RICHMOND — A state transportation board Wednesday advanced plans for a controversial project to build a parkway connecting Prince William and Loudoun counties.

The Commonwealth Transportation Board, in a 15 to 1 vote, endorsed a master-plan study that looked at potential improvements along the state’s North-South Corridor, a 45-mile route connecting the two Northern Virginia counties.

The vote was denounced by opponents of the Bi-County Parkway, who said board’s decision is a sign that the state is moving forward with the 10-mile road, which would skirt Civil War sites to connect I-66 in Prince William with Route 50 in Loudoun.

Board member W. Sheppard Miller III, of Virginia Beach, voted against moving forward, saying the board’s resolution did not adequately rule out toll roads, which he opposes.

A total of 15 people appeared before the board to comment, and several of them urged Virginia Department of Transportation (VDOT) officials to delay the adoption of the corridor study, saying that the state has not been transparent about its plans.

“This impacts tens of thousands who are unaware,” said Tom Thompson, who lives near the site of the proposed parkway.

Gary Garczynski, who lives in Woodbridge and represents the board’s Northern Virginia district, said the vote was a small step in a years-long process for the parkway.

“It’s not a foregone conclusion, and I regret to say that a lot of people think it is,” Garczynski said. “From my perspective, that’s just not true. We have a long way to go.”

The North-South corridor is one of 12 designated regions in which state transportation funding priorities are established. The improvements, including the Bi-County Parkway, are designed to improve traffic flow, spur economic development and provide better access to Dulles International Airport, supporters say.

Del. Timothy D. Hugo, a Republican Party leader who represents parts of Fairfax and Prince William counties, was among those who attended the meeting to object to the proposed road.

The board has “created a firestorm. . . . The rationale provided by VDOT [for the parkway] changes every time,” Hugo said. “These people deserve a straight answer.”

Residents say they worry about increased traffic and the fact that the parkway would run through a protected rural area with a rich Civil War history.

The board’s vote Wednesday came after a month’s delay. Concerns were raised by Rep. Frank R. Wolf (R-Va.), who wrote a letter to Gov. Robert F. McDonnell (R), to say that the state’s process lacked transparency and that more public input was needed. Six Republican state legislators, led by Hugo, have announced that they oppose the road and the state’s handling of the process.

Stewart Schwartz, president of the Coalition for Smarter Growth, has questioned whether a plan for north-south improvements is necessary.

“You started with a conclusion and went backwards,” he said of the adopted study. ““We will look back and realize that we have gained no ground and squandered billions.”