Wash. Rev. Code § 39.92.050
Local governments are authorized and encouraged to enter into interlocal agreements to jointly develop and adopt with other local governments the transportation programs authorized by this chapter for the purpose of accomplishing regional transportation planning and development. Local governments shall also seek, to the greatest degree practicable, consistency among jurisdictions in the terms and conditions of their programs for the purpose of increasing fairness and predictability on a regional basis. Local governments shall seek comment, in the development of their programs, from other affected local governments, state agencies, and governments authorized to perform public transportation functions. Local governments are also encouraged to enter into interlocal agreements to provide technical assistance to each other, in return for reasonable reimbursement, for the purpose of developing and implementing such transportation programs.
RCW 39.92.050