While it's probably not a wise thing to do, "flipping off" a cop with your middle finger is not reason enough to get you pulled over and arrested.
The Huffington Post reports that a 14-page opinion released by the U.S. Court of Appeals for the 2nd Circuit ruled Jan. 3, that the "ancient gesture of insult is not the basis for a reasonable suspicion of a traffic violation or impending criminal activity."
The "ancient gesture", however, is what got John Swartz in trouble in May 2006. The Huffington Post reports that Swartz and his wife "Judy Mayton-Swartz sued two police officers who arrested Swartz in May 2006 after he flipped off an officer who was using a radar device at an intersection in St. Johnsville, N.Y. Swartz was later charged with a violation of New York's disorderly conduct statute, but the charges were dismissed on speedy trial grounds."
In July 2011, a federal judge in the Northern District of New York granted summary judgement to the police officers but the Jan. 3 decision erased that and ordered the lower court to take up the case again.
So, tell us - would you give a cop the middle finger?