The real estate market may have slowed, but state and county officials are moving full steam ahead on badly needed road improvements.

The Prince William Board of County Supervisors broke ground last week on the Prince William Parkway, a new highway that will swoop from Davis Ford Road near Old Bridge Road, past Potomac Mills Mall to Interstate 95 at Horner Road.

The Virginia Department of Transportation completed hearings last week on the Route 234 Bypass, which would link Route 234 south of Manassas with Interstate 66 just west of the Manassas National Battlefield Park.

Major landowners along both proposed roads are working out final details on agreements under which they would pay additional property taxes to speed up the building of the roads.

Unless the landowners chip in, the roads probably would not be built until the next century.

Local officials heralded these developments as a welcome change from the 1980s.

Road construction lagged far behind residential growth in Prince William for most of the decade. The county's population grew by 55 percent, to more than 230,000, during the period, and most major roads became seriously crowded.

The causes were twofold, according to local officials. They complain that Prince William did not get its fair share of road money from the state Transportation Department and that county voters turned down bond measures, limiting Prince William's ability to finance major construction.

When local legislators tried to get more state money, "we ran into a stumbling block. The county wasn't willing to help itself," said Del. Harry J. Parrish (R-Manassas).

But local officials said they believe the county turned the corner with changes in the way the state allocates transportation dollars and the approval of a $66 million road bond in 1988.

That new money allows the county to take advantage of the slow economic climate to prepare for more growth in the future.

"This is an opportunity to take a deep breath and catch up and do transportation planning," said County Executive James H. Mullen.

Since the private sector has slowed its efforts, many road construction and general contracting firms are willing to work for less.

"It's a good time to be going out to bid," said Supervisor Kathleen K. Seefeldt (D-Occoquan). "It's never going to get any cheaper."

County motorists are just beginning to see the benefits of the 1988 bond money.

A $5 million project to widen Sudley Road to six lanes is two-thirds finished and should be completed by December, according to Thomas Blaser, director of transportation.

The 1988 bond also included $30.8 million for the eight-mile parkway, which will be four lanes between Old Bridge and Minnieville and six lanes from Minnieville to I-95.

The entire project, including interest on the bonds, is expected to cost $40 million, and six major landowners along the route formally notified the supervisors last month that they were willing to pay extra taxes for 35 years to pay off the bonds.

Under the agreement the supervisors are expected to approve next month, which would be the second of its kind in Virginia, the landowners would pay 20 cents more than the standard rate of $1.36 per $100 of assessed value every year, which would cover 85 percent of the cost of the project, including part of an interchange with I-95, said the landowners' attorney, Curtis Coward.

The state would build the rest of the interchange as part of the plan to extend the reversible car pool lanes on I-95, said county Public Works Director Robert W. Wilson.

Last year, public works officials said they hoped to have the parkway completed by the end of 1991, but Blaser said last week that a more realistic schedule calls for the first section of the project -- from Minnieville to Telegraph Road -- to be finished by the winter of 1991 and the entire road to be completed in December 1992.

The $110 million Route 234 Bypass will take somewhat longer to put together.

The state Transportation Department probably won't approve the final design until early 1991, said spokeswoman Joan Morris, and the landowners have not yet formally asked the supervisors to impose an additional tax.

Finding money for the road's $10 million interchange with I-66 has also been problematic. Proponents had hoped for a federal demonstration grant to pay for most of the cloverleaf, but Congress adjourned without acting on the request.

Coward, who also represents the Route 234 landowners, said he expects the U.S. Department of the Interior to ask for the interchange money in its 1992 budget.

Still, local officials said they are encouraged by the county's progress on road construction.

"It's possible if all this gets up and running by 1993 we will be in better shape than we would have been if growth had continued at a 15 to 20 percent rate, as it has," Mullen said.