The provisions of § 46.2-888 shall not apply to any rural mail carrier stopping on the highway while collecting or delivering the United States mail at a mailbox, provided there is lettered on the back of the vehicle operated by such rural mail carrier, or lettered on a sign securely attached to and displayed at the rear of such vehicle, in letters at least four inches in height the following words and groups of words, which may be in any order: CAUTION FREQUENT STOPS U.S. MAIL Additionally, the provisions of § 46.2-888 shall not apply to such rural mail carrier so stopping if, in lieu of such sign, the vehicle has, and is using, supplemental turn signals mounted at each side of the roof of the vehicle. Between the lights on the assembly shall be mounted a sign with the words “U.S. Mail”, or at least one flashing amber warning light, mounted on the roof or rear of the vehicle, to be used in conjunction with a rear-mounted “U.S. Mail” sign. The roof-mounted “U.S. Mail” sign required by the foregoing provisions of this section shall be yellow with black letters at least four inches in height, and the lights shall be of the type approved by the Superintendent of State Police. The lettered sign shall be displayed only when the vehicle is engaged in the collection or delivery of the United States mail. Nothing in this section shall be construed to relieve any such mail carrier from civil liability for such stopping on any highway if he is negligent in so doing, and if the negligence proximately contributes to any personal injury or property damage resulting therefrom.