The use of a seat belt of the type which is standard equipment shall not violate this article if (i) the affected child is at least four years old but less than eight years old and (ii) any physician licensed to practice medicine in the Commonwealth or any other state determines that use of a child restraint system by a particular child would be impractical by reason of the child’s weight, physical fitness, or other medical reason, provided that any person transporting a child so exempted shall carry on his person or in the vehicle a signed written statement of the physician identifying the child so exempted and stating the grounds for the determination.