The so-called community benefits agreement, part of the airport expansion plan approved by the City Council this week, is the largest of its kind in the nation and the latest example of a tactic increasingly employed by advocacy groups to extract concessions from developers and public entities proposing large projects on scarce urban land.

Groups using the same approach are now negotiating deals with public officials and private developers in New York, Seattle, Denver, Miami and New Haven, according to Madeline Janis-Aparicio, executive director of the Los Angeles Alliance for a New Economy, which was instrumental in winning the concessions.

The money will pay for soundproofing schools in the airport's flight paths, training new workers, replacing diesel airport vehicles with cleaner-burning equipment and studying the health effects of expanded airport operations.

The measures secured some labor and political support for the airport expansion project, which has been the subject of 10 years of controversy and tens of millions of dollars of studies. The agreement also immunizes the city against the threat of costly and time-consuming litigation - at least from the 25 community groups that signed the agreement.

The parties to the agreement called it an unprecedented deal to address a host of traffic, air pollution, noise and economic concerns raised by the huge airport project, which will take a decade to complete if it wins federal approval next year. Mayor James K. Hahn called it a national model for airport development and a way to "enhance the quality of life for those living around the airport."

But it was also the product of cold calculation on the part of the mayor and officials of Los Angeles World Airports, the public authority that runs the city's four airports. It is, in essence, a pre-emptive out-of-court settlement of potential lawsuits that could block or delay the project.

"This is not simply a case of 'Here's a half a billion dollars, go away,' " said Danny Tabor, of the LAX Coalition for Economic, Environmental and Educational Justice, who helped negotiate the agreement. "This buys them some very necessary legal peace. The school districts came to table with the means and the intent to sue. The environmental groups have a history of opposing mayors' projects for LAX. This agreement says we won't sue if they fulfill their commitments."

In exchange for not bringing legal action, the coalition gets a large basket of benefits. The package includes as much as $200 million to install soundproofing, air-conditioning and air filter systems for schools near the airport.

It also includes money for a study of the long-term health effects of noise and air pollution on those living near a major airport, as well as to reduce diesel emissions by using electricity to power the mobile corridors linking planes and terminals.

Jerilyn Lopez Mendoza, the Los Angeles policy director for Environmental Defense, one of the parties to the agreement, said her group had frequently resorted to lawsuits to achieve gains like those reached by negotiation in the airport deal.

"We certainly hope to create a non-litigation model," she said. She added that joining two dozen other community groups with varying agendas gave each group more clout than it would have wielded on its own.

The alliance, which put together the community benefits agreement, is a nonprofit organization. In 1999, it negotiated with the developer of the Staples Center in downtown Los Angeles to win green space and affordable housing around the arena, and earlier this year it worked with residents of Inglewood, a low-income suburb, to defeat plans to build a Wal-Mart store there.

The alliance was a pioneer in this kind of work, which combines the tools of community organizing with the hard-nosed tactics of political and economic pressure. "These agreements completely redirect the priority of economic development toward raising the quality of life in communities," said Ms. Janis-Aparicio of the alliance. "It's a big switch in the mindset of all stakeholders."

She said the alliance had negotiated six such agreements earlier with private developers, but that this one was the first with a public entity and by far the largest.

But the agreement with the community groups does not ensure smooth flying for the airport project. The Los Angeles County Board of Supervisors has authorized a lawsuit against the project because of concerns about unchecked growth in vehicle and passenger traffic. Several communities not covered by the $500 million settlement are also considering lawsuits to block the expansion.

And many officials are expressing reservations about a plan to move all passenger and baggage check-in to a remote terminal about a mile east of the airport, part of a second stage of the expansion. After checking in, passengers would take an automated rail system to the gates.

Jack Weiss, one of three people on the 15-member Los Angeles City Council to vote against the airport plan this week, said the remote terminal would make the airport much more vulnerable to terror attacks.

"Mayor Hahn has proposed spending $8 billion in public funds on a passenger check-in facility that not only won't protect against terrorism, but it may make for a more attractive terrorist target because it will concentrate all the passengers in one location," Mr. Weiss said. He said a recent Rand Corporation study had pointed out the security risks of the new passenger terminal.