Headlight flashing: What it really means and is it illegal?

Flashing headlights can actually mean a few different things, depending on the context.
Fortunately, it only takes a second to check the two most common things the other driver is trying to communicate: Your own headlights, and the potential dangers ahead of you on the road (and despite what you may have heard, it's not a gang initiation ritual.)
But a quick flash of the headlights typically means the presence of law enforcement ahead, with radar, trying to catch speeders.
So, the question becomes, is it legal to flash your headlights for this reason?
In some states it used to be illegal to flash your headlights to warn other drivers about a speed trap, but for drivers in Missouri that changed back in 2014. A federal court in St. Louis ruled that drivers are allowed to flash their lights to warn other motorists of nearby police and speed traps. The court ruled that it’s a first amendment right.
The American Civil Liberties Union (ACLU), has stepped in numerous times across the country representing individuals concerning this subject. The particular ruling for Missouri stemmed from a 2013 case in Ellisville when a driver received a citation for flashing his headlights “to notify motorists of a radar set up ahead.”
"The police cannot retaliate against drivers who have done nothing wrong and are simply exercising their right to communicate with other drivers," says Jeffrey A. Mittman, executive director of the ACLU of Missouri.
Each state has different rules regarding flashing lights as a warning. However, with the Supreme Court ruling in Spence v. Washington, it’s safe to say outdated municipal ordinances about this issue don’t hold water.