WEST PALM BEACH, FLA. — A driver in the next lane is moving his lips. Is he on a hands-free cellphone? Talking to someone in the car? To himself? Singing along to the radio?

If lawmakers follow the advice of a federal board, police officers will have to start figuring that out — somehow.

The National Transportation Safety Board said this week that drivers should not only be barred from using hand-held cellphones, as they are in several states, but also from using hands-free devices. No more, “Sorry, I’m stuck in traffic” calls, or virtually any other cellphone chatter behind the wheel.

Though no state has yet implemented such restrictive rules, the NTSB’s recommendations carry weight that could place such language into future laws, or motivate the federal government to cut funding to states that don’t follow suit.

Many of the men and women patrolling the nation’s streets and highways wonder how they would sort the criminally chatty from the legally chatty.

“It would be almost impossible to determine if someone was talking on a phone or exercising their vocal cords,” said Capt. Donald Melanson of the West Hartford, Conn., police department, which took part in a national pilot program aimed at cracking down on drivers’ cellphone use. “That would be much more difficult to enforce, almost to the point where it would be impossible.”

Officer Tom Nichols of the Port St. Lucie, Fla., police said a law written like the NTSB suggests would be difficult to enforce because so many variables would be at play.

“If you identify someone who has a hands-free set hooked up to their ear that doesn’t mean they are talking on the phone,” he said. “They could be talking to a passenger. They could be talking to a child in the back. They could be singing.”

Police could end up turning to technology for help. They might even end up with the cellphone equivalent of a radar speed gun.

Fred Mannering, a Purdue University civil engineering professor who is associate director of the Center for Road Safety, said that since all cellphones emit signals, a simple Bluetooth detection device could spot them.

Computers are already common in patrol cars, and Mannering said a relatively cheap add-on could fit them to track cellphone signals.

“It would be really easy for police to have a computer on board and pick up those signals,” Mannering said, “but it is sort of Big Brother.”

The NTSB’s proposal, announced Tuesday as a unanimous recommendation of its five-member board, urges all states to impose total bans except for emergencies. It cited deadly crashes caused by distracted drivers across the country, and noted that many studies have shown that hands-free cellphones are often as unsafe as hand-held devices.

The recommendation poses an astounding number of questions. What about chauffeurs and traveling salesmen who spend their entire day on the road? And roadside Amber Alert and Silver Alert notifications that implore drivers to call in if they spot a specific vehicle? What comes of phone lines dedicated to those “How’s My Driving?” signs on trucks? How will you let someone know you’re stuck in traffic?