MARIETTA — County leaders now say the $1.8 million Alternatives Analysis transportation study will be done by September, which is about five months earlier than originally expected, but still after the July 31 regional transportation tax vote.

If voters in metro Atlanta agree on July 31 to pay a 1 percent sales tax for 10 years, the money will go for a list of specific projects. But the biggest line-item on Cobb’s list is Project 35, enhanced premium transit service from Acworth to the MARTA Arts Center Station estimated to cost $695 million. The initial plan is for bus service, though that could morph into light rail, depending on the AA study.

It is the only project on the $6 billion metro area project list that could be altered by the results of a study, though some DeKalb residents are hoping to convert a proposed bus line along Interstate 20 to rail if the tax passes.

The AA study, meanwhile, will recommend the single best type of mass transportation for Cobb and the best route for it, and the study will include a 20-year financial plan for the type of transit it recommends.

Chairman Tim Lee vowed that the five elected commissioners, not simply the study, will have the final say on what is actually done, though he did not know how soon any action might happen.

If the tax fails, all bets are off. Without a source of money to cover any of the costs, neither bus nor rail would happen anytime soon.

But do Cobb voters know what they’re getting with Project 35 when they vote on July 31?

Lee thinks they do.

“The project on the TIA is a project that makes sense on its own,” Lee said.

Interestingly, Faye DiMassimo, Cobb’s transportation director, said the county never asked Croy Engineering — the company that last August won the contract to perform the study — to have it done before the July 31 vote on the Transportation Investment Act because there simply wasn’t time.

“The amount of work left to do requires that amount of time,” DiMassimo said. “It couldn’t be done by July 31.”

On May 29, the county announced some early findings of the AA, namely narrowing down the best types of mass transit to light rail, express bus and bus rapid transit. Possible routes were narrowed down to Cobb Parkway (Highway 41) or Interstate 75.

As for how that portion of the study could be done sooner when other parts could not, DiMassimo said: “The entire team just performed the work in a shorter period of time.”

In fact, she said, the timing of the AA study and the TIA vote were “never intended to be compatible.”

“This project started well before the Transportation Investment Act was ever even passed,” DiMassimo said. “It was never on a schedule that worked with the Transportation Investment Act schedule.”

The TIA legislation was signed into law on June 2, 2010, a month after the Federal Transit Administration announced that money for studies like the AA was available. The county then applied for the federal money in July 2010, and learned it had won the $1.36 million federal grant in December 2010. The grant required $340,000 in local matching funds, of which the county paid half and the rest came from the cities of Marietta, Acworth, Kennesaw and Smyrna, Cobb’s two community improvement districts and Kennesaw State University.

Chairman Lee said that once the study is complete, the Board of Commissioners will vote on accepting it.

But actually implementing the results, or seeking federal funding to that end, will require additional votes by the commissioners, Lee said.

He is unconcerned that the AA won’t be done before the July 31 vote.

“The AA will hopefully, if the voters pass the TIA, help fine-tune what’s best for the northwest corridor,” Lee said.

When the AA is finished in September, the results will be presented at a commission work session, DiMassimo said. The results will also be presented to the Atlanta Regional Commission, since any plans for seeking federal funding would require the preferred alternative fit into the ARC’s Regional Transportation Plan.

Two county commissioners, Helen Goreham of west Cobb and JoAnn Birrell of northeast Cobb, said it’s too early to determine whether they will vote to implement the AA recommendations.

But east Cobb Commissioner Bob Ott, who has been an outspoken critic of the TIA and light rail in Cobb, does not put much stock in the study.

It’s not a true alternatives analysis, he said, because federal rules make it nearly impossible for technologies like maglev to be studied because they are considered “unproven” in this country.

“A lot of these grants or studies require that you do it the federal way,” Ott said. “The minute you fall into that, you’re going into the direction of light rail, bus rapid transit or express bus.”

As for the tax vote, he says it would be better to wait, given that residents will also be voting on a chairman on July 31, he said.

“If the TIA is defeated on the 31st, that’s going to make it very interesting to see what happens with the AA, because it doesn’t have any money,” Ott said.