I heard that if you flash lights to warn drivers of a speed trap you can get a ticket.
If true what law does warning others violate?
I believe in some jurisdictions such actions are considered as interfering with law enforcement. Exactly which law it violated would depend on the jurisdiction.
Many states have had the courts rule that flashing to warn is protected under free speech. Try this:
Hell, holding a written sign is free speech - but it hasn’t stopped people from getting cited.
I wonder what would happen if I just walked out to the nearest street with a speed trap sign, knowing that there was not a speed trap nearby. Would I get busted, too?
There is at least one smart phone map app (waze is the one I know of) that has interactive input from users who can warn other users of speed traps, among other road hazards. Prosecuting that would be pretty difficult, I think.
A defence might be ‘I was not warning the other driver of a speed trap. It appeared to me that he was speeding, and as a safe driver who follows speed laws, I was trying to get him slow down.’
It worked here, but that was 30 years ago: the law may well have changed again.
The man’s defense was “But I always flash my headlights at people I see speeding”, and the courts believed him, and the defense was accepted.
Of course, 40 years ago it used to be common to flash your headlights at people to warn them of hazards, or just to say hello.
And back in the day, it was entirely credible that someone might be speeding on a poorly sign posted road, and not know they were speeding unless a friendly person flashed at them.
You can also flash to let people merge (which I do on occasion).