It is illegal to flash your high beams at oncoming drivers for any reason
(1) Whenever a motor vehicle is being operated on a roadway or shoulder adjacent thereto during the times specified in RCW 46.37.020, the driver shall use a distribution of light, or composite beam, directed high enough and of sufficient intensity to reveal persons and vehicles at a safe distance in advance of the vehicle, subject to the following requirements and limitations: (2) Whenever a driver of a vehicle approaches an oncoming vehicle within five hundred feet, such driver shall use a distribution of light, or composite beam, so aimed that the glaring rays are not projected into the eyes of the oncoming driver. The lowermost distribution of light, or composite beam, specified in RCW 46.37.220(2) shall be deemed to avoid glare at all times, regardless of road contour and loading. (3) Whenever the driver of a vehicle approaches another vehicle from the rear within three hundred feet such driver shall use a distribution of light permissible under this chapter other than the uppermost distribution of light specified in RCW 46.37.220(1). [1963 c 154 § 17; 1961 c 12 § 46.37.230. Prior: 1955 c 269 § 23; prior: 1947 c 267 § 5, part; Rem. Supp. 1947 § 6360-25a, part; RCW 46.40.140, part; 1933 c 156 § 3, part; 1929 c 178 § 5, part; 1927 c 309 § 22, part; RRS § 6362-22, part.] In summary, Washington law prohibits flashing one’s high beams within 400 feet of another vehicle, including using the beams to signal for any reason. Flashing high beams can have contradictory meanings and confuse other drivers even though your intentions may be harmless and misinterpretation of the flashing driver’s intent can cause accidental crashes. In many other states high beams can be used to request a slow driver to pull over to allow passing. Use of high beams for the purpose is also illegal in Washington State.