Tenn. Code Ann. § 55-9-411

The commissioner of safety is authorized to design or develop standards for a safety warning device or flashing signal light that may be used by rural mail carriers when delivering mail. Rural mail carriers may submit for approval to the state official any such device or light and, if approved for safety reasons by the official, may attach and use the device or light on top of any vehicle used by them while the vehicle is in use delivering mail. The device or light may not be used on the vehicle except when the vehicle is in use delivering mail.