Tenn. Code Ann. § 55-8-144: Signals by hand and arm or signal device

(a) Any stop or turn signal required by this chapter shall be given either by means of the hand and arm or by a signal lamp or lamps or mechanical signal device approved by the department of safety as provided in § 55-8-143 , except as otherwise provided in subsection (b).

(b) Any motor vehicle in use on a highway shall be equipped with, and required signal shall be given by, a signal lamp or lamps or mechanical signal device approved by the department when the distance from the center of the top of the steering post to the left outside limit of the body, cab or load of the motor vehicle exceeds twenty-four inches (24"), or when the distance from the center of the top of the steering post to the rear limit of the body or load thereof exceeds fourteen feet (14'). The latter measurement shall apply to any single vehicle, also to any combination of vehicles.