MARIETTA — While debate over the July 31 Transportation Investment Act grows ever louder, a $1 billion project to add reversible toll lanes to Interstates 75 and 575 in Cobb is quietly moving ahead.

It’s called the Northwest Corridor project and it will be the most expensive road project in the state’s history, according to the Georgia Department of Transportation. Its price tag equals one-eighth of the entire TIA project list, though the Northwest Corridor project is entirely separate from the proposed TIA sales tax that voters will decide on July 31.

Construction could begin in late 2013 with the reversible lanes open to traffic by 2018.

“What sold me on it is the importance of that corridor and the importance of relieving congestion,” Gov. Nathan Deal said. “It’s not a political thing. It’s just one of the facts of life when dealing with the realities of a problem you’re presented with, and trying to figure out the best solution to it.”

The project entails two new lanes along the west side of I-75 between its interchanges with Interstate 285 and Hickory Grove Road. The lanes will be separate from the existing interstate. Both lanes will carry traffic southbound during morning rush hours and northbound in the evening rush. On I-575, one new reversible lane will be added in the center median from the I-75 interchange and Sixes Road.

Toll amounts have not been set, and will vary depending on traffic volume. The toll lanes, which are intended as an option for reliable trip times for drivers when the regular lanes are congested, will only be open during peak travel times.

Deal made clear the Cobb project is different than the toll lanes introduced in Gwinnett and DeKalb counties.

“These are new lanes,” Deal said. “The conversion of the HOV lane to a HOT lane on I-85 in Gwinnett County took an existing lane and simply converted it into a toll lane. … I do not approve of taking an existing lane and converting it to a toll lane. I think toll lanes should only be new construction and new lanes.”

Planners say drivers will be able to get on or off the toll lanes at six points along I-75, and at three points along I-575.

On I-75, the access points will be at I-285; Terrell Mill Road; Roswell Road; I-575; Big Shanty Road; and Hickory Grove Road.

State Sen. Lindsey Tippins (R-west Cobb) said the reversible toll lanes “will bring a tremendous amount of relief.”

“You double your capacity of what conventional construction would bring, because you have pretty much two lanes going in each direction at peak times,” Tippins said.

The state had planned to allow private companies to build the project and give them control of the tolls and lane operations. Deal canceled that plan after he took office in 2011, an action that Tippins praises.

“I have a great deal of respect for the governor and his intestinal fortitude to have canceled the other format … and take back the sovereignty of the road to the state of Georgia instead of being under the thumb of what would have been an entity that’s based in Spain. We’d have been under their thumb for up to 60 years. That did not bode well for the state of Georgia,” Tippins said.

The state senator said the project is not controversial because it is “so obviously beneficial.”

“You don’t generate a great deal of opposition and resistance when you have clarity of what a project is going to do and you spell out what it’s going to do, and when it’s going to do it,” Tippins said. “Any rational mind would realize that adding two lanes of interstate capacity in each direction at peak times, it’s kind of like killing rattlesnakes, I’m not sure there’s a wrong way to do that.”

The Northwest Corridor project is expected to cost almost $1 billion. Earlier this year, the Georgia General Assembly designated $300 million of state gasoline taxes carried over from previous years to the project. The state has been approved to apply for a $270 million low-interest federal loan, and GDOT has put in $200 million from its construction budget.

A yet-to-be-named private partner will front 10 to 20 percent of the project cost, to be repaid by the state.

Deal said credit must be given to the members of the General Assembly who were willing to put the money in the supplemental budget for 2012.

“This I-75 corridor north is … not only for the metropolitan area, not only for Cobb County, but for the greater good of our state, because it is a major corridor where goods that are manufactured are moving through it, goods that are bringing raw materials to our manufacturers are using that thoroughfare. I just simply have to congratulate the members of the General Assembly for being willing to do that, and I’m sure that there will be occasions in the future where they will be asked to make similar decisions,” Deal said.

Deal called it a major departure from doing business as usual.

“I hope that it is a signal of a new era of cooperation. Not only between the General Assembly and the Department of Transportation, but also with local communities as well, and to me, that is the most long term positive result that we can have, is to put aside local interests as being the only reason that you vote for or against something and look at the greater good of our state.”