The Revised Code of Washington, 46.61.140(1), states that whenever a roadway has been divided into two or more clearly marked lanes for traffic the following rules in addition to all others consistent herewith shall apply: “(1) a vehicle shall be driven as nearly as practicable entirely within a single lane and shall not be moved from such lane until the driver has first ascertained that such movement can be made with safety.”
This statute shows that the Washington legislature recognizes that a person may go outside their lane of travel and not be in violation of the law. Therefore, the law anticipates that will not be able to constantly maintain their lane of travel, brief drifting over the line will happen.
The Washington Court of Appeals has addressed this lane change statute in the case State v. Prado. In this case, the driver crossed over lane dividers by two tire widths for one second while on a highway off ramp. The Court decided that the Washington legislature's words “as nearly as practicable” show that brief incursions over lane lines will happen. Therefore, if a vehicle crosses over lane dividers momentarily, it is not a legitimate basis for a traffic stop in violation of a lane travel statute because legislature indicated intent to avoid penalizing brief, momentary, and minor deviations of lane lines.
This does not go to say that if the lane travel is accompanied by any other traffic violation or driving patterns that lead an officer to believe that the driver may be impaired, then there may be a legitimate basis for a traffic stop. It does mean, however, that only briefly crossing outside your own lane is not a permissible basis for a traffic stop.