The Idaho Transportation Department (ITD) is responsible for the establishment and classification of a state highway network, including 5,000 miles (8,000 km) of roads that are classified as Interstate Highways, U.S.

During the 1920s, in lieu of numbering its highways, Idaho had a system of lettered Sampson Trails.

[2] They were marked by businessman Charles B. Sampson of Boise at no expense to the state, using orange-colored shields.

[3] By 1929, the trails system had included 6,500 miles (10,500 km) of marked highways that covered most of the state.

[5][6] In 1978, the ITD began using brown state highway markers to denote scenic Idaho highways,[7] in addition to the main highway markers that featured a black background and white lettering and the name "IDAHO" in black lettering inside a white geographic outline of the state.