Lax oversight and consistently low estimates of expenses have allowed the Springfield interchange reconstruction to triple in cost, with the latest projections "approaching $1 billion," according to a federal report to be released Monday.

The two-year study by the U.S. Department of Transportation's inspector general predicts that the interchange, one of the nation's most complex highway projects, will not be finished by its target date of 2007. The inspector general's office calls for a federal review normally reserved for "high-risk" or "troubled" programs.

The report, the first extensive outside audit of one of Virginia's most expensive highway projects, says the cost increases have soaked up money that could have financed other road construction in Northern Virginia. More than 70 percent of the state's proposed road projects for the region from 1994 to 2000 were delayed or canceled, in part because of the Springfield interchange's escalating costs, the report says.

"Cost increases, especially on large, high-priority projects such as Springfield, have repeatedly resulted in the state canceling or delaying many smaller planned projects," the report states.

The inspector general's office was critical of the Virginia Department of Transportation and the Federal Highway Administration for failing to adequately manage a project that represents a huge investment by the state and federal governments.

The report said the cost estimate for the Mixing Bowl is $676.5 million and will probably rise. It compared the cost overruns to those at other mega-projects across the country and urged VDOT and the Federal Highway Administration to adopt management controls that have helped to bring down costs elsewhere.

It has become common for the nation's largest public works projects to vastly exceed their original cost estimates. In the process, their completion dates recede and the programs soak up local and federal money that had been destined for other improvements.

The price of downtown Boston's Central Artery/Ted Williams Tunnel, known as "the Big Dig," has soared from $2.3 billion in 1984 to $14.6 billion. The staggering size of the miscalculation and the strain that it put on local and federal transportation programs were unique. But costs of big projects such as the Corridor H highway in West Virginia and the Alameda Corridor rail line in California have grown well beyond engineers' original projections.

The Springfield project was designed to improve the safety of an interstate choke point through which hundreds of thousands of vehicles swirl each day. The merges are so intricate and hazardous that the interchange is widely known as the Mixing Bowl.

When the construction that began in 1999 is completed, more than 30 ramps, 50 bridges and 41 miles of roadway will loop over and around Interstates 95 and 395 and the Capital Beltway. In some places, the interchange will be 24 lanes wide.

Rep. James P. Moran Jr., the Virginia Democrat who requested the inspector general's report in the summer of 2000, called the report "damning" because it documents the impact the increased costs of the Mixing Bowl project have had on Northern Virginians' ability to move around freely.

"This is a case study in poor management, poor fiscal management and probably poor upfront estimating," Moran said. He said he was disappointed that the report was not released before this month's vote on a sales tax increase for regional transportation improvements, which voters rejected.

"That was our only hope for making up transportation funding," he said.

Virginia officials said the report accurately reflects the project's problems, though they strenuously disagreed with the $1 billion cost projection.

Gov. Mark R. Warner (D), who slashed VDOT spending by nearly $3 billion, to $7.2 billion, shortly after entering office, issued a statement last night promising to follow the report's recommendations.

He said the report "confirms our diagnosis of the problems and validates our solutions: better project management and better financial management."

VDOT Commissioner Philip A. Shucet, who took over the agency in April, said the department revamped the way it estimates costs and is developing a finance plan.

"The Springfield interchange will be delivered in the year 2007, and it will certainly be less than $700 million. It is not going to be a billion-dollar project," he said. "I'm just not going to allow it."

The project's costs have climbed steadily since the first $220 million price tag advertised at public hearings in January 1994. The latest increase came in March, when VDOT officials said the price had jumped almost $100 million.

The inspector general's report says the Mixing Bowl's price tag increased in part because of decisions by politicians and transportation officials to expand its scope, adding lanes, ramps and sound walls.

But the report says most of the increase -- nearly $237 million -- is the result of poor management as well as costs that state engineers should have been able to anticipate as early as 1994.

The original estimate "severely understated project costs because it excluded known, planned, and easily predictable costs that are standard cost elements in major highway construction projects," the report said.

The report said federal highway officials did not detect that Virginia's transportation projects, including the Mixing Bowl, were underfunded. That meant VDOT was free to use whatever estimates it wanted on the project with little fear of being challenged by the federal government, the report said.

The Federal Highway Administration "has established few requirements to ensure that reasonable cost estimation is an integral part of any state's highway project management or planning processes," it states.

Bill Outlaw, a spokesman for the Federal Highway Administration, referred yesterday to a response to a draft of the report that Federal Highway Administrator Mary E. Peters wrote in September.

Peters wrote that federal officials are reviewing VDOT's schedules for the project and its finance plan to outline "reasonable cost estimates, adequate funding and reliable schedules to meet the financing requirements."

Some officials who represent Northern Virginia said the report confirms their fears about the Mixing Bowl and its impact on other transportation projects.

"I guess the bad news is clear, and there's enough blame to go around," said David Marin, a spokesman for Rep. Thomas M. Davis III (R-Va.). "Officials somehow failed to take inflation into account with the original cost estimates, and there's been no coherent strategy for integrating schedules with multiple contractors."

The U.S. Department of Transportation's inspector general says that the Mixing Bowl will not be finished by 2007 and that its cost could approach $1 billion.