As a New Yorker, you know all about noise. You live with it every day. Last year, the New York City Police Department's Quality of Life Hotline fielded more than 90,000 complaints about noise, from boom boxes to jackhammers to car alarms.

Even Mayor Michael Bloomberg raised his voice over the din last year when he launched Operation Silent Night, a crackdown on cacophony in the city. Armed with sound meters, police officers concentrated on 24 neighborhoods within the five boroughs and began ticketing motorists for loud stereos, loud engines and loud horns.

The mayor's call for quiet raises some serious but seldom-asked questions for drivers. When is honking necessary? When is it legal? And when is it just noise?

Horns by the Book

Article 9, Section 375, Paragraph 1 of New York State's Vehicle and Traffic Laws requires every motor vehicle to have "a suitable and adequate horn or other device for signaling, which…shall not be used other than as a reasonable warning nor be unnecessarily loud or harsh." Obviously, the key words "reasonable warning" leave room for interpretation.

With horns-as with so much else in life-context is everything.

"The idea behind the law is that you use your horn only when there is imminent danger of a collision," says Frank Niland of the Club's Traffic Safety Department. "For example, when a pedestrian steps off the curb into your path without looking at you, or when a driver about to change lanes doesn't see you. In those cases, you have to alert others to your presence."

Because the words "reasonable warning" imply danger, one particular use of the horn irks some noise-abatement advocates: the honk that accompanies the "lock" button on some remote keyless entry systems.

"Why should a car honk just as confirm-ation that the doors are locked?" asks Ted Rueter, director of Noise-Free America, a national grass-roots organization fighting noise pollution. "I can't believe that the engineers who dreamed this up couldn't have designed a better alternative-say, a little red light on the controller to indicate that the remote is working. In most states, it's illegal to use a horn except to signal danger. Using a horn in any other circumstance diminishes its significance for safety. People become inured to honking."

Tooting Taboos

By confining the horn to a warning device, the law also seems to preclude other kinds of honking-those at the driver's discretion. Safety experts urge you to think before blowing your horn in the following instances:

• As a substitute for your brakes. "Sometimes drivers blow their horns instead of slowing down," says Niland. "You see this most often when they're approaching an intersection and the light changes. They go to the horn as if to say 'Out of my way!' when they should go to the brakes." Naturally, adds Niland, you should sound the alarm at an intersection in a true emergency-when your brakes fail, for example, or when you're sliding on ice.
• As a greeting or farewell. "Once, a friend of mine picked me up at an airport in South Carolina," says Noise-Free America's Rueter, "and he greeted me by blowing his horn. I couldn't believe it! I was coming to town to talk about noise!" As a greeting or farewell, a wave works just fine.
• As an expression of anger. "It's always a misuse of the horn to honk in a traffic jam or in response to another driver honking his horn," says Niland. An exchange of sonic blows can escalate into an exchange of physical blows-and an incident of road rage.
  
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  Then there are those gray areas-situations that defy simple rules and test even Miss Manners' sense of etiquette. Take bicycles, for example. Most adult cyclists resent a toot from a car overtaking them. Unless they're speeding down a long downhill stretch with the wind in their ears, they can usually hear the tires humming and engine purring behind them.

However, younger cyclists are another matter entirely. Kids tend to focus on their immediate path of travel and often become oblivious to dangers around them. A judicious toot serves as a polite warning.

"Ideally, you should sound the horn when you're about a half-block away-not one or two car lengths," says Niland. "And it should be a casual, friendly tap. If you blast the horn at close range, you'll startle the cyclist. He may look over his left shoulder in surprise and steer inadvertently into your path. Worse, he may lose control and fall directly in front of you."

But no situation sparks more controversy than sitting behind a laggard at a green light. How long should you wait before tooting to alert a preoccupied driver that the light has changed?

Here in New York, most drivers seem to rigidly enforce the "two-second rule"- anyone who doesn't move within that time receives a blast. Niland believes you should at least double that time before giving the horn a light tap. "Using the horn any sooner becomes an aggressive act," he says.

Rueter sums up the anti-noise arguments this way: "Drivers often use honking as a substitute for thinking. In fact, it should never be a substitute for common courtesy, controlling your emotions or being willing to wait."