Developers in Fairfax County have grumbled loudly in recent years over the millions of dollars they have had to agree to invest in major road improvements to get giant projects approved by the county's board of supervisors.

However, those road improvements have proved to be the keys to the vitality of congested areas such as Tysons Corner and the best guarantees possible of the economic viability of most of the individual projects, development and land-use lawyers said this week.

The theory behind this "proffers" system is that large commercial or residential developments may increase traffic problems, so the builder or developer should help to alleviate these problems by making or funding road improvements to gain county approval for the development.

"Without the transportation proffers in recent years, I doubt seriously if Fairfax County would be riding the crest of the economic boom it is today," said John T. (Til) Hazel, an attorney and developer whom some of his fellow developers consider to be the father of the proffer system.

Wayne Angle, vice president of Homart Development Corp., builders of the 107-acre Tysons II mixed-use project, said the $20 million that his company is pouring into road improvements negotiated under the proffers process helps to ensure the economic viability of the future of Tysons II.

"From a business standpoint, we had to correct traffic problems in the Tysons II area," Angle said.

Shiva Pant, head of Fairfax County's transportation planning office, said the proffer system is a way of "putting in writing the results of negotiations between a developer and the county."

"It's a two-sided thing," Pant said. "The county can't tell the applicant what he has to do. At the same time, the proffer system makes it binding because promised road improvements become part of the land records. If a developer sells the land, the proffers go with the land," Pant said.

According to Hazel, the proffer system is legal under legislation enacted by Virginia in the mid-1970s after intense lobbying in the days "when Fairfax wasn't allowed in the state capitol building." Pant said other jurisdictions across the country that must depend on covenants or agreements between developers and elected officials envy the legal status of the system used by Fairfax County.

Meanwhile, developers are adjusting to the demands of the system, which they say is not always as voluntary as it appears to be. In many cases, developers base their proffers on suggested traffic-improvement needs included in preliminary staff reports.

"It is always hard to be the person who has to come up with the money. But we know it is part of the process," said Alan Fink, a Cen-tennial Development Corp. executive. Fink was involved in recent negotiations that led to approval of Centennial-Gateway, a large mixed-use complex in Fairfax Center. Centennial coughed up more than $11 million in specific road improvements and another $5 million for the area's general road bond.

"Tysons II carried the burden of what is and what will be developed in the Tysons area," Angle said, but whether that is fair is not a question in Angle's mind. He said he considers it an economic necessity because potential tenants were worried about traffic at Tysons.

A recent report on transportation in Fairfax published by the county's economic development authority said roads being built by developers not only benefit those developments but may spur growth in areas affected by the traffic improvements. Major improvements now under way in the I-495 and Route 50 corridor are causing large traffic jams daily, but those projects are generating a revitalization of what is known as the Merrifield corridor of the county, an area between Tysons Corner and Fairfax Hospital.

The dollars for those improvements are being provided by developers of Fairview Park, also known as Cadillac-Fairview, on what was known as the old Chiles tract. Hazel said approval of Cadillac-Fairview was really the beginning of today's proffer system.

Hazel's own development company -- Hazel-Peterson Cos. -- already has built major components of a transportation network that it proffered in the process of getting approval for Fair Lakes, a major mixed-use development anchoring development at Fairfax Center. A 1.5-mile segment of the Springfield Bypass, which runs through Fair Lakes, has been built. This week, work is scheduled to begin on a complicated overhaul of the I-66 interchange that was part of the Hazel-Peterson proffers.

For Tysons II, the proposed traffic improvements this week have become very real. The road network for the Tysons II project has been cut and graded. Permits to pave the roads were filed with Fairfax County more than two months ago. As soon as those permits are granted, workers will begin paving International Drive from Route 123 to the Rotunda, a high-rise residential development. Angle pointed to the rapidly moving bulldozers that are pushing aside 10,000 to 12,000 cubic yards of dirt a day in anticipation of starting construction of the regional shopping mall that is a major component of the Tysons II project.

"We're running about three weeks ahead of schedule," Angle said. He credits much of that scheduling to the grading contractor, S. W. Rodgers & Co. Inc. of Manassas.

Developers are recognizing that, no matter how costly traffic improvements may appear at the time a developer is trying to get a rezoning approved, it helps to have future traffic needs projected along with specific plans for the road network that will be needed to accommodate that traffic.