RCW 36.81.121 requires the preparation and annual updating of a six-year comprehensive transportation program. The program shall be adopted by the county legislative authority at any time before adoption of the annual budget and shall include all anticipated road and bridge construction projects, capital ferry expenditures, paths and trails projects and any other specified capital outlays for the following six-year period. The purpose of this chapter is to implement these statutory requirements with assurance that the program is based on a realistic assessment of available funding during the program period.