I've never driven in a state where the simple technique of interstate driving is so utterly and completely ignored. If you drive in Oregon outside of the "urban" areas (Portland, Salem, Eugene), you'll find traffic in the right lane, with folks pulling in to the left lane to pass, and then returning to the right, just as God intended. Not so in Washington. Those who wanted to drive just a bit faster than traffic had to weave dangerously back and forth between the lanes, while Mr. and Mrs. Oblivious sail along for miles in the left lane.
Get a clue. Outside of urban areas, you should never be in the left-most lane of an interstate unless you are in the process of passing someone. Notice that this means you must be going FASTER than the car you are trying to pass, another tidbit that escaped many of the drivers I saw yesterday.