Whenever any physician licensed to practice medicine in the Commonwealth or any other state determines, through accepted medical procedures, that use of a child restraint system by a particular child would be impractical by reason of the child’s weight or height, physical unfitness, or other medical reason, the child shall be exempt from the provisions of this article. Any person transporting a child so exempted shall carry on his or her person or in the vehicle a signed written statement of the physician identifying the child so exempted and stating the grounds therefor.