Wikipedia:WikiProject U.S. Roads/Idaho/All-time list

Sources include: Years before 1951 are therefore not the exact year a change was made.
 * 1937 Rand McNally (anything on this map through 39 is marked 1930s; a 1932 Montana map shows numbers through 33, but not 47, and the original clustering is assumed to stop with 39)
 * 1939 Rand McNally
 * 1940 Rand McNally
 * 1946 Rand McNally (smaller scale, some routes missing)
 * official maps back to 1947
 * 1956 Gousha
 * meeting minutes back to 1951
 * traffic flow maps back to 1959
 * county maps back to 1962

In 1953 a number was assigned to every unnumbered state highway, but these don't seem to be listed in one place. At the same time, duplicate numbers were changed. A year of 1953 below means that it was probably numbered then.