Wash. Rev. Code § 36.120.080
If the voters approve the plan, including creation of a regional transportation investment district and imposition of taxes and fees, the district will be declared formed. The county election officials of participating counties shall, within fifteen days of the final certification of the election results, publish a notice in a newspaper or newspapers of general circulation in the district declaring the district formed, and mail copies of the notice to the governor, the secretary of transportation, the executive director of the regional transit authority in which any part of the district is located, and the executive director of the regional transportation planning organization in which any part of the district is located. A party challenging the procedure or the formation of a voter-approved district must file the challenge in writing by serving the prosecuting attorney of the participating counties and the attorney general within thirty days after the final certification of the election. Failure to challenge within that time forever bars further challenge of the district's valid formation.
RCW 36.120.080
Findings-2006 c 311: See note following RCW 36.120.020.