Except as otherwise provided, each required signal given by hand and arm shall be given from the left side of the vehicle in the manner specified in this section. A left turn signal is given by the hand and arm extended horizontally. A right turn signal is given by the hand and arm extended upward; except that a bicyclist may extend the right hand and arm horizontally to the right. A stop or decrease in speed signal is given by the hand and arm extended downward.