br />ACTION REQUESTED: Maintain the existing 50 Miles per Hour Speed Limit <br />LEAD STAFF: Joshua Fredrickson, County Engineer. <br />RECOMMENDATION: <br />Maintain the existing 50 miles per hour speed limit on Thrall Road MP 0.05 to MP 1.10. <br />BACKGROUND: <br />The State of Washington sets the maximum speed limit for county roads at Revised Code of <br />Washington Chapter 46.61.400. <br />The State of Washington authorizes local authorities to establish or alter maximum speed limits at <br />Revised Code of Washington Chapter 46.61.415. <br />Kittitas County Code manages 35 miles per hour speed limited roads at Chapter 10.08.020. <br />Thrall Road (MP 0.05 to MP 1.10) posted speed limit is currently 50 miles per hour. <br />Kittitas County utilizes roadway segments as the basis of setting service levels. Service levels are <br />used to identify congestion and impacts to roadway surface condition along with assigning risk <br />associated with roadway usage. We assign alphabetic designations: A, B, C, D and F. We require <br />mitigations improving roadways segments that drop below service level C. A represents free flowing <br />traffic with no delays in the road segment. Service Level F represents a traffic jam. <br />Public Works utilizes national engineering standards when evaluating speed limits on County Roads. <br />The standard engineering practice is to collect vehicle speed data and process the data determining the <br />85d' percentile speed. Traffic engineers rely on the 85th percentile rule to help establish speed limits <br />on non -local streets. Typically, the speed limit is set to the speed that separates the bottom 85% of <br />vehicle speeds from the top 15%. Statistically, the 85th percentile speed is slightly greater than the <br />speed that is one standard deviation above the mean of a normal distribution. This is the assumed <br />national standard representing most drivers negotiating the roadway. <br />Public Works designs roadways pursuant to national standards (American Association of State <br />Highway and Transportation Officials) for: horizontal and vertical curves and associated roadway <br />geometrics. <br />Page 1 of 4 <br />PUBLIC WORKS BOARD OF COUNTY COMMISSIONERS ENGINEER'S REPORT <br />