The Fairfax County Board of Supervisors unanimously rejected last night plans for a $250 million corporate park on the Capital Beltway that drew vigorous opposition from a powerful coalition of residential neighborhoods.

The action came as a surprise to some observers; earlier in the day the supervisors had deferred consideration of another major issue -- an employe pay raise -- until after the Nov. 6 elections.

The site for the proposed development -- the 128-acre Chiles Tract at the Beltway and Rte. 50 -- has been called one the most desirable large vacant locations remaining for commercial and industrial purposes in the Washington area.

Plans to develop it foresaw 8,500 jobs and attracted wide attention because the developer said he would impose an unusual array of controls to limit traffic.

The firm, Western Development Corp., which has been active in Georgetown's redevelopment, said it would subsidize van pools and buses, provide a shuttle service to a nearby future Metro station and even subsidize a health club to get people to work early and keep them there after the evening rush hour.

But the supervisors, after listening to a parade of residents express fears that they would be overwhelmed with traffic, said no in an 8-to-0 vote. A motion by Supervisor John Shacochis (R-Dranesville) to defer action for several months failed to win a second.

Supervisor James M. Scotts (D-Providence), in whose district the Fairlake Corporate Park would be built, said he was unconvinced that the size of the project would be limited by traffic volume as Western claimed.

"If ever there was a case where a developer had a chance to prove himself, this was it," Scott said.

A court fight is a foregone conclusion. Attorney William H. Hansbarger, who represented Western, said he already has aproval from the land's owner, the Chiles family, which he also represents, to go to court.

He said the supervisors' decision was "arbitrary and capricious."

The residents who fought Fairlake were jubiliant about the outcome. William Herbert Jr., president of Save and Conserve Our Residential Neighborhoods, the coalition, said: "We defeated a $250 million project with our $100 budget."

The supervisors made their decision even as they are actively courting industry to come to the county.

"No doubt this is exactly what we are looking for in the way of development," Supervisor Marie B. Travesky (R-Springfield) said, "but it is in the wrong place." She invitedWestern to come to her district.

Another supervisor, Martha V. Pennino (D-Centreville), said, "The concept is very exciting but it has some insurmountable problems." She, too, invited Western to move to her district.

With the profusion of offers and counteroffers that were made as negotiations continued up to and even past a midnight deadline last Friday, it was not completely clear just where the impasse between Western and the county government developed. One main issue appeared to be the maximum number of cars that would be permitted at Fairlake.

Western said it would hold the number of cars to 2,340 at the peak of the morning rush hour if it could develop a maximum of 2.4 million square feet. The county staff said the State Department of Highways and Transportation had set a limit of 1,960 cars at the peak of the rush hour.

Fairlake, as Western proposed to develop it, would have included a 32.5-acre park and a 21-acre lake, around which some of the office buildings would be constructed. Western's president, Herbert Miller, said he already has a commitment from a major corporation to relocate its regional headquarters to 500,000 square feet of space at the proposed site.

Western said Fairlake would generate $5 million in annual revenues for Fairfax and $9 million for the state.

While many nearby residents spoke against the project, former state delegate Carrinton Williams (D), who used to represent the area, spoke for it. "When you get a developer to promise to do what this developer has promised," Williams said, "I say, Glory, Glory Hallelujah, you should grab him in both arms and embrace him."

Of all the supervisors, the most vulnerable to the coalition's lobbying effort was Scott, who is in what some political observers say is a close race with his Republican opponent Gwen Cody. Coalition officials have said privately they told Scott in so many words that rejection of the Western proposal was a condition for their support in the election.

"I don't think political pressure was a problem for myself or any of the other supervisors," said Scott, "because the applicant made it easy by not complying with the master plan."

The intersection of Rte. 50 and the Beltway has become on of the most sought-after locations for industrial development in the county.

The big problem, though, has always been traffic. Rte. 50 is already jammed at rush hours, and Mobil, which will eventually bring 1,800 workers to the site, will generate considerably more traffic.