A key panel of Virginia transportation officials recommended yesterday that the state enter into negotiations with a private firm to build toll lanes on a 13-mile stretch of the Capital Beltway, despite doubts from some panel members about how practical the project is.

The proposal would add two lanes on each side of the Beltway -- and separated from other traffic -- between Springfield and Georgetown Pike. The high-occupancy toll lanes -- or HOT lanes -- would be the first of their kind in the Washington region. Under the plan approved yesterday, the lanes would be free for carpools of three or more people, but others would pay for the privilege of riding in them. Tolls would increase with the amount of traffic to keep the lanes from clogging.

The project would be built and mostly paid for by Fluor Daniel, a private firm that submitted the unsolicited proposal to Virginia officials.

With yesterday's approval, the proposal moves to Virginia Transportation Commissioner Philip A. Shucet, who will decide whether to build the lanes. State officials said that no private plan has been denied once it has been approved by the advisory panel and that Shucet and Gov. Mark R. Warner (D) have spoken in favor of HOT lanes.

The addition to the Beltway would be the first of what Virginia and Maryland leaders hope will be an extensive network of congestion-priced highways across the region. Virginia officials also are considering HOT lanes on portions of Interstates 95 and 395, and Maryland officials announced plans this month to build "express toll lanes" on their part of the Beltway, I-270, the Baltimore Beltway and I-95 north of Baltimore.

Congestion-priced lanes, sometimes derided as "Lexus lanes" and a perk for the rich, are gaining favor across the nation as traffic planners cast about for some way to give drivers options. Officials in Texas and California praise the benefits of HOT lanes in their states, while Minnesota, Colorado, Washington and others are considering proposals.

Officials in Maryland and Virginia have said they turned to HOT lanes largely because they don't have the money to make serious transportation improvements without charging drivers.

Nonetheless, officials have embraced the concept as a creative way to give motorists relief from some of the region's worst traffic tie-ups. They predict that most drivers will use them at some point -- parents when they're late picking up children from day care, contractors who want to schedule more jobs and the daily commuter who is simply sick of sitting in bumper-to-bumper traffic.

Backers also say the lanes would allow for bus service that is not possible now because chronic traffic jams make scheduling haphazard at best. And they say drivers on regular lanes would benefit if traffic is moved into HOT lanes.

But during a meeting yesterday that lasted more than three hours, members of the advisory panel said they lacked critical information about the specifics of the project's design, scope, financing and appeal to drivers.

"We don't know exactly what we're doing," said Gerald P. McCarthy, a panel member who also serves on the Commonwealth Transportation Board, which decides what roads are built. "We don't have a good definition of this project. I don't think we're at a point where we can honestly tell the commissioner to go forward."

Tom Farley, VDOT's Northern Virginia administrator, also voiced misgivings. "I'm not comfortable in terms of not having answers," he said. "At the same time, if it's up or down, my answer is up."

A major snag centered on a request from transportation officials that the lanes include ramps to Tysons Corner and other key exits that are not currently in the proposal. That would add as much as $150 million to the project's $693 million cost. A Fluor official said after the meeting that his company would not pay that.

VDOT officials told panel members that the state does not have the $91 million needed to foot its share of the Fluor proposal -- much less the additional cash if the project is broadened.

Connecting to Tysons also raised questions about whether the lanes are designed to encourage carpooling and mass transit or to turn a profit.

Members agreed that linking to Tysons would add many more carpoolers, but they split over whether that was good for moving people or whether carpoolers would overwhelm the lanes and leave little room for paying customers. Fluor is relying on toll funds to repay the bonds secured to build the project. It hopes to make a profit through a negotiated fee from the state.

Others worried that demand will not meet projections after a study by the Metropolitan Washington Council of Governments found that many drivers on that part of the Beltway take short trips and that backups are confined to certain spots, not the whole highway.

"We've got to be able to get people to go into these lanes and feel like it's worth their while," said Ron Kirby, transportation planning director for the council and a panel member. "If [congestion] is just in one or two segments, you have to wonder."

Fluor officials, who predict that motorists traveling the whole 13 miles would save 10 minutes, dismissed the concerns. They said that drivers are motivated more by a desire to keep moving than how far they're going.

Gary Groat, director of project development at Fluor, added that company officials "don't expect that [carpoolers] will ever be a problem" and that the "original concept, we think, still has the potential of working."

Many members who expressed concern about the proposal, including Kirby, said they voted for it because they liked the concept and would trust VDOT to handle the specifics. The proposal passed by a vote of 8 to 3.

Shucet did not say how he would respond to the panel's recommendation. "I will make two or three days free in my calendar where I'll sit and review very thoroughly the information the panel forwards to me," he said.

The decision on the HOT lanes will go to state Transportation Commissioner Philip A. Shucet.