It depends. In State v. Prado 186 P.3d 1186 (WA 2008) Washington Courts for the very first time took up what it meant to drive “as nearly as practicable" in one’s own lane. The pertinent statute, RCW 46.61.140(1), provides:
“Whenever any 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.”
In Prado, Tonelli Prado's car on I-5 crossed an eight-inch white line dividing the James Street exit lane from the adjacent lane by approximately two tire widths for one second.
The Court held: “Washington State's requirement that automobile drivers remain within a single lane of travel "as nearly as practicable" does not impose strict liability. (my emphasis) A vehicle crossing over a lane once for one second by two tire widths does not, without more, constitute a traffic violation justifying a stop by a police officer.”
I have been able to use Prado to get some great resolutions or dismissals for clients. The reality is that the trooper or officer must have enough to make that initial contact with you - whatever it may be.