This is the relevant law in Maine:

>6. Ways with speed limit of 65 or more miles per hour. An operator driving on a limited-access way with a speed limit of 65 or more miles per hour is restricted in ordinary operation to the right-hand lane and may use adjacent lanes for overtaking and passing another vehicle, but must return to the right-hand lane at the earliest opportunity. This requirement does not apply to an authorized emergency vehicle, or to a vehicle otherwise directed by posted signs, a law enforcement officer or a highway maintenance crew.

Basically, the right lane is the lane you're supposed to drive in. The middle lane is the passing lane, and the left lane is an extra passing lane that you can use if you need to. Drivers in the middle or left lanes are supposed return to the right-hand lane at the earliest opportunity.

Treating it that way goes hand-in-hand with prohibiting big trucks from using the left lane. They still have a passing lane that they can use, because the travel lane is still the far right.

Only about 10% of the drivers down in that part of the state understand this, though, so good luck.