Wonderful replies above…
It’s difficult to deal with these kinds of things as discrete instances without knowing more about one’s general way of dealing with things. (and no I’m not asking for details)
My personal experience is that, while even a saint can be moved to anger at times, most of us struggle with anger issues to one degree or another.
It is the anger issue that is more of a concern than the specific act of “flipping someone off” or whatever.
While I agree with the others here that this does not quite rise to the level of mortal sin, I would say that it is a serious sin…and one that should be confessed.
We need the grace of confession to combat our “character flaws” (anger, greed, envy, pride etc.) which are often the underlying causes of many of our sins…
Peace
James