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Speed limit on section of U.S. 20 in Eastern Idaho increases to 70 mph

The speed limit on U.S. 20 between Idaho Falls and Ashton increased to 70 mph Wednesday (March 22), for both passenger vehicle traffic and commercial trucks.

The speed-limit changes stem from engineering and traffic studies completed Feb. 21 for the 50-mile stretch of U.S. 20. Results indicate that 85 percent of drivers travel the route at 70 mph or below. Setting speed limits according to this "85th-percentile speed" results in the lowest crash rates.

Idaho Transportation Department (ITD) officials discussed the proposed speed-limit increases with county officials and law enforcement prior to formalizing the changes. ITD recommended the changes to the Idaho Transportation Board and received approval March 16.

"Raising the speed limit to 70 mph will further traffic safety and mobility," said District 6 Traffic Engineer Ben Burke. "Posted speed limits can’t replace common sense, however. Drivers should always slow down if necessary to meet hazardous conditions."

Speed limits are the maximum speed motorists can legally travel under ideal roadway and weather conditions. Limits encourage better traffic flow by reducing the variances in speed from one vehicle to the next.

Crews also adjusted the speed limits on each side of the stretch of U.S. 20 for improved transitions. For instance, the speed limit between Lindsay Boulevard and Riverside Drive in Idaho Falls was lowered from 55 to 45 mph. See the speed-limit map here for more details.

ITDt uses traffic studies to determine what speed motorists consider reasonable, convenient and safe for existing driving conditions. Speed limits are intended to supplement driver judgment and are set at a level that studies indicate drivers will adhere to voluntarily.

In 2014, the Idaho Legislature amended the "Basic Rule and Maximum Speed Limits" in Idaho Code to allow the Idaho Transportation Board to raise interstate speed limits to 80 mph and other state highway speed limits to 70 mph, when concluding that the increases are in the public interest based on engineering and traffic studies.

Last year, ITD raised the speed limit to 70 mph on U.S. 20 between Idaho Falls and Butte City, on Idaho 22 between its junction with Idaho 33 and Dubois, and on Idaho 28 between Mud Lake and Leadore. The transportation department also raised the speed limit to 70 mph on a 15-mile section of U.S. 95 in northern Idaho.