Washington Revised Code Title 46. Motor Vehicles § 46.61.610. Riding on motorcycles

A person operating a motorcycle shall ride only upon the permanent and regular seat attached thereto, and such operator shall not carry any other person nor shall any other person ride on a motorcycle unless such motorcycle is designed to carry more than one person, in which event a passenger may ride upon the permanent and regular seat if designed for two persons, or upon another seat firmly attached to the motorcycle at the rear or side of the operator.  However, the motorcycle must contain foot pegs or be equipped with an additional bucket seat and seat belt meeting standards prescribed under 49 C.F.R. Part 571 for each person such motorcycle is designed to carry.