As of 2010 the answer is no. It is recommended to use your turn signals when changing lanes, but not a law.
You could be hitting the flash feature on the turn signal arm when you use it.
We could use a little more info here. The turn signal light? In front? In the rear? The turn signal switch?
Bad flasher unit or turn signal switch
It is more than necessary. It is required by law to use your indicator any time you change lanes or make a turn.
Reason number one - it is a law. Reason number 2 - it allows other drivers, pedestrians, and cyclists to know what your intentions are.
Every time you turn left or right, or switch lanes on a highway you should use your turn signal in your car.
No.
Bad bulb or crossed wires somewhere. If it only happens, say, when you use the left turn signal, but not when you use the right turn signal, it's probably one of the left-side bulbs.
In many cases, the brake light and the turn signal use the same bulb.
If you mean turn signal hand signals, then yes.
These are two separate circuits. They use separate flasher relays. They use separate fuses. They use separate switches. Check your signal fuse and signal flasher relay.
When merging, you should use a turn signal. In any situation in which your movement could affect another car, you should signal. Not to do so is a moving violation.