RCW 46.61.425 (WASHINGTON CODE, NOT CALIFORNIA)
Minimum speed regulation — Passing slow moving vehicle.
(1) No person shall drive a motor vehicle at such a slow speed as to impede the normal and reasonable movement of traffic except when reduced speed is necessary for safe operation or in compliance with law: PROVIDED, That a person following a vehicle driving at less than the legal maximum speed and desiring to pass such vehicle may exceed the speed limit, subject to the provisions of RCW 46.61.120 on highways having only one lane of traffic in each direction, at only such a speed and for only such a distance as is necessary to complete the pass with a reasonable margin of safety.
(2) Whenever the secretary of transportation or local authorities within their respective jurisdictions determine on the basis of an engineering and traffic investigation that slow speeds on any part of a highway unreasonably impede the normal movement of traffic, the secretary or such local authority may determine and declare a minimum speed limit thereat which shall be effective when appropriate signs giving notice thereof are erected. No person shall drive a vehicle slower than such minimum speed limit except when necessary for safe operation or in compliance with law.