Actually, a cop can arrest you under many things for that act. It's called Inciting Speeches and Gestures. That particular gesture is in and of itself, intended to get someone fired up, so it is considered to be a crime in many places, especially when directed at a law enforcement officer. Also, why would you do so when the cop has the law on his/her side. They will just "casually" follow you until you do mess up, and trust me, you will at some point. Do you know that there are over 200 reasons for PC (probable cause) dealing with vehicles. Leaving your high beams on too long at night, loud exhaust, bad tail/brake light, cracked windshield, bald tires, weaving, crossing the center line, no seatbelt, just to name a few. It's not suggested, trust me.