When David Hackbart was trying to parallel park on Murray Avenue in Squirrel Hill last April, another car pulled up behind him, blocking his path.

Incensed, Mr. Hackbart flipped the other driver off. As he was doing it, though, he heard a voice say, "Don't flip him off."

Before Mr. Hackbart, 32, of Regent Square, saw who had spoken, he turned his middle digit on that person, too.

It was a city police officer, and Sgt. Brian Elledge promptly cited Mr. Hackbart with disorderly conduct.

Now, Mr. Hackbart has filed a federal lawsuit against Sgt. Elledge and the city of Pittsburgh.

The claim alleges violations to Mr. Hackbart's First Amendment right to be free from criminal prosecution or "retaliated against in any way for engaging in constitutionally protected speech."

Witold Walczak, the legal director for the American Civil Liberties Union of Pennsylvania, which filed the lawsuit on Mr. Hackbart's behalf, said one of the reasons for it is to teach law enforcement officials a lesson.

"Police officers need to understand that they are not Miss Manners," he said. "Using profanity under the First Amendment is not, and cannot be, a crime."

There have been at least a half-dozen of these types of cases in the area in recent years, he said. In just three that he's handled, Mr. Walczak said two settled for $10,000 and one went to a jury trial, which sided with the plaintiff.

"These happen with great frequency," he said. "For whatever reason, the police don't seem to get it."

Arthur Hellman, a constitutional law professor at the University of Pittsburgh, said that the Supreme Court has long held that some forms of conduct -- such as flipping the bird -- are seen as a form of speech.

"They're intended as communication, and recognized by the audience," he said. "Everybody knows what the message is."

The only time that type of speech is unprotected, the professor continued, is if it is seen as "fighting words," or intended to provoke retaliation.

But, what would likely work in Mr. Hackbart's favor, Mr. Hellman said, is that the courts have also found that "fighting words" used against police officers are generally protected anyway.

"Police officers are expected to be thick-skinned," Mr. Hellman said. "It's just part of their jobs, unfortunately, that they have to take a certain amount of verbal abuse."

According to the nine-page complaint, Sgt. Elledge was upset with Mr. Hackbart.

"During the vehicle stop, Sgt. Elledge demanded Plaintiff's driver's license and social security number and continued to shout at Plaintiff: "You don't flip me off!" it read.

After he was cited, Mr. Hackbart was found guilty by a district judge in June and was ordered to pay fines and costs of $119.75. Later, when Mr. Hackbart appealed, the district attorney's office withdrew the charge.

He is seeking compensatory and punitive damages, as well as court costs and attorney's fees. Mr. Hackbart was living in Florida at the time, so he had to pay $240 for a plane ticket and he lost $300 in wages for taking time off.