Halfway through the two-year widening of the Dulles Toll Road, state transportation officials are proposing that the four-lane commuter highway be expanded to eight lanes instead of six.

The change, expected to be discussed tomorrow by Virginia's Commonwealth Transportation Board, would add about $3 million to the project's estimated $45 million cost but would not necessarily delay its fall 1991 completion date, officials said yesterday.

The proposal surfaced Monday at a Transportation Board subcommittee meeting in an attempt to ensure that car-pool restrictions will be imposed once the widening is completed.

Some had suggested that connections between the toll road and the parallel Dulles Airport Access Road might have safety problems if car-pool restrictions were implemented on the toll road. Some officials said yesterday that eight lanes and a new ramp might resolve those problems.

"I think there's a workable package there," said airport authority General Manager James A. Wilding. However, Transportation Board member Mark Warner called the plan "more of a contingency than a proposal."

In recent months the transportation corridor between the Capital Beltway and Dulles International Airport has been a focus of increased attention. With some studies projecting the toll road will carry more traffic than either Interstate 95 or I-66 within two decades as the area around the airport prospers, efforts to get more people into car pools, an express bus service and possibly a rail system in the corridor have taken on greater urgency.

The proposal by a private firm to extend the Dulles Toll Road from the airport to Leesburg also has moved ahead. The current 15-mile stretch of the toll road, operating at a profit, carries more than 60,000 vehicles a day, far exceeding projections when the road opened six years ago.

The proposal to make the toll highway eight lanes -- four in each direction, with one lane limited to "high-occupancy vehicles" at peak hours -- might make car-pool restrictions "more palatable" to the public and the various public agencies that would have to approve the designation, said Mary Anne Reynolds, a spokesman for the Virginia Department of Transportation.

Fairfax County Board Chairman Audrey Moore lauded the proposal yesterday. Noting that Northern Virginia officials have been lobbying for more car-pool and mass transit facilities, she said the proposal shows "the message got through."

When state officials authorized the current widening work, they told contractors to build three lanes in each direction but to make the shoulders wide and strong enough to permit eventual transformation to four lanes. On one stretch of I-95 in Northern Virginia, for example, pavement that is considered part of the highway shoulder becomes an official extra traffic lane during rush hours.

Transportation Department officials said yesterday it was unclear whether the eight-lane toll road proposal would be approved tomorrow by the policy-making Commonwealth Transportation Board. Road designers "are scurrying in Richmond" to come up with revised plans before the meeting, Reynolds said, and the increased cost could be a factor in any decision.

Most of the added cost would go to build a flyover ramp allowing access road traffic to go directly to Route 7 in Tysons Corner without merging onto, then exiting the toll road.

Officials said yesterday that rushing construction of the ramp to keep the overall project on schedule could raise its cost from about $2 million to nearly $3 million.