In preparing their respective six-year programs relating to rural arterial improvements, counties shall select specific priority improvement projects for each functional class of arterial based on the rating of each arterial section proposed to be improved in relation to other arterial sections within the same functional class, taking into account the following:

(1) Its structural ability to carry loads imposed upon it;

(2) Its capacity to move traffic at reasonable speeds;

(3) Its adequacy of alignment and related geometrics;

(4) Its accident experience; and

(5) Its fatal accident experience.

The six-year construction programs shall remain flexible and subject to annual revision as provided in RCW 36.81.121.