 SH-1 was originally created in the 1920s as part of Sampson Trail B, which ran from Boise north to Lewiston, Coeur d'Alene, before entering British Columbia at Porthill.Later in the mid-1930s, Idaho adopted a new numbered highway system with SH-1 as one of the first highways.The International Selkirk Loop, which uses SH-1, was formed in 1999.