164All signals herein required to be given by hand and arm shall be given from the left side of the vehicle in the following manner and such signals shall indicate as follows:
(a)
to indicate a left turn the left hand and arm shall be extended horizontally,
(b)
to indicate a right turn the left hand and arm shall be extended upward, and
(c)
to indicate a stop or decrease speed the left hand and arm shall be extended downward.