Evaluation of the Impacts of Differential Speed Limits on Interstate Highways in Idaho Grant uri icon



  • In 1998, the Idaho legislature passed a bill that set the maximum Interstate highway speed limit at 65 mph for trucks weighing more than 26,000 lbs and having 5 or more axles. This created a differential speed limit between those trucks and all other vehicles which have a 75 mph speed limit on most sections of the Interstate system. In 2000, the National Institute for Advanced Transportation Technology (NIATT) at the University of Idaho published a report evaluating the impacts of the reduced Interstate highway truck speeds. The Idaho legislature is now considering whether the difference in speed limits should either be removed or reduced, and the Chairman of the Senate Transportation Committee has asked ITD to form a team to study the issue and "agree on an acceptable path forward on speed limits to reduce the gap between automobiles and large trucks, primarily on the interstate." The purpose of this research is to refresh the NIATT report to include data from the 11 years since the last report. There has also been considerably more national study of the issue since 2000, and the national findings will be incorporated and compared with the Idaho-specific study.

date/time interval

  • January 1, 2012 - January 1, 2013