You can't flash to tell an oncoming car to dim their lights?
You’re not supposed to. Of course, I’ve never seen that one enforced except for once, when a guy flashed his high beams at an oncoming motorist that was highbeaming him. The motorist wouldn’t dim his lights, so he hit his high beams a second time. Motorist still wouldn’t dim his lights, so the guy just switched to high beam like the other motorist.
Then, the blue lights came on atop the other motorist’s car. Don’t remember if the guy was cited or not, but he was definitely warned not to do it in the future.
Actually, I read a little and it is illegal to have them on in my state within 500 feet of an oncoming car.
I do it anyway when someone high beams me, though, and in case of road hazard or emergency vehicles.