RCW 46.04.304
Moped.
"Moped" means a motorized device designed to travel with not more than three sixteen-inch or larger diameter wheels in contact with the ground, having fully operative pedals for propulsion by human power, and an electric or a liquid fuel motor with a cylinder displacement not exceeding fifty cubic centimeters which produces no more than two gross brake horsepower (developed by a prime mover, as measured by a brake applied to the driving shaft) that is capable of propelling the device at not more than thirty miles per hour on level ground.
The Washington state patrol may approve of and define as a "moped" a vehicle which fails to meet these specific criteria, but which is essentially similar in performance and application to motorized devices which do meet these specific criteria.