RCW 46.04.710

"Wheelchair conveyance" means any vehicle specially manufactured or designed for the transportation of a physically or medically impaired wheelchair-bound person. The vehicle may be a separate vehicle used in lieu of a wheelchair or a separate vehicle used for transporting the impaired person while occupying a wheelchair. The vehicle shall be equipped with a propulsion device capable of propelling the vehicle within a speed range established by the state patrol. The state patrol may approve and define as a wheelchair conveyance, a vehicle that fails to meet these specific criteria but is essentially similar in performance and application to vehicles that do meet these specific criteria.