It's not against the law to flip off a cop:
U.S. District Judge David S. Cercone ruled David Hackbart exercised his constitution right to free speech in 2006 when he angrily displayed his middle finger to another driver during a parking dispute in Squirrel Hill and when he displayed the same gesture to Officer Brian Elledge, who told Hackbart to stop.
“The United States Supreme Court has long recognized that non-verbal gestures and symbols may be entitled to First Amendment protection,” Cercone wrote in his 19-page opinion and order filed Monday. “Moreover, several courts, including federal and state courts in Pennsylvania, have found that the expressive use of the middle finger is protected speech under the First Amendment.”
http://www.pittsburghlive.com/x/pittsburghtrib/news/breaking/s_617562.html