Does flashing your headlights mean ‘go ahead’?

Sometimes drivers at a four-way stop flash their lights to signal others to go ahead. That leaves room for miscommunication, Mr. Robinson said, especially if a driver intends the signal to mean that others should stay put.

Robert K. Gillmer, the president of the Driving School Association of the Americas in North Wales, Pa., near Philadelphia, said that if the other person is slow to respond to the signal, the one yielding might then decide to go.

“If that person doesn’t react fast enough, they could change their mind,” Mr. Gillmer said. “To play it 1,000 percent safe,” he recommended against using headlights to signal other drivers. “By you doing no action, it’s better that you don’t have somebody misinterpret what you are saying,” he said.

In other cases, drivers might use their lights to indicate that it’s safe to pull into traffic or make a turn.

“Flashing your head beams to say ‘It’s O.K. to come out’” when the other driver cannot see oncoming traffic is fairly common, Sergeant McDonnell said. But he cautioned drivers to do so at their own peril, “because you are putting your safety in the hands of that other driver’s courtesy.”

What are other headlight no-nos?

Experts advised against using headlights to warn drivers about a speed trap.

Mr. Van Tassel said that AAA does not encourage that kind of behavior and that motorists should drive within the speed limit. He noted that the effects of a crash at 65 miles per hour are much worse than one at 55 m.p.h.

Some suggested that using headlights in this way could be interpreted by the authorities as obstructing governmental administration, though Sergeant McDonnell said he didn’t know if he would go that far.