Unhinged.

Rachel is a divorced single mother whose bad day gets even worse. She's running late to drop her son off at school when she honks her horn impatiently at a fellow driver during rush-hour traffic. After an exchange of words, she soon realizes that the mysterious man is following her and her young son in his truck. A case of road rage quickly escalates, at horrifyingly psychotic proportions, into full-blown terror as Rachel discovers the psychopath's sinister plan for revenge. He is single-mindedly determined to teach her a deadly lesson.