It's a code all drivers share: a flash of the high beams is a warning there's a speedtrap ahead. But when police in Florida saw Eric Campbell doing it he got slapped with a $115 ticket.
"The police officer saw me blinking and he was upset, you could tell in his voice, he was professional, he wasn't rude, but you could tell he was irritated," Campbell said.
The cop clearly figured there ought to be a law, but in Florida there isn't. So Campbell's fine for speed trap warning was overturned in court.
In some states however it is illegal to warn other drivers there's a speed trap ahead. Arizona and Alaska forbid flashing of high beams, while the states of New Jersey, New York, Ohio, Pennslvania, Tennessee and Virginia allow it. California allows it. In fact, the state's highway patrol says it supports any action that gets motorists to abide by the speed limit.
"We just want people to slow down, whether it's through enforcement or through word of mouth, as long as people slow down, they're driving safely, that's all we really are concerned with," said Officer Ming Hsu with the California Highway Patrol.
But that wasn't the case in Texas this summer, when Houston resident Natalie Plummer got into big trouble.
"I got handcuffed, put in the back of a cop car, taken to jail for 12 hours, for holding a sign on a sidewalk. "
In June, she held up a hand-drawn sign on the side of a Houston street about a block in front of a speed trap. Police officers questioned her, threatened to charge her with obstructing justice and eventually arrested her.
"I asked the lady who was booking me, so, what is my charge, what am I here for exactly, and she said, oh, well, she looks at the paper, and says, oh, well, it says here you're here for walking on the street or something," Plummer said.
Plummer claims she stayed on the sidewalk the whole time and accuses the cops of restricting her right to free speech. Her case was eventually dismissed but it focused attention on police efforts to clamp down on a traditional driver to driver courtesy.
"What they're being ticketed or arrested for are things other than the actual alert to other drivers."
Loyola law professor Jessica Levinson says police often have to stretch to find a legal reason to go after drivers who warn of speedtraps.
"It feels like there's something unfair about the cops saying, you're not allowed to tell people where we are," Levinson said.
But in the era of the smartphone the tussle between police and drivers is moving beyond headlight flashing. Now there's an app for that.
Red light camera, coming up.
"So you just heard a warning of a red light camera and you can actually see it in the middle of the road here"
It's called Trapster - a free smartphone app that now has 16 million users who can report and map speed traps, red light cameras and other road hazards for all other Trapster users to see.
"Right, just tap the report button, and then tap the red light camera in the top right there ... set it wherever the trap is and you hit the check box to confirm and bam, you've just reported a red light camera."
"Is this legal? Do I have to hide this if a policeman pulls me over?"
"No, not at all," said Sean Farrell, the app's creator. "There's no laws against mobile apps"
"If law enforcement feels that this is not helping them, then I think, yes, the app could face some sort of question, but they're going to have to, you know, whoever prosecutes the app company, is going to have to come up with a good reason as to why this is hindering law enforcement," Levinson said.
No one has attempted that yet, so Trapster continues to grow, and with each touch of a button, the days of flashing headlights seem to be dimming into the past.