Wash. Rev. Code § 47.17.055
A state highway to be known as state route number 12 is established as follows:
Beginning at a junction with state route number 101 at Aberdeen, thence easterly by way of Montesano and Elma to a junction with state route number 8 in the vicinity of Elma; also
From that junction with state route number 8 in the vicinity of Elma, thence southeasterly to a junction with state route number 5 in the vicinity north of Centralia; also
Beginning at a junction with state route number 5 in the vicinity south of Chehalis, thence easterly by way of Morton and White Pass to a junction with state route number 410 northwest of Yakima; also
From that junction with state route number 410 northwest of Yakima, thence southeasterly to a junction with state route number 82 at Yakima; also
Beginning at a junction with state route number 182 near Pasco, thence southeasterly by the most feasible route by way of Wallula to Walla Walla, thence northerly by way of Dayton to a junction with state route number 127 at Dodge; also
From that junction with state route number 127 in the vicinity of Dodge, thence easterly by the most feasible route by way of Pomeroy and Clarkston to the Washington-Idaho boundary line.
RCW 47.17.055