Idk about elsewhere but in the Midwest If you are driving and a semi wants to merge into your lane in front of you, you can flash your lights (off and back on, NOT high beams) a couple times to let them know they are clear of obstacles. Often they will turn off their lights (specifically tail lights or running lights) a couple times as a thank you.

I’ve never driven truck, but my step-dad did and I watched him signal this way for someone once and I’ve done it ever since. He said it makes it a lot easier as a driver, and gives me warm fuzzies when they do it back.