THORNBURG, VA., OCT. 22 -- The good news is that highway crews finished work today on the final stretch of I-95 to be widened to six lanes between Washington and Richmond.

The bad news, for some, is that construction may resume again as early as next October on car-pool lanes in Prince William County.

But for now, for the first time this decade, there is no construction on Virginia's busiest highway.

When I-95 opened in 1963 "no one dreamed of the traffic volumes we're experiencing today," said David R. Gehr, director of operations for the Virginia Department of Transportation.

By the early 1970s, highway planners realized that the original four lanes, two in each direction, were not enough. (The sections between the District and Triangle, in Prince William County, and between Ashland and Richmond were originally built with six lanes).

So plans were drawn to widen the remaining 55 miles, between Triangle and Ashland, to six lanes.

Construction took seven years, though frequent travelers on the road, including Fairfax-to-Richmond commuter Vivian E. Watts, the state's secretary of transportation, might swear it has been an eternity. During that time, traffic increased by about 25 percent.

About 40,000 vehicles travel the Thornburg-to-Fredericksburg section of I-95 each day; by contrast, more than 172,000 vehicles were recorded daily between the Capital Beltway and the Springfield/Franconia exit of I-95, according to 1986 figures from the Transportation Department.

In addition to the costs associated with lengthened travel time -- the ordinary two-hour Washington-to-Richmond trip often took much longer -- and the $170 million paid to eight contractors for 15 separate projects, the project also cost the lives of two workers. A highway department inspector was run over by an asphalt truck and a contractor's employe fell off a bridge into the Rappahannock River.

No serious accidents involving motorists were atttributed to the construction, "just a bunch of fender-benders" and bruised tempers, said R.M. Douglas, a Transportation Department inspector.

Shortly before noon today, after a news conference in a Stuckey's parking lot here, the final 5,000-pound concrete barrier was lifted onto a truck, the last blinking warning sign was folded up and, for the first time, traffic between Washington and Richmond whizzed by three abreast in both directions.

The 19.2-mile HOV extension in Prince William had been scheduled to be put out for bids this month, but a cutback in federal funds has delayed the $183.7 million project for at least a year.

Construction will last about three years, but Gehr said the delays won't be as bad as those that accompanied the widening because the road already is six lanes wide.

The first phase will be a 4.1-mile stretch from the Beltway south to Accotink Creek.

The HOV (high occupancy vehicle) extension calls for two barrier-restricted, reversible lanes to be added in the median.

Still to come on Virginia's portion of the East Coast's principal north-south highway are a 600-vehicle commuter parking lot in Fredericksburg at Rte. 1, for which the state will acquire 13 acres next year; completion of a cloverleaf interchange at Rte. 610 in Stafford County in 1990; an upgrade for the Atlee-Elmont interchange in Hanover County, set for 1991, and routine maintenance.

And when will six lanes no longer be enough?

"Not for a long, long time," said Gehr, perhaps echoing the promise of another official a quarter of a century ago.