The left lane is for passing.
In a little more detail: The left lane is for passing, not for cruising. The left lane is for passing, not for people who think they might be the fastest car on the road. The left lane is for passing, not for getting a better view of traffic on the opposite side of the interstate. The left lane is for passing, not so you can have your random pick of lanes for the hell of it.
And if you are passing multiple slow cars all at once and see someone coming up behind you going much faster than you, get over to the right lane for a minute to let that person by, then finish your passing.
I thought we were all taught this by our parents and by drivers' ed — apparently not.
That is all.
(We just returned from a little road trip to Raleigh, with a combined 13 hours of driving over the weekend.)
(p.s., Dad, sometime I'll post something about people who think they're too good for turn signals...)