In order for someone to be proven negligent in an auto accident in which an injury occurs, five things would have to be true.
Duty – The other person, a.k.a the defendant, owed a duty of care to you, the plaintiff. This is true every time a person drives a car. A duty is owed to all the other drivers that you drive as safely as possible.
Breach of Duty – By failing to exercise reasonable care, the defendant put you in jeopardy. If the defendant was speeding, inattentive or under the influence, these would all qualify as actions which breached the duty of care.
Cause in Fact – The actions of the defendant directly caused harm to you and/or your property, and would not have happened otherwise if the defendant wasn’t caring less.
Proximate Cause – This means that the potential harm caused by the plaintiff should have been foreseeable, which may be the most difficult aspect of negligence to prove, depending on the situation.
All four of these things would have to be true in a negligence claim stemming from an injury accident. Deciding how much they were at fault may be another issue.