331.13  SIGNALS BEFORE CHANGING COURSE, TURNING OR STOPPING.
   (a)   No person shall turn a vehicle or move right or left upon a highway unless and until the person has exercised due care to ascertain that the movement can be made with reasonable safety, nor without giving an appropriate signal in the manner hereinafter provided.
   (b)   When required, a signal of intention to turn or move right or left shall be given continuously during not less than the last 100 feet traveled by the vehicle before turning.
   (c)   No person shall stop or suddenly decrease the speed of a vehicle without first giving an appropriate signal in the manner provided herein to the driver of any vehicle immediately to the rear, when there is opportunity to give a signal.
   (d)   Any stop or turn signal required by this section shall be given either by means of the hand and arm, or by signal lights that clearly indicate to both approaching and following traffic the intention to turn or move right or left, except that any motor vehicle in use on a highway shall be equipped with, and the required signal shall be given by, signal lights 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 such motor vehicle exceeds 24 inches, 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 14 feet, whether a single vehicle or a combination of vehicles.
   (e)   The signal lights required by this section shall not be flashed on one side only on a disabled vehicle, flashed as a courtesy or "do pass" signal to operators of other vehicles approaching from the rear, nor be flashed on one side only of a parked vehicle except as may be necessary for compliance with this section.
(ORC 4511.39)
Penalty - see Sections 307.01 and 307.02