Funds available for expenditure by the board pursuant to RCW 36.79.020 shall be apportioned to the five regions for expenditure upon county arterials in rural areas in the following manner:

(1) One-third in the ratio which the land area of the rural areas of each region bears to the total land area of all rural areas of the state;

(2) Two-thirds in the ratio which the mileage of county arterials and collectors in rural areas of each region bears to the total mileage of county arterials and collectors in all rural areas of the state.

The board shall adjust the schedule for apportionment of such funds to the five regions in the manner provided in this section before the commencement of each fiscal biennium.