Wash. Rev. Code § 36.79.050
At the beginning of each fiscal biennium, the board shall establish apportionment percentages for the five regions defined in RCW 36.79.030 in the manner prescribed in RCW 36.79.040 for that biennium. The apportionment percentages shall be used once each calendar quarter by the board to apportion funds credited to the rural arterial trust account that are available for expenditure for rural arterial and collector projects and for construction of replacement bridges funded by the federal bridge replacement program on access roads in rural areas. The funds so apportioned shall remain apportioned until expended on construction projects in accordance with rules of the board. Within each region, funds shall be allocated by the board to counties for the construction of specific rural arterial and collector projects and construction of replacement bridges funded by the federal bridge replacement program on access roads in rural areas in accordance with the procedures set forth in this chapter.
RCW 36.79.050