There is hereby established the Lewis and Clark highway. It shall be composed of the following existing routes:
(1) from the Idaho state line west of Lolo Hot Springs, Montana, to the junction with U.S. highway 93 at Lolo;
(2) thence north from Lolo on U.S. highway 93 to Missoula;
(3) thence east from Missoula on U.S. highways 12 and 10 to Garrison;
(4) thence east from Garrison on U.S. highway 12 through Forsyth and Baker to the North Dakota state line.