The father
Chad was a father and had a loving family that he held above everything else. So in his mind, when he saw this happening all he could think about was the fact that Emma might crash into someone or something and seriously hurt people. Just imagine if it was another father.
Not even a father, but a mother or a son or daughter to another family? In his mind, that would be the worst thing that could ever happen to him, so he wanted to make sure that he would try his best to prevent it. He felt that he had an obligation to help.