What is the ruling on insulting a driver with "son of a whore" and what is its expiation?

What you have done by insulting a man and accusing his mother of fornication is a great evil and an act of slander (qadhf), which is one of the major sins. You must hasten to repent. The principle is that you should seek their forgiveness, but if you cannot reach them, then you must increase your supplications for forgiveness (istighfar) and pray for them. You must be careful not to fall into such acts in the future, and to control your anger, while guarding your tongue, as the Prophet, peace and blessings be upon him, said: "Are people thrown into the Hellfire on their faces—or on their noses—except because of the harvest of their tongues?"