A Defensive Driver Always Signals
During the question and answer portion of any group presentation that I give, someone always asks about the use of turn signals. They are either curious to know what they must do or are being sarcastic about the drivers that they see around them that never signal. It's usually the latter but when asked to articulate, most drivers don't know exactly what is required of them by law and what a defensive driver will choose to do for safety.
Regardless of the fact that you may be the only vehicle on the highway, you must always signal a start from a stopped position or when making a lane change. If you are turning, you are only required to signal if your turn will affect surrounding traffic. Oddly enough, a semaphore arm may still be a legal method of giving a signal in addition to hand and arm signals or signal lights.
In my defensive driving classes I was told that I must always signal any start, turn or lane change. A lane change was considered to have happened if I moved more than half a vehicle width to the left or right. This meant that I had to signal left and then right if I moved partially out of my lane to drive safely around an obstruction at the side of the road.
If you always signal correctly and make a driving error you will show your intention to surrounding traffic. This may be enough to prevent a collision.
Finally, ask any emergency vehicle driver and they will tell you that when they are asking for right of way with lights and siren activated, signal your intention to get out of the way and then follow that signal without fail. They will worry about getting around you safely after that.