I'm going to flip off a girl texting while driving
Are you doing that because you think she's in the wrong?
Yes she could have gotten many people killed from texting while driving.
Texting while driving is a serious offense. Next time call the police on her to let them handle it, and let that rage within you go. Offensive gestures and actions are not yours to give, your anger is going to be the death of you one day.
She is putting people at risk. I have the right to be angry.
I agree with you that texting and driving is extremely dangerous. But flipping her off while driving is only going distract her more and you will also be distracted. I would recommend just letting this go because there is no positive outcome.