It is illegal to honk your car horn except in an emergency.

The Law

To combat the relentless auditory assault of city traffic, New York City introduced rules stating drivers can only use their horn to warn of imminent danger. Using your horn to express anger, greet a friend, or urge the car in front of you to move faster is technically a violation punishable by a hefty fine. Of course, a brief walk down any Manhattan avenue will show you just how frequently this rule is ignored.