Snopes .com has this as ’ False ’ and calls it an Urban Legend .
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My dad got a ticket for flashing brights to warn oncoming traffic of a speed enforcement officer, I do flash the brights when a truck has a lane change symbol on to let the driver know I see your intentions and you are clear, usually get the light blink thank you also. @jtsanders
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That was urban story around here also…totally false.
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I actually flashed my lights this morning on the way to work! A semi was in the left lane and I followed behind for 1/4 mile or so. I was in the right lane. Eventually, I decided to pass him on the right. Of course, as soon as I sped up, he put his right signal on. So I slowed a bit, flashed my lights, let him get in the right lane, and passed him on the left. He blinked his tail lights as he turned into the right lane. So truckers must still get it.
Just us old ones. 
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All still accepted “signals” to me.
Does anybody have a memory of the unwritten light code truckers used to follow? I recall it was fairly short but comprehensive. Things like 1 flash means? 2 flash means? Hi/ low rapid flash means? Lights on / off or off/ on means?