Wash. Rev. Code § 47.04.040
Upon and after April 1, 1937, all rights-of-way of any primary state highways, together with all appurtenances thereto, the right or interest in or to which was, or is, in any county, road district, township, local improvement district, or other highway or road district or political subdivision of the state of Washington shall be and the same is hereby transferred to and vested in the state of Washington for use in conjunction with such primary state highways under the department of transportation.
All public highways in the state of Washington which have been designated to be primary state highways or secondary state highways or classified as primary roads and which have been constructed and improved and maintained for a period of seven years prior to April 1, 1937, at the expense of the state shall operate to vest in the state of Washington all right, title, and interest to the right-of-ways [rights-of-way] thereof, including the roadway and ditches and existing drainage facilities, together with all appurtenances thereto and no informalities in the records of title to such public highways shall be construed to invalidate or vacate such public highways or to divest the state of Washington of any right, title and interest in the right-of-way thereof.
RCW 47.04.040