Metro Atlanta is in for another cold reality check Thursday when the Atlanta Regional Commission shows how to chop the region's $22.9 billion wish list of transportation projects in half.

Faced with a gargantuan list of transit, interstate, roadway and pedestrian projects covering a 10-county area, the elected officials charged with paring them to $6.1 billion by mid-October asked the ARC's staff of experts to try whittling the list down to about $11.5 billion by Thursday.

What comes next will depend on the reception the ARC's trimmed list receives.

The "first pass," as ARC Chief of Transportation Planning Jane Hayse terms it, could well become the first round of cuts to a list that voters will either accept or reject in a July 2012 sales tax referendum. Norcross Mayor Bucky Johnson, chair of the regional roundtable of mayors and county commission chairs, said the executive committee could look at the results and tell ARC to keep cutting.

They also could say that they'll take it from there, or they could say to start all over, Johnson said. Members will have two weeks to digest the suggested list before their next meeting July 21. A final draft, cut to $6.1 billion, is due to the full roundtable Aug. 15. The roundtable must approve a final list for the ballot by Oct. 15.

The ARC suggestions will show where obstacles lie within the projects submitted by counties, cities, community improvement districts, MARTA and the state Department of Transportation. While impact and practicality will be guiding principles, politics will play a big role, too, as the ARC's staff has met with planning, transportation and public works directors from all 10 counties and the city of Atlanta to hear what they consider "must haves," Hayse said.

“The list captures the priorities of all the local governments,” she said. “The list includes projects that are the most efficient at managing congestion. We've also looked at the impact to employment centers.

"But keep in mind, this list has got to be halved again," Hayse said, "and then we'll have the must haves of the must haves."

If approved by voters, $6.1 billion in transportation projects -- funded by a 10-year penny sales tax -- could dramatically alter how metro commuters move between suburbs, job centers and the city core. In addition, the tax will raise $1.1 billion to be spent by cities and counties on smaller local projects.

Still, it would make only a dent in the region's estimated $120 billion in transportation needs over the next 30 years.

Now is the time when expectations are tempered by what’s actually possible when ordering from a $22.9 billion menu with just $6.1 billion in one’s wallet.

For example, Cobb County's desired high-capacity transit line running as far north as Kennesaw State University -- connecting the county to MARTA's Arts Center station -- would cost $1.5 billion, nearly a quarter of what the penny tax would bring in for regional projects. Gwinnett County's five-piece transit line from the Doraville MARTA station to Gwinnett Arena is estimated at $1.1 billion. The Beltline streetcars: $1.6 billion.

Hayse would not say Wednesday which projects would be staying and which would be going for the suggested list, as it was still being finished.

Kennesaw Mayor and executive committee member Mark Mathews, who made the motion to ask the ARC to take a stab at the list, said what's produced will be a big step.

"ARC has the tools and the modeling capabilities to run the forecasts on whatever it is we need," he said. "We've got to start making serious cuts, and it's time."

The ARC is the region's official planning agency.  Though the roundtable members are the decision makers, they're getting technical help from the ARC's staff.

The executive committee told the ARC to consider four factors: congestion relief, economic development, deliverability and regional equity. The first two can be done with computer modeling and statistical analysis, Hayse said, but the second two are trickier.

On deliverability, the roundtable has said a requirement for the final list will be projects that can be finished within 10 years -- the length of time that the penny tax will be collected -- but Hayse said it's unclear whether they'll consider projects that can be mostly finished during that time frame. Legally, projects must be under way within 10 years to be included.

A study by the Georgia Regional Transportation Authority, another major reality check on the sales tax list, found several big-ticket transit projects couldn't be finished in that time frame. The first phase of the Cobb transit line, going only as far as Cumberland Mall, could take until 2026, the study found. Building the Gwinnett line to the Gwinnett Arena could take until 2024.

MARTA's proposed $1.03 billion I-20 line to Candler Road could take 10 years and six months, GRTA's study found.

As for equity, the roundtable asked for data on how much each of the 10 counties would contribute in sales taxes to the $6.1 billion. Johnson said that's not to say that the proportional shares would determine proportional projects. One of Douglas County's most crucial projects, he pointed out, is improvements to the I-20/I-285 interchange, which is in Fulton County.

"They just want to be sure that the revenue is spread equally around the region," Johnson said.

TRANSPORTATION PROJECTS IN PLAY

A $22.9 billion wish list of transportation projects will be chopped to $11.5 billion Thursday in a dry run by Atlanta Regional Commission staff, which is making suggested cuts to help a regional roundtable of elected officials decide how to ultimately cut the list to $6.1 billion.

An advance copy of the suggested list wasn't available Wednesday, and ARC Chief of Transportation Planning Jane Hayse gave no indication what might stay or go. Here, however, are some of the big-impact, big-dollar projects sure to be in play. Not everything can stay; some could be funded in pieces.

The Cobb County line

Cost: $1.5 billion

Description: Connecting Cobb County to MARTA through high-capacity transit, extending to Cumberland Mall, then on to Kennesaw State University.

Possible pieces? Going only as far as Cumberland Mall would bring the price down to $917 million, including operational costs. Different alignment options could be explored.

The Gwinnett County line

Cost: $1.11 billion

Description: A light rail line running, in five sections, from the Doraville MARTA station to Gwinnett Arena.

Possible pieces? An alternative initial step is packaging a $146 million heavy rail extension from Doraville to Norcross with a $100 million study for the entire corridor.

The Clayton County line

Cost: $157 million -- one leg of a $463.6 million line

Description: A weekday commuter rail connecting to downtown Atlanta's new Multi-Modal Passenger Terminal, by way of an existing Norfolk Southern line, to the Hampton area of Henry County.

Possible pieces? Already a piece of a larger project running to Macon.

MARTA’s Lindbergh to Emory line (Clifton Corridor)

Cost: $1.1 billion

Description: A new MARTA line from the existing Lindbergh Center station to Emory University, and on to Clairmont Avenue and North Decatur Road. A new Armour Yard connection would link to MARTA's red and gold lines, giving access to Doraville, North Springs and the trains at Hartsfield-Jackson airport.

Possible pieces? Running the line as far as Emory would cost $685 million. Without the Armour Yard airport link, it would cost $838 million.

MARTA’S I-20 line from Central Atlanta to Candler Road

Cost: $1.03 billion

Description: A new MARTA line connecting downtown Atlanta to south DeKalb County, ending at a new station at Candler Road.

Possible pieces? As an alternative, for $522.5 million, MARTA's existing blue line could be extended from the Indian Creek station to a new station at Wesley Chapel Road near I-20. The I-20 line could be added as bus rapid transit, for the time being, for $93.2 million to $197 million.

Beltline streetcars

Cost: $1.6 billion

Description: Surface-track streetcars running along the Beltline, a network of parks and trails circling the city of Atlanta inside I-285. The streetcars would connect to MARTA stations.

Possible pieces? Northeast Atlanta to downtown: $234.3 million; south Buckhead to Midtown: $430.3 million; southeast Atlanta to Midtown: $407.5 million; southwest Atlanta to Midtown: $508.5 million

Ga. 400/I-285 interchange

Cost: $500 million

Description: Reconstructing one of the most congested interchanges in the Southeast.

Possible pieces? Funding could be limited to environmental studies and right-of-way acquisition.

I-285 North and Hammond Drive transit

Cost: $1.02 billion

Description: Buses running within fixed guideways, with limited stops, along the top of I-285, connecting to Hammond Drive and Perimeter Center in Dunwoody. Or could be light rail.

Possible pieces? The western half likely cannot be completed in 10 years, according to GRTA. The eastern half might, or phases could be done.

Gwinnett’s Ronald Reagan Parkway Extension from Pleasant Hill Road to I-85

Cost: $300 million

Description: Completing a long-desired plan to extend a four-lane, median-separated and grade-separated freeway to I-85.

Possible pieces? Not likely, but could be limited to funding environmental studies and right-of-way acquisition.

Tara Boulevard Super Arterial in Clayton County

Cost: $254.2 million

Description: Upgrading Tara Boulevard into a "Super Arterial," with grade separations at major cross streets and collector-distributor systems at arterial and access roads.

Possible pieces? Could be broken up into three to five segments.