[Oh Haha]

Holy crap!!! This was the situation I was in just last week.

Same setup except that I am the car in the right lane coming up on the slower moving car. Four lane-divided highway.

I look to my left too see a car behind me so I wait before signaling to move left. I slow before reaching the car in front of me.

The guy in the left lane actually slowed a bit and gave me time to move to the left and around the other car. I was amazed that somebody else was paying attention. I gave him a wave before moving back to the right in front of the slower car.

I generally will do the same thing. If I see a car is going to be forced into the left lane I will give them a moment to do that. If they see the opportunity that's great. If not, I will speed up slightly so they might know that I will make room.