RCW 36.75.060: County road districts.

For the purpose of efficient administration of the county roads of each county the board may, but not more than once in each year, form their respective counties, or any part thereof, into suitable and convenient road districts, not exceeding nine in number, and cause a description thereof to be entered upon their records.

Unless the board decides otherwise by majority vote, there shall be at least one road district in each county commissioner's district embracing territory outside of cities and towns and no road district shall extend into more than one county commissioner's district.