Wash. Rev. Code § 46.04.552
"Special mobile equipment" means every vehicle not designed or used primarily for the transportation of persons or property and only incidentally operated or moved over a highway, including but not limited to: Ditch digging apparatus, well boring apparatus and road construction and maintenance machinery such as asphalt spreaders, bituminous mixers, bucket loaders, tractors other than truck tractors, ditchers, leveling graders, finishing machines, motor graders, road rollers, scarifiers, earthmoving carryalls and scrapers, power shovels and draglines, and self-propelled cranes and earthmoving equipment. The term does not include house trailers, dump trucks, truck mounted transit mixers, cranes or shovels or other vehicles designed for the transportation of persons or property to which machinery has been attached.
RCW 46.04.552
Effective date-1963 c 154: See note following RCW 46.37.010.