The Idaho Transportation Department 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. highways, and state highways.
Instead of numbering its highways, Idaho had a system of lettered Sampson Trails in the 1920s. They were marked by Charles B. Sampson of Boise at no expense to the state. By the mid-1930s, the state had adopted a more standard system of numbered state highways.
The following trails were marked:
A, Boise to Wyoming via Twin Falls and Pocatello; became US-30 and US-30N
B, Boise to Canada via Lewiston and Coeur d'Alene; became US-30, US-30N, US-95, and SH-1
C, Pocatello to Montana via Idaho Falls; became US-91 and US-191
D, Boise to New Meadows via Banks; became SH-15 (today's SH-55)
E, not on the 1926 map
F, Boise to Blackfoot via Stanley; became SH-21, SH-17, US-93 (today's SH-75), and SH-27
G, Mountain Home to Picabo via Fairfield; became SH-22 (today's US-20)
H, Bliss to Trude via Arco; became SH-24, SH-23, SH-22, SH-29, and an unnumbered road (today's US-20 and US-26)
I, Pioneer to Idaho Falls; became an unnumbered road
J, Nevada to Shoshone via Twin Falls; became US-93
K, Utah to McCammon via Preston; became US-91
L, not on the 1926 map
M, Washington to Montana via Coeur d'Alene; became US-10
N, Bonners Ferry to Montana; became US-2
O, Oregon to Caldwell; became SH-49 and SH-18
P, Utah to Downey; became SH-36
Q, Declo to Fairfield; became SH-25 and SH-46
R, not on the 1926 map
S, Oregon to Nampa; became unnumbered road and SH-45
T, Star to Grimes Pass; became SH-16, unnumbered road, and SH-17
U, Utah to Montpelier; became SH-35
V, Hailey to Stanley; became US-93 (today's SH-75)
W, not on the 1926 map
X, not on the 1926 map
Y, Bonners Ferry to Canada; became US-95
