A person may not turn a vehicle at an intersection, unless the vehicle is in the position required by § 21-601 of this subtitle. A person may not turn a vehicle to enter a private road or driveway or otherwise turn a vehicle from a direct course or move it right or left on a roadway or from a shoulder or bikeway onto a roadway, unless the movement can be made with reasonable safety. A person may not, if any other vehicle might be affected by the movement, turn a vehicle until he gives an appropriate signal in the manner required by this subtitle. When required, a signal of intention to turn right or left shall be given continuously during at least the last 100 feet traveled by the vehicle before turning; except that a bicyclist may interrupt the turning signal to maintain control of the bicycle. If there is an opportunity to signal, a person may not stop or suddenly decrease the speed of a vehicle until he gives an appropriate signal in the manner required by this subtitle to the driver of any other vehicle immediately to the rear. The signals provided for in § 21-605(b) and (c) of this subtitle: May be used to indicate an intention to turn, change lanes, or start from a stopped, standing, or parked position; and May not be flashed as a courtesy or “do pass” signal to the driver of any other vehicle approaching from the rear.