A Virginia Supreme Court decision barring the use of pledge bonds to pay for new roads has dealt a blow to the state's plans to speed highway projects and could significantly delay such major undertakings as the widening of Rte. 28 through Fairfax and Loudoun counties, according to transportation and legal specialists.

Although state transportation officials said the decision last week will have little effect, others said it means that the state has lost a key mechanism for financing such projects as the $392 million widening of Rte. 28, which require enormous sums of money at the outset. The project is regarded as critical to easing traffic gridlock in the Dulles Airport area.

"We were very disappointed," said Curtis M. Coward, a lawyer who serves on the Governor's Commission on Transportation in the Twenty-First Century, which first proposed the use of pledge bonds. "If nothing is done and no corrective action is taken, we have lost the principal means by which we can accelerate the delivery of roads."

Pledge bonds would have allowed the state to raise large amounts of money by pledging gasoline taxes and other highway revenues to pay off the principal and interest. Their use for transportation projects was approved by the General Assembly in September 1986 along with a special package of taxes earmarked for transportation projects. No further legislative or voter action would have been required before pledge bond money could have been spent.

Last Wednesday, the state Supreme Court barred that avenue of convenience by ruling 6 to 1 that pledge bonds are unconstitutional in Virginia.

As a result, legal specialists said, the state's primary options for raising money for new roads will be limited to less convenient general obligation bonds or revenue bonds.

General obligation bonds are paid from general tax revenues and must be approved by the General Assembly and the voters, who often say no. Most jurisdictions like to reserve their general obligation bonding authority for school construction and similar projects likely to win approval.

Revenue bonds are simply user fees and take money from a specific source such as tolls to pay off a debt. Revenue bonds were used to build the Dulles Toll Road, for example.

The pledge bond decision comes at a time of shrinking federal resources for new highway construction. Under the 1987 Surface Transportation Assistance Act, Virginia's share of federal highway funds declined by $103 million from the previous year, forcing the state to delay plans to extend commuter lanes on I-95 south into Prince William County, among other things.

The combination of reduced federal aid and the illegality of pledge bonds has prompted calls for a rescue plan during the next session of the General Assembly, which will convene in January.

"If we are going to build roads, we have got to find some way to finance them," said John Lynch, a member of the Fairfax County Goals Commission, an influential citizens panel appointed by the Board of Supervisors to study growth in the county.

State transportation officials played down the effects of the court's decision, saying that they had never counted on the pledge bonds to provide money for road construction.

"Anticipating the court case . . . we did not anticipate using pledge bonds for any project in the six-year program," said state Transportation Commissioner Ray D. Pethtel. He said the decision "eliminates one option" but that "there are other ways the state can do debt financing."

That optimism was not universally shared. "I'm sure {the decision has} got everyone discouraged," said Fairfax County Executive J. Hamilton Lambert. "Without the availability of being able to borrow money up front, it will slow down certain projects."

Virginia has always been reluctant to borrow to build new roads, operating under a pay-as-you-go philosophy. As a consequence, growth has often outpaced highway capacity, particularly in Northern Virginia. The special legislative session in 1986 was Gov. Gerald L. Baliles' attempt to rectify that problem; under the tax package an additional $422 million a year was to be made available for transportation projects.

If the money for the Rte. 28 project were to come solely from the annual allocation of state funds for primary roads in Northern Virginia, which now stands at $18 million, widening it would be next to impossible, transportation specialists said. Moreover, the price would keep escalating because of increases in the construction and right-of-way costs.

"Under pay-as-you-go financing, that road is never going to get improved," said Shiva Pant, the Fairfax director of transportation. "It is a setback, but it will be compensated for by figuring out some other strategy, which may affect the timing of construction."

Next week, the Fairfax and Loudoun county boards of supervisors will vote on whether to establish a special transportation tax district along the Rte. 28 corridor, under which commercial landowners would be assessed a surtax to help pay for the widening. Since it will be years before the tax district itself could pay for the road, pledge bonds would have been sold to provide the up-front money.

Pethtel said the state is exploring the possibility of financing the road with revenue bonds, which could then be paid off with money from the tax district, much as other roads pay for themselves with tolls. But it is not clear whether the state could legally use the tax district revenues to retire the debt on revenue bonds, he said.

Even before the pledge bond decision, the state had scaled back some of its plans for Northern Virginia. In response to the loss of federal highway funds, it has delayed by two years plans to extend commuter lanes on I-95 south from the Capital Beltway to Triangle, a distance of 19 miles.

Lynch, the commission member, said the group is considering other possible options, such as raising real estate and personal property taxes, imposing a capital gains tax on real estate transfers, or permitting localities to raise the sales tax by a half-cent.

Local use of the sales tax would require approval by the General Assembly, which traditionally has reserved the sales tax for statewide purposes.

"This is out of the question," said Del. Dorothy S. McDiarmid (D-Fairfax), chairman of the Virginia House of Delegates Appropriations Committee. She said the Northern Virginia delegation will meet soon to address the bond problem. "We will certainly do whatever is necessary," she said.