Wash. Rev. Code § 36.75.035
A county pursuant to chapter 36.88 RCW, or a service district as provided for in chapter 36.83 RCW, may, with the approval of the state department of transportation, improve or fund the improvement of any state highway within its boundaries. The county may fund improvements under this section by any means authorized by law, except that expenditures of county road funds under chapter 36.82 RCW under this section must be limited to improvements to the state highway system and shall not include maintenance or operations. Nothing in this section shall limit the authority of a county to fund cooperative improvement and maintenance agreements with the department of transportation, authorized by RCW 36.75.030 or 47.28.140.
RCW 36.75.035
County road improvement districts and service districts may improve state highways: RCW 36.83.010 and 36.88.010.