Flipping off cops is legal, though we don't recommend it.   

According to this REPORT, Greg Bombard flipped off an officer during a traffic stop who then arrested him for disorderly conduct.  Mr. Bombard got $175,000 for his troubles.  No admissions of wrongdoing are part of the deal.

All things given equal, we'd rather you not make any statements to police if you're involved in a traffic stop.  And by "statements" we also mean flipping them off.  But if you do and get arrested for it, give us a call.