I have never ever seen such a sign.
I live in Texas, and you will find those signs on just about every 2-lane highway. The DOT and the State Highway Patrol have recently been trying to educate people that the signs mean precisely what they say - "Left Lane for Passing Only", and it is simply not permissible to stay in the left lane and cruise.
It seems most people can't seem to mentally process what the signs means, and therefore ignore them. Neither of my two sons had the topic covered when they took their driver training classes, which is a terrible omission.
As I said earlier, they way to know you're doing it right is that at no driver should ever have to pass you on the right.
It means a lot of lane changing. Quite a lot in moderate traffic, but that's okay. It keeps you alert and cuts down on dangerous highway hypnosis.
In heavy traffic, you will find you spend a lot of time in the left lane as part of the line moving passed the slower traffic in the right lane.
What state are you in where you've never seen a sign like this?
“In heavy traffic, you will find you spend a lot of time in the left lane as part of the line moving passed the slower traffic in the right lane.”
Not in Houston. In fact, so many people default into the left lane that during heavy traffic hours the right lane is actually moving 20 mph faster in some spots. Yet people still pack it into the left lane.