RCW's

RCW 46.04.336 Motorized foot scooter.
"Motorized foot scooter" means a device with no more than two ten-inch or smaller diameter wheels that has handlebars, is designed to be stood or sat upon by the operator, and is powered by an internal combustion engine or electric motor that is capable of propelling the device with or without human propulsion.
For purposes of this section, a motor-driven cycle, a moped, an electric-assisted bicycle, or a motorcycle is not a motorized foot scooter.
MAJOR NOTE: Serious amendments to Motorized Foot Scooter Laws, like having to stand, instead of being able to sit, and a 20 MPH cap go into effect July 27th, 2009. See this PDF doc.
RCW 46.04.169 Electric-assisted bicycle
"Electric-assisted bicycle" means a bicycle with two or three wheels, a saddle, fully operative pedals for human propulsion, and an electric motor. The electric-assisted bicycle's electric motor must have a power output of no more than one thousand watts, be incapable of propelling the device at a speed of more than twenty miles per hour on level ground, and be incapable of further increasing the speed of the device when human power alone is used to propel the device beyond twenty miles per hour.
RCW 46.04.332 Motor-driven cycle.
"Motor-driven cycle" means every motorcycle, including every motor scooter, with a motor that produces not to exceed five brake horsepower (developed by a prime mover, as measured by a brake applied to the driving shaft). A motor-driven cycle does not include a moped, a power wheelchair, a motorized foot scooter, or an electric personal assistive mobility device.
RCW 46.04.330 Motorcycle.
"Motorcycle" means a motor vehicle designed to travel on not more than three wheels in contact with the ground, on which the driver rides astride the motor unit or power train and is designed to be steered with a handle bar, but excluding a farm tractor, a power wheelchair, an electric personal assistive mobility device, and a moped.
The Washington state patrol may approve of and define as a "motorcycle" a motor vehicle that fails to meet these specific criteria, but that is essentially similar in performance and application to motor vehicles that do meet these specific criteria.
(1) No person shall operate a moped upon the highways of this state unless the moped has been assigned a moped registration number and displays a moped permit in accordance with the provisions of RCW 46.16.630.
(2) Notwithstanding any other provision of law, a moped may not be operated on a bicycle path or trail, bikeway, equestrian trail, or hiking or recreational trail.
(3) Operation of a moped, electric personal assistive mobility device, or an electric-assisted bicycle on a fully controlled limited access highway is unlawful. Operation of a moped or an electric-assisted bicycle on a sidewalk is unlawful.
(4) Removal of any muffling device or pollution control device from a moped is unlawful.
(5) Subsections (1), (2), and (4) of this section do not apply to electric-assisted bicycles. Electric-assisted bicycles and motorized foot scooters may have access to highways of the state to the same extent as bicycles. Subject to subsection (6) of this section, electric-assisted bicycles and motorized foot scooters may be operated on a multipurpose trail or bicycle lane, but local jurisdictions may restrict or otherwise limit the access of electric-assisted bicycles and motorized foot scooters, and state agencies may regulate the use of motorized foot scooters on facilities and properties under their jurisdiction and control.
(6) Subsections (1) and (4) of this section do not apply to motorized foot scooters. Subsection (2) of this section applies to motorized foot scooters when the bicycle path, trail, bikeway, equestrian trail, or hiking or recreational trail was built or is maintained with federal highway transportation funds. Additionally, any new trail or bicycle path or readily identifiable existing trail or bicycle path not built or maintained with federal highway transportation funds may be used by persons operating motorized foot scooters only when appropriately signed.
(7) A person operating an electric personal assistive mobility device (EPAMD) shall obey all speed limits and shall yield the right-of-way to pedestrians and human-powered devices at all times. An operator must also give an audible signal before overtaking and passing a pedestrian. Except for the limitations of this subsection, persons operating an EPAMD have all the rights and duties of a pedestrian.
(8) The use of an EPAMD may be regulated in the following circumstances:
(a) A municipality and the department of transportation may prohibit the operation of an EPAMD on public highways within their respective jurisdictions where the speed limit is greater than twenty-five miles per hour;
(b) A municipality may restrict the speed of an EPAMD in locations with congested pedestrian or nonmotorized traffic and where there is significant speed differential between pedestrians or nonmotorized traffic and EPAMD operators. The areas in this subsection must be designated by the city engineer or designee of the municipality. Municipalities shall not restrict the speed of an EPAMD in the entire community or in areas in which there is infrequent pedestrian traffic;
(c) A state agency or local government may regulate the operation of an EPAMD within the boundaries of any area used for recreation, open space, habitat, trails, or conservation purposes.
RCW 46.04.1695 Electric personal assistive mobility device (EPAMD).
"Electric personal assistive mobility device" (EPAMD) means a self-balancing device with two wheels not in tandem, designed to transport only one person by an electric propulsion system with an average power of seven hundred fifty watts (one horsepower) having a maximum speed on a paved level surface, when powered solely by such a propulsion system while ridden by an operator weighing one hundred seventy pounds, of less than twenty miles per hour.

