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.
The ghostery addon does that whole decline process automatically, and also randomizes data that is mandatory to use the site, to help hide who you actually are. I never see these popups anymore, and I have cookies set to clear on browser close, which I do at least once a day to clean up whatever’s left over.