Washington Revised Code Title 46. Motor Vehicles § 46.61.470. Speed traps defined, certain types permitted--Measured courses, speed measuring devices, timing from aircraft

(1) No evidence as to the speed of any vehicle operated upon a public highway by any person arrested for violation of any of the laws of this state regarding speed or of any orders, rules, or regulations of any city or town or other political subdivision relating thereto shall be admitted in evidence in any court at a subsequent trial of such person in case such evidence relates to or is based upon the maintenance or use of a speed trap except as provided in subsection (2) of this section. A “speed trap,” within the meaning of this section, is a particular section of or distance on any public highway, the length of which has been or is measured off or otherwise designated or determined, and the limits of which are within the vision of any officer or officers who calculate the speed of a vehicle passing through such speed trap by using the lapsed time during which such vehicle travels between the entrance and exit of such speed trap.

(2) Evidence shall be admissible against any person arrested or issued a notice of a traffic infraction for violation of any of the laws of this state or of any orders, rules, or regulations of any city or town or other political subdivision regarding speed if the same is determined by a particular section of or distance on a public highway, the length of which has been accurately measured off or otherwise designated or determined and either: (a) The limits of which are controlled by a mechanical, electrical, or other device capable of measuring or recording the speed of a vehicle passing within such limits; or (b) a timing device is operated from an aircraft, which timing device when used to measure the elapsed time of a vehicle passing over such a particular section of or distance upon a public highway indicates the speed of a vehicle.

(3) The exceptions of subsection (2) of this section are limited to devices or observations with a maximum error of not to exceed five percent using the lapsed time during which such vehicle travels between such limits, and such limits shall not be closer than one-fourth mile.