A recent Ask Yahoo question was, “Is using a turn signal while changing lanes the law or just common courtesy?
The short answer: It’s the law.
More specifically, the Uniform Vehicle Code states,
For vehicles equipped with mechanical or electrical turn signals, a signal of intention to turn or move right or left when required shall be given continuously during not less than the last 100 feet traveled by the vehicle before turning.
as well as
No person shall stop or suddenly decrease the speed of a vehicle without first giving an appropriate signal in the manner provided herein to the driver of any vehicle immediately to the rear when there is opportunity to give such signal.
That second part is particularly common and I don’t understand the logic of people who do it. A blinker is meant as an indication that you’re going to turn. I’ve seen people come to a complete stop in the middle of a lane, only to turn on their blinker right before they turn. It’s an exercise in futility at that point because I could have already smashed into the back of them by that point.
I wish more people would follow the simple order of operations for using a blinker. First, turn on your blinker to indicate you’re going to turn and to warn people you’ll be slowing down, then and only then should you slow down to make the turn.