Do not let it go one day and jump down the children's throats the next. If you are having a particularly bad day and cannot tolerate even a small amount of typical car fighting, let your children know in advance: "I have a huge headache today and I will not be able to stand any fighting at all. I need you to cooperate and control yourselves in the car. This is your fair warning." If you say it like you mean it, they will get the message and comply.