 Although US 12 was not extended into Washington until 1967 , portions of it have been part of Washington 's state highway system since as early as 1905 . The last part of the highway to open was over White Pass in 1951 , although it was added to the state highway system by the legislature in 1931 . Most of the route ( except for the approximately 160 miles ( 260 km ) between Elma and Naches ) had been part of the U.S. Highway System since its inception in 1926 as part of U.S. Route 410 . The portion between Napavine and Grand Mound was also designated a U.S. Highway in 1926 as part of U.S. Route 99 . 
