 When the U.S. Highways were first established in 1926 , US 12 ended in Miles City , Montana , and most of US 12 's current routing in Washington was followed by US 410 . U.S. Route 12 was extended westward in stages ; an extension to Lewiston , Idaho , was approved on June 19 , 1962 . At various times in the early 1960s , the states of Idaho , Washington , and Oregon submitted plans for further westward extension to the American Association of State Highway Officials ( AASHO ) to either Vancouver , Washington or Boardman , Oregon — or , a plan submitted in 1963 had the highway going only as far as Pasco — but all of these plans were rejected . Washington introduced a new system of sign route numbers in 1964 . While US 410 was still signed as such , present US 12 became State Route 8 between US 410 at Elma and US 99 / I @-@ 5 at Grand Mound and State Route 14 between US 99 / I @-@ 5 at Chehalis and US 410 at Naches . The extension of US 12 to Aberdeen was finally approved on June 20 , 1967 , with US 12 taking over much of US 410 's former routing . However , where US 410 had crossed the Cascades at Chinook Pass , US 12 used the all @-@ weather White Pass , replacing SR 8 and SR 14 . Signs were changed in late December 1967 , and the bypassed segments of US 410 became a new SR 8 between Elma and Olympia , and SR 410 between Tacoma and Naches . 
