A signal of intention to turn right or left shall be given continuously during not less than the last fifty feet traveled by the vehicle before turning. The signals herein required shall be given either by means of the hand and arm or by a signal .lamp or signal device of a type approved by the State Department of Roads and Irrigation. The hand-and-arm signals herein required shall be given from the left side of the vehicle in the following manner:
(a) Stop, or decreased speed, hand and arm extended downward;
(b) Left turn, hand and arm extended horizontally;
(c) Right turn, hand and arm extended upward.
(1958 Code § 16.50)