(CBS News) It's a code all drivers share: A flash of the high beams is a warning there's a speed trap ahead.
But when police in Florida saw one man doing it, he got slapped with a $115 speeding ticket.
His case was overturned, but in some states, it is illegal to warn other drivers there is a speed trap ahead. Arizona and Alaska forbid flashing of high beams, while the states of New Jersey, New York, Ohio, Pennsylvania, Tennessee and Virginia allow it.
In California, 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," said Officer Ming Hsu. "Whether it's through enforcement or word of mouth, as long as people slow down, they're driving safely, that's all we really are concerned with."
Loyola law professor Jessica Levinson says police often have to stretch to find a legal reason to go after drivers who warn of speed traps.
"It feels like there's something unfair about the cops saying, you're not allowed to tell people where we are," she 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.
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.
"If law enforcement feels that this is not helping them, then I think, yes, the app could face some sort of question," Levinson said. "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."
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.