The novel financing plan for widening Virginia Rte. 28, widely touted as a model for transportation improvements around the region, is threatened by disagreements over several key issues, according to sources.

Passed this year by the legislature, the plan was hailed as the solution for one of the most congested roads in Northern Virginia. It allowed for creation of a special tax district under which commercial landowners along Rte. 28 would help pay for widening the road in the corridor of booming development near Dulles International Airport.

In recent weeks, however, several important stumbling blocks to the plan have emerged, prompting gloomy assessments of its future.

"If this thing falls apart, it's going to send a very bad message to the legislature," said Tom Hyland, a legislative aide to state Sen. Charles Waddell (D-Loudoun), one of the chief architects of the tax district legislation. "And that is that the local governments and the developers are not yet able to work together to deal with these local options."

Among the major points of dispute:Before they agree to the tax district, developers along the corridor want a commitment that their tax burden will not increase beyond the level stipulated in the legislation, which allowed for a surtax of up to 20 cents per $100 of assessed value. But Fairfax County officials, fearing that such economic variables as a recession could change the assumptions on which the tax plan rests, say they cannot guarantee that the rate will never have to be raised. There is mounting concern that the Rte. 28 project, which will cost an estimated $392 million, will siphon funds from other much-needed projects in Northern Virginia -- especially if the tax district fails to generate as much money as hoped. Last month, a citizens panel, the Fairfax County Goals Commission, questioned whether the Rte. 28 project "should be given priority over other areas fully developed with more urgent traffic problems." Even if the tax plan goes forward, some people fear that massive development along Rte. 28 will create more traffic jams on adjacent streets. In order to finance the entire project with the proposed surtax, developers along the corridor would have to build between 30 million and 40 million square feet of new office space, roughly the equivalent of two Tysons Corners. Developers and state and county officials have not agreed on how quickly and in what stages the widening project should proceed. Developers want a commitment that the road will be widened to eight lanes with 13 major interchanges; state and county officials say they cannot commit beyond the first phase, which calls for widening the road to six lanes with three major interchanges.

Opinions vary on the seriousness of the disagreements. "I'm not anticipating the kinds of problems that I've heard suggested," said Curtis M. Coward, an attorney for developers along the Rte. 28 corridor. "It will be an extraordinary deal for the county. If there are any anxieties, we can reconcile them."

But county officials said that the issues are significant and that the tax district is by no means a sure thing. "If they can't be resolved, they are substantial enough for us to raise red flags to the Board of Supervisors," said Denton Kent, the deputy Fairfax County executive.

No one disputes that something needs to be done quickly. The section of Rte. 28 in question runs north-south for about 15 miles between Rte. 7 in Loudoun County and I-66 in Fairfax County.

In some places, the two-lane road has reached levels of congestion that traffic engineers describe as unacceptable. By 2010, according to state traffic projections, Rte. 28 will carry 89,000 vehicles a day between Rte. 50 and the Dulles Access Road.

The tax plan was aimed at improving the road before development makes it impassable, thereby heading off a situation that has plagued other areas of Northern Virginia.

Under the legislation passed this year, Fairfax and Loudoun counties were authorized to create the special tax district if it was agreed to by commercial landowners representing at least 51 percent of the acreage along the Rte. 28 corridor. Money from the tax district would help pay off bonds to finance the project.

After lengthy negotiations this summer, developers and state and county officials agreed to a formula under which the developers would contribute 80 percent of the cost and the state would pay 20 percent.

Last month, a petition signed by developers representing 57 percent of the land in the tax district was presented to the county, along with two key stipulations.

First, the developers wanted a guarantee that the county would never raise the transportation surtax, an option that would have to be approved by the General Assembly.

Second, the developers wanted assurances that the state and county are committed to completing the eight-lane version of the road, rather than stopping at six lanes. Developers say that six lanes would fill with traffic as soon as the road was opened.

"The concurrent resolution sets forth what we feel is a very fair position," said Victor Trapasso, chief executive officer of G.T. Investments Co., the developer of Renaissance Park on Rte. 28. "Now if the county says that is not acceptable, the only recourse we have is to go back and see if we have more negotiating ability. I honestly don't know what the answer would be."

But county officials say they cannot agree to either condition because of limited road funding alternatives.