A plan to add four toll lanes to a 14-mile section of the Capital Beltway is on track after approval by a regional planning body last month, and the lanes could open to drivers by 2010.

The project, proposed to ease congestion in Northern Virginia and being watched closely by Maryland transportation officials, would be the first major refinement of the Beltway in 30 years.

Now that the transportation planning panel of the Metropolitan Washington Council of Governments has found that the lanes would not harm the region's air quality, two other steps, expected to come late this year and early next, represent the last major hurdles before construction can begin.

The first is a detailed traffic and revenue study being done by the private firms behind the project -- Fluor Enterprises Inc. and Transurban Inc. -- that is due by the end of the year. The results are eagerly anticipated because there have been some questions about whether enough drivers would be willing to pay the tolls after an independent report found that most drivers on that portion of the Beltway travel short distances.

The project also must pass a required federal review, scheduled to be finished by early next year.

Once those steps are completed, the companies can meet with state officials to work out final details.

The two sides would have to agree on what kind of entrances and exits would be used for toll lanes, regular lanes and roads that connect to the highway. The firms have proposed several "slip ramps," which are essentially merge lanes between toll and regular lanes. Because the toll lanes are planned for the middle of the highway, drivers would have to cross traffic in the regular lanes to exit the Beltway.

Virginia officials prefer separate entrance and exit ramps that would take drivers directly from the toll lanes to connecting roads such as Route 7 and Route 50. But those ramps would cost tens of millions of dollars. Fluor and Transurban have said they are not interested in paying for them, and the state does not have the money, so it is not clear what kind of deal could be worked out.

Assuming those issues are resolved, Fluor officials said they could start work in a little more than a year and open new lanes to the public by 2010. State officials are equally optimistic.

"The project has cleared its major planning and environmental hurdles," said Virginia Transportation Secretary Pierce R. Homer. "The private marketplace still has to assess the financing of the project, and we're confident that will occur. We're still on schedule for 2010 operation for the first major improvements to the Beltway in over three decades."

The high-occupancy toll, or HOT, lanes would stretch between Springfield and Georgetown Pike, near the Maryland border. They would be free for vehicles carrying three or more people, while other drivers could pay a toll that would increase with traffic volume. It's unclear what the tolls would be, but studies have indicated they could exceed $10 for the full 14 miles during peak hours.

The $900 million cost of the lanes would be funded entirely by the private companies in exchange for a share of toll revenue. The companies would also operate and maintain the lanes, though the state would control toll rates.

The lanes would be the first of their kind in the Washington region. Similar projects are in operation in California, Texas and Minnesota, while several other states are exploring the concept.

Virginia officials are also considering two proposals -- one by Fluor and Transurban -- to add HOT lanes on Interstates 95 and 395 between Spotsylvania County and the District. They have signaled their intention to seek deals on other roads as well, while Maryland officials are studying a similar concept.

The Beltway project has drawn a measure of national acclaim. The American Road and Transportation Builders Association selected it last month as one of the top three public-private ventures for 2005, along with projects in Texas and Louisiana.

To win, "a project has to either push the envelope for the field of public-private partnerships and innovative financing or have national or regional significance," said Michael F. Martin, a managing director of the association, in a statement announcing the award. "In the case of Virginia's I-495 HOT lanes, it does both."