Statistics show that most accidents are caused by the driver being distracted. A distraction can come in many forms when behind the wheel of your car, one of them is anger. Being human people get angry and being angry in your car is just one more distraction you do not need. It could cause you to make some rash decisions. Avoiding a driver behaving aggressively is also a major distraction that could cause actions putting you and the drivers around you in a serious situation.
Aggressive driving is defined as: "A form of automobile operation in which an operator will deliberately behave in such a manner as to increase the risk of an automobile accident" (Wikipedia). Aggressive driving includes behaviors such as speeding, running red lights, frequent or sudden lane changes, following too close, flipping off the other driver, and just general stupidity.
A driver acting aggressively becomes not only a danger to themselves and the driver who is the target of their anger but everyone around them. Aggressive behavior and distractions lead to mistakes and accidents. Incidents of aggressive driving can also quickly escalate in to "Road Rage" which usually leads to physical violence. According to the statistics aggressive driving is a key factor in 56% of crashes with fatalities. Insurance payouts to claimants of accidents caused by others bad behavior, especially uninsured drivers is passed on through insurance policy rates.