For the purpose of apportioning rural arterial trust account funds, the state is divided into five regions as follows:

(1) The Puget Sound region includes those areas within the counties of King, Pierce, and Snohomish.

(2) The northwest region includes those areas within the counties of Clallam, Jefferson, Island, Kitsap, San Juan, Skagit, and Whatcom.

(3) The northeast region includes those areas within the counties of Adams, Chelan, Douglas, Ferry, Grant, Lincoln, Okanogan, Pend Oreille, Spokane, Stevens, and Whitman.

(4) The southeast region includes those areas within the counties of Asotin, Benton, Columbia, Franklin, Garfield, Kittitas, Klickitat, Walla Walla, and Yakima.

(5) The southwest region includes those areas within the counties of Clark, Cowlitz, Grays Harbor, Lewis, Mason, Pacific, Skamania, Thurston, and Wahkiakum.