I'm reasonably certain the answer is "no":
RCW 46.61.405, emphasis mine:
Whenever the secretary of transportation shall determine upon the basis of an engineering and traffic investigation that any maximum speed hereinbefore set forth is greater than is reasonable or safe with respect to a state highway...the secretary may determine and declare a reasonable and safe lower maximum limit...which shall be effective when appropriate signs giving notice thereof are erected.
RCW 46.61.410, emphasis mine:
(1)(a) Subject to subsection (2) of this section the secretary may increase the maximum speed limit on any highway or portion thereof to not more than seventy-five miles per hour in accordance with the design speed thereof (taking into account all safety elements included therein), or whenever the secretary determines upon the basis of an engineering and traffic investigation that such greater speed is reasonable and safe under the circumstances existing on such part of the highway.
(b) The greater maximum limit established under (a) of this subsection shall be effective when appropriate signs giving notice thereof are erected...
RCW 46.61.415, emphasis mine:
Whenever local authorities in their respective jurisdictions determine on the basis of an engineering and traffic investigation that the maximum speed permitted under RCW 46.61.400 or 46.61.440 is greater or less than is reasonable and safe under the conditions found to exist upon a highway or part of a highway, the local authority may determine and declare a reasonable and safe maximum limit thereon...
(5) Any altered limit established as hereinbefore authorized shall be effective when appropriate signs giving notice thereof are erected.
A speed-limit sign is necessary to give force to a speed limit other than the defaults in RCW 46.61.400, but it does not in and of itself create a speed limit.
However, from a practical standpoint, obeying one is a good idea, so long as it looks like a genuine speed-limit sign: RCW 46.61.050 places the burden of proof on the driver to show that the sign is invalid, rather than on the prosecution to show that it's valid:
(3) Whenever official traffic control devices are placed in position approximately conforming to the requirements of this chapter, such devices shall be presumed to have been so placed by the official act or direction of lawful authority, unless the contrary shall be established by competent evidence.