Back then, it sounded like a lot of money: a quarter of a billion dollars to rebuild a Northern Virginia highway interchange so baffling to motorists that it had become known as the Mixing Bowl.

A brochure handed out to the public in 1994 explained why the Virginia Department of Transportation needed to fix the interstate junction at Springfield: "These high volumes of traffic, coupled with difficult weaving and merging movements, result in traffic delays and higher than normal accident rates."

That June, the Commonwealth Transportation Board approved Alternative 12, a $241 million plan to make the interchange safer while keeping open one of the most heavily traveled corridors in the nation.

As grand and costly as the plan was, it was nothing compared with today's reality: The $676 million Springfield interchange is Virginia's biggest, most complex and most expensive highway project in the Washington suburbs.

At a time when the state's financial problems are forcing cuts in transportation spending, an enormous portion of the available money goes to this single effort.

A federal audit of the project released last week illustrates how finances for a massive state project can spiral out of control. It also offered a glimpse at the impact one costly project can have on a region's effort to improve mobility.

Today, the Washington region is actively considering several such large-scale efforts: a rail line through Tysons Corner and the Dulles area, a rail line across Montgomery and Prince George's counties, and a highway linking Interstates 270 and 95.

Last month, Maryland transportation officials opened a new set of bids for reconstruction of the Woodrow Wilson Bridge, the region's most expensive public works project, and were relieved to see that they were only $18 million over engineers' estimates. Potential problems with the one-mile bridge still loom, and completion of the first span is at least a year behind schedule.

At Springfield, VDOT set out to improve the safety of an interchange where more accidents occurred than on any other area of road in Virginia. Construction began in April 1999.

Now, according to the audit issued by the U.S. Department of Transportation's inspector general, the Mixing Bowl's cost is "approaching $1 billion." The audit criticized VDOT and federal highway officials for low-balling the 1994 estimate and then failing to control costs.

The report found that:

* VDOT, acting largely at the behest of Fairfax County officials, added more ramps from the interchange to Franconia and Old Keene Mill roads. The project grew to include the widening of Loisdale Road and Commerce Street, improvements thought to help traffic flow through downtown Springfield.

Added cost: $30 million.

* Demands from the public for more noise walls were heeded, and bridges that engineers had said could be redecked were replaced instead. More retaining walls were built to allow extra work on utilities.

Added cost: $54.3 million.

* When widening Franconia Road required taking land from Lee High School, the Fairfax County School Board demanded compensation. VDOT agreed to replace athletic fields, tennis courts and a stadium press box.

Added cost: $500,000.

* Fairfax County supervisors demanded broader efforts to ease congestion during construction. That led to an increased presence by Virginia State Police, expanded local bus service and improvements to roads in the area. The state also financed an information center in Springfield Mall.

Added cost: $31.2 million.

These and other additions made at the request of local officials increased the project by $140 million, the report states.

Katherine K. Hanley (D), chairman of the Fairfax County Board of Supervisors, said those items should have been included in the original estimate.

"Some of those things, like noise walls and traffic management, shouldn't have had to be added," she said. "The fact is that they are important to the project."

Hanley's point is made repeatedly by the federal audit, which said the $241 million estimate by VDOT in 1994 "severely understated project costs because it excluded known, planned, and easily predictable costs that are standard cost elements in major highway construction projects."

The most basic of those omissions, the report states, was inflation.

From 1994 to 2001, VDOT did not assume any increase in the cost of materials or labor for the life of the project. In 2001, at the urging of federal auditors, the department began assuming a modest 3 percent increase in prices each year.

Added cost: $44 million.

Some planning and design costs, which are a staple of every large road project, were left out of the estimate, the federal audit found. And incentive payments to contractors for finishing their work early were not counted toward the overall cost of the project.

Added cost: $65.6 million.

The costs that federal auditors believe should have been anticipated total $236.5 million.

In addition, contractor overruns from early phases of construction and scheduling delays could add nearly $59 million to the tally, the audit says.

"The project could experience significant future cost increases due to uncertainties about the costs of ongoing construction problems and the complexity of the latter phases of the project," the report concludes.

VDOT lowered the cost of the interchange project by $84 million in August 2000 by deciding that construction of ramps to connect carpool lanes on I-95 to future carpool lanes on the Capital Beltway would no longer be considered part of the project. Construction of the ramps was included in the original $241 million program, and they are still planned. But development of carpool lanes on the Beltway stalled for financial reasons.

According to the federal audit, a true comparison between the original cost of the interchange and the current price tag would include the cost of the ramps.

"VDOT's decision [to eliminate the ramps] was based primarily on financial considerations," the report states.

While the price tag for the Mixing Bowl soared, Northern Virginia fell further behind in dealing with traffic congestion. This year, Gov. Mark R. Warner (D) slashed the state's $10 billion road program by a third, saying there was not enough money to complete all the work.

The audit report says the interchange project is partly to blame.

Seventy percent of the 152 road projects in Northern Virginia between 1994 and 2000 were delayed or canceled at least in part because of increased spending on the Mixing Bowl, the report says.

VDOT's road-building plans, according to the inspector general's office, were not "realistic or achievable because VDOT underestimated costs for Springfield and many other projects."

The report says VDOT did not use a scheduling system sophisticated enough to track construction involving multiple contracts so that the agency could reliably estimate completion times. With work so big and complex, conflicts involving overlapping construction schedules must be identified and resolved, the audit says.

Such management changes have helped control costs in other large projects, the report's authors say.

The Federal Highway Administration, which is responsible for reviewing and approving states' transportation plans, did not detect Virginia's problems, the report says. The inspector general said it did not ensure that reasonable cost estimates were an integral part of planning and managing a state highway project.

The report says VDOT should prepare -- and federal highway officials should review -- an overall financing plan for the project. Such documents are usually required for projects that cost $1 billion or are designated "high risk" or "troubled."

State transportation officials said they have already agreed to all of those recommendations and are nearly finished with the financing plan.

"The work's been done," VDOT Commissioner Philip A. Shucet said. "I have fixed the budget for this project at $676 million. I have made it clear that's not an estimate -- that's a budget."

According to a federal audit, the Mixing Bowl's cost is "approaching $1 billion."Some planning and design costs were left out of the original estimate for the Mixing Bowl project, an audit says.