Washington Revised Code Title 46. Motor Vehicles § 46.61.305. When signals required--Improper use prohibited

(1) No person shall turn a vehicle or move right or left upon a roadway unless and until such movement can be made with reasonable safety nor without giving an appropriate signal in the manner hereinafter provided.

(2) A signal of intention to turn or move right or left when required shall be given continuously during not less than the last one hundred feet traveled by the vehicle before turning.

(3) No person shall stop or suddenly decrease the speed of a vehicle without first giving an appropriate signal in the manner provided herein to the driver of any vehicle immediately to the rear when there is opportunity to give such signal.

(4) The signals provided for in RCW 46.61.310 subsection (2), shall not be flashed on one side only on a disabled vehicle, flashed as a courtesy or “do pass” signal to operators of other vehicles approaching from the rear, nor be flashed on one side only of a parked vehicle except as may be necessary for compliance with this section.