The California Vehicle Code states that turn signals must be used 100 feet before turning or changing lanes. The purpose of doing that ahead of the actual move is to warn other drivers, and pedestrians as well, of the intended move.
From my experience, most drivers do not signal at all or do it during or after they make the lane change or turn, which is no warning at all. Moreover, a considerable number of the drivers who do not warn of their intended lane change or turn recklessly cut in front of the car they are passing and zoom on to tailgate the next car in front of them.
So, please, drivers, use your turn signals as a warning of an intended move on the road and in parking lots. Not only would it polite and respectful, but it would also be safer and lawful.
Jim Flanagan, North Fork