In 2018 the Washington Legislature amended the statute so that the prevailing wage would be assessed based on the highest wage set by collective bargaining agreements in the county.
Historically, the prevailing wage was calculated by averaging the wages within a certain industry and county. However, in 2018 the Washington Legislature amended the statute so that the prevailing wage would be assessed based on the highest wage set by collective bargaining agreements in the county. The amendment (RCW 39.12.015(3)) reads as follows:
(3)(a)…the industrial statistician shall establish the prevailing rate of wage by adopting the hourly wage, usual benefits, and overtime paid for the geographic jurisdiction established in collective bargaining agreements…
(b) For trades and occupations in which there are no collective bargaining agreements in the county, the industrial statistician shall establish the prevailing rate of wage by…conducting wage and hour surveys.
So, for example, if union engineers bargain for a wage, that is the wage all engineers in the county must be paid on public projects. The legislature passed this law for the sake of efficiency because it took significant resources for the Industrial Statistician to compute the prevailing wage for every trade and every county, but the law has significant knock-on effects.