Politicians, bureaucrats and developers celebrated the opening of the first segment of a widened Route 28 in western Fairfax County yesterday and congratulated one another for setting aside their differences to make it happen.

The Route 28 improvement project has generated interest nationwide because of its innovative funding: landowners and developers in the corridor formed a special district and agreed to tax themselves to pay for 80 percent of the $120 million cost to widen the road from two lanes to six.

"This is as good an example of the admonition, 'Put your money where your mouth is,' as I've ever seen," state Transportation Secretary John G. Milliken said at a ribbon-cutting ceremony. "This is a project that would not have been built in this century at all if it had not been for the unique funding."

Sen. Charles L. Waddell (D-Loudoun), chairman of the Senate Transportation Committee and chief sponsor of state legislation creating the special tax district in 1987, said the opening of the road was "a dream come true . . . the culmination of an idea for a new way to meet our critical transportation needs."

At the time that landowners agreed to help pay for the road, state highway planners did not expect to expand Route 28 until the next century, posing a bottleneck that threatened one of the region's hottest commercial real estate markets.

Since then, land values have fallen sharply along Route 28 and the Dulles Toll Road. Despite that slump, property owners said they do not expect the current downturn to affect the tax district's ability to pay back bonds on the road project.

When the first phase is completed in July 1991, Route 28 will be a major six-lane road complete with cloverleaf intersections stretching 14.3 miles between Route 7 in Loudoun County in the north and Interstate 66 in Fairfax County in the south. State transportation officials said Phase I was expected to cost about $138 million, but is about $18 million below budget.

The part of the road opened yesterday runs 3.9 miles from Route 50 north to Horsepen Run, a stream about halfway between the Dulles Toll Road and Redskins Park in Herndon. The total cost for the section, including engineering, land acquisition and construction, is expected to be about $16.5 million.

A 4.3-mile segment between I-66 and Route 50 and a 3.4-mile stretch between Sterling Boulevard and Route 7 in Loudoun are scheduled to open by the end of the year, according to state highway officials.

Located on the eastern boundary of Dulles International Airport and at the western gateway of the Dulles Toll Road corridor, planners and builders expect the Route 28 corridor to be one of the hottest development spots along the eastern seaboard.

One reason is that Dulles is one of the few airports in this part of the country capable of major expansion. It is in the midst of a $700 million facelift that will more than double the length of the main terminal, permit four times the number of international arrivals, add runways, a series of midfield terminals, more parking and underground people-movers.

According to state figures, the special tax levied against land owners in the district -- a surcharge of 20 cents per $100 of assessed value -- totaled $4.7 million in 1989, about $800,000 more than originally expected. This year, the district was expected to collect about $5.2 million, but state officials say revenue will actually be about $7 million.

Annual debt service on the bonds is about $12 million for the next 30 years; revenue from the district is projected to reach that level in 1996 or 1997. In the meantime, the state is making up the shortfall.

"The district at the current level of development can't support the bonds. It has to grow," said Fran C. Steinbauer, president of Advisory Group Ltd. and a key organizer of district landowners. Even though development in the corridor has been sharply lower recently, Steinbauer said, cyclical real estate markets were accounted for in the financial analysis done for bond justifications.