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Question:

You are approaching an intersection and you want to turn left. You cannot see any other vehicles around. Do you need to use your turn signals?

A Yes, in urban or metropolitan areas, you must signal for 100 feet, and on four-lane highways where the speed limit exceeds 40 mph, you must signal for 200 feet.
Explaination

In urban or metropolitan areas, you must signal for at least 100 feet. If you are on a four-lane highway with a speed limit over 40 mph, you must signal for at least 200 feet. [Signaling, 10.4 Turning, 10. Rules of the Road, Colorado Driver Handbook]