A one finger salute doesn't convince a bad driver to avoid bad driving in the future or even returning and apologizing for his stupidity. Thus the gesture is useless. Instead, try waving and smiling at him. Then he will think, "I wonder if that is the neighbor's boy that I just cut off. I think he knows me." That will create more guilt in him.
When someone gives me the one finger salute, I respond with a smile and a friendly wave. There is nothing more disconcerting than to have one's insult misunderstood.