When you're involved in a car accident, if may seem obvious whose fault it was. However, even if someone else was responsible, that doesn't mean that they can be held liable for it. Certain things have to be proven in order for a person to be considered negligent in civil court. This applies not just to car accidents but to other types of personal injury cases as well.
The first question is whether the defendant breached a legal duty of care that he or she owed to the plaintiff. For example, a person is expected to drive safely. The standard for whether a person breached that duty is what a reasonably prudent person would do in a situation.