Depends apparently. Recent ruling.
He may have missed the side road, or he may have been fucking with the friend.
Hell, driving a sedan around so many large cars means sometimes you can’t tell the difference between their high and low, being frequently blinded.
I haven’t been in the situation in ages, but when I lived in the northeast I flashed and have been flashed, and I sure knew what it meant. I had no idea of whether the car going the other way was speeding of not, but it seemed like a courtesy so they wouldn’t get startled. If I came across a car going so fast as to be unsafe I wouldn’t flash, but that has never happened.
Only on highways - not on streets in town because people shouldn’t speed there.
I can remember my Mom in the 70’s flashing and being flashed, as a warning about speed traps. But anymore, a headlight flash means: 1, “Go ahead, I’ll let you in to traffic ahead of me”; 2, “Hey, dim your lights, willya? You’re blinding me”; or 3, “Oi! Dumbass! Yer driving inna dark! Turn yer damn lights on!”
At least where I live.
I would never warn another person because if you’re speeding, you’re breaking the law and therefore I don’t care if you get caught in your choice. I speed a little every day and I know the risk I am taking.
I flash for a lot of reasons but speed traps are not one of them… I’ll admit I have sometimes flashed speeders when there isn’t a speed trap, just to keep them honest. 
Oh yes, I’d flash. Speed traps are, very nearly without exception, about generating ticket revenue for the city/town.
Guess it depends on your region. I learned about flashing-headlights-for-cops as a child (dad did it) and it’s a very common thing here. I would and have done so in the past (regardless of speed of other driver, as I can’t judge it). I never know exactly how long after passing the cop I should flash for, though.
Anecdote: Once someone flashed headlights at me on the road I lived on. Odd, since I’d really never seen a cop there. Turned out cows were in the road, and no cops were around. Flashing also occasionally done by drivers to indicate there is an accident ahead.
The only time I flash is to request that the oncoming driver stop blinding me. I have no patience with speeders - they endanger me and everyone else on the road. Just the other day, I was going with the flow on the interstate and some moron came zipping along, weaving in and out to get ahead of everyone else. I wish one of the other cars had been an unmarked police vehicle.
No - I meant on. I turn them off once I have crested the hill.
Do you turn them on while approaching the crest of the hill (and potentially other car) but turn them off (to low-beam) before cresting the hill or on meeting the other driver? Because if you don’t I agree with Scabpicker, you’re blinding other drivers.
But then I drive on narrow country roads a lot and never fail to understand why other drivers need to see the other car before dipping their lights, its not really difficult to tell when the other vehicle is about to come into view.
I used to flash for speed traps. Now I only flash for deer next to the road or something else people should pay attention for.
I always flash. Why, just the other day I was over by the local High School going commando in my trench coat, and… oh wait… never mind.
Every time I see the thread title, I momentarily think of a recent Facebook(?) post I saw where all the comments were telling the woman how beautiful she was but I wondered whether they also noticed that it seemed like a cold day and she could use more padding and didn’t she notice that before posting?
Ooooo, please stop doing this. As Disposable Hero explained, you’re almost certainly blinding other drivers worse than you otherwise would be.
The only way this could work to not blind the oncoming driver is for a few degrees, and only if you did it while actually cresting the hill at exactly the right moment. I think that’d be really hard to time properly, even if it would be possible. The rest of the time, they’re getting at least the low beams just like they normally would.
If your high beams are adjusted properly, they’re still directed down toward the pavement. So your high beams still hit the oncoming driver in the eyes, just a little earlier, and probably for longer. You’re also doing it with a more focused beam of light, which is going to blind the oncoming driver worse.
OMG, I laughed out loud. Yes there are places where safety is the point of the exercise. But it’s more the exception than the rule where I live.
I flash my lights when I get the chance.