ston , ▁Washington , ▁thus ▁being ▁marked ▁as ▁part ▁of ▁the ▁Lewis ▁and ▁Clark ▁Tra il . ▁ ▁The ▁east ▁end ▁of ▁the ▁highway ▁in ▁the ▁state ▁is ▁at ▁Clark ston , ▁where ▁the ▁highway ▁cross es ▁the ▁S nake ▁River ▁into ▁Id aho ▁at ▁Lew ist on . ▁ ▁The ▁Washington ▁section ▁of ▁US ▁ 1 2 , ▁other ▁than ▁a ▁conc urrency ▁with ▁Inter state ▁ 5 , ▁is ▁defined ▁at ▁Washington ▁Re vised ▁Code ▁§ ▁ 4 7 . 1 7 . 0 5 5 . ▁ ▁Id aho ▁ ▁US ▁ 1 2 ▁enters ▁the ▁state ▁at ▁Lew ist on , ▁crossing ▁the ▁S nake ▁River ▁from ▁Clark ston , ▁Washington . ▁It ▁asc ends ▁the ▁Clear water ▁River , ▁concurrent ▁with ▁US ▁ 9 5 ▁for ▁ ▁. ▁ ▁It ▁reduces ▁to ▁a ▁two - lane ▁und iv ided ▁highway ▁with ▁signs ▁that ▁read ▁" w inding ▁road ▁next ▁ 9 9 ▁miles " ▁and ▁goes ▁on ▁to ▁O ro f ino , ▁continu ing ▁up ▁the ▁middle ▁fork ▁of ▁that ▁river ▁to ▁Low ell , ▁the ▁j unction ▁of ▁the ▁L och sa ▁and ▁Sel way ▁R ivers . ▁ ▁It ▁continues ▁up ▁the ▁L och sa ▁and ▁clim bs ▁to ▁L olo ▁Pass ▁at ▁the ▁Mont ana ▁border . ▁This ▁portion ▁of ▁the ▁highway ▁is ▁also ▁designated ▁as ▁part ▁of ▁the ▁Lewis ▁and ▁Clark ▁Tra il . ▁Most ▁of ▁the ▁highway ▁in ▁Id aho ▁is ▁within ▁the ▁Clear water ▁National ▁Forest . ▁The ▁eastern ▁section ▁of