Local elected officials are forging ahead with plans to extend the Mark Clark Expressway across Johns and James islands, with no idea how they would pay for about 25 percent of the controversial project.

Local road priorities

Priority list for long-range transportation projects*:1. U.S. Highway 17 Alternate (North Main Street in Summerville)2. Interstate 26 and 526 interchange3. Traffic signal control systems (regionwide)4. U.S. Highway 52 and 176 interchange5. S.C. Highway 41 (U.S. Highway 17 to Joe Rouse Road)6. Ashley Phosphate Road (Cross County Road to Northwoods Boulevard)7. Jedburg Road (U.S. Highway 78 to Wildgame Road)8. Savannah Highway (Orleans Road to Wesley Drive)9. Aviation Avenue connector to Ashley Phosphate Road10. Sheep Island Parkway11. St. Andrews Boulevard (Ashley River Road to Wesley Drive)12. Michaux Parkway (Dorchester Road to International Boulevard)13. Main Road (Savannah Highway to River Road)14. Folly Road (James Island Connector to Patterson Avenue)15. Mark Clark Expressway extension*There are 71 projects on the complete list.Berkeley-Charleston-Dorchester Council of Governments

The state Department of Transportation’s most current estimate for the completion of Interstate 526 is $556 million, but only $420 million is available for the project, leaving a $136 million shortfall.

That shortfall is more than the cost of rebuilding four downtown schools that would fail in a serious earthquake.

Charleston County Council Vice Chairman Elliott Summey, who supports the completion of I-526, said he is certain that it could be done with the $420 million currently available for the road.

Dana Beach, director of the Coastal Conservation League, who opposes it, said the project could cost as much as $600 million, and the overrun likely would be passed on to local taxpayers because federal and state coffers for road projects are empty.

Beach also said he doesn’t think it’s a coincidence that last week, at Summey’s urging, a County Council committee considered a transportation sales tax increase for road projects the day before a group of powerful local politicians “declared their eternal commitment to I-526.”

He was referring to a Friday news conference where local mayors and state-level leaders pushed for the project to move forward, saying most of their constituents supported it. “You don’t have to be a detective to figure out what’s going on there,” Beach said.

Summey last week proposed that Charleston County go to voters with a referendum this fall to boost the transportation sales tax from a half to a full cent. Council’s Finance Committee instead voted to start meeting with citizen groups to get their input on the possibility of a transportation sales tax increase.

Summey said the move was to pay for critical projects, and was not about funding I-526.

Supporters of the controversial I-526 project say it is essential to relieve severe traffic congestion in the growing region, improve hurricane-evacuation routes and link local communities.

Opponents say the project will promote urban sprawl, and that other local road projects are more important to the area than I-526.

Summey, who also is chairman of the Berkeley-Charleston-Dorchester Council of Governments’ regional transportation committee, said he has done his own research with independent engineering firms, which have told him they could complete I-526 with the money available.

He adamantly disagrees with the DOT estimate, as do other local leaders who support the project, he said. “We don’t understand where that number comes from,” he said. “I wish somebody would show me a budget.”

Summey said last week that when the transportation committee in June updated the cost estimate to complete the project, it effectively removed any risk that additional expenses would be passed on to the DOT.

If the project were to run over $420 million, the difference would have to be covered by the Council of Governments’ federal allocation for road projects, or from local and state sources, he said, but he could not specify which sources. He also said I-526 could be completed in phases, as money becomes available.

The Council of Governments receives about $17 million in federal money each year for transportation projects in the tri-county area. The group has a prioritized list of 71 projects; I-526 ranks 15th.

County Council in January voted to turn over control of the long-stalled road project to the DOT, but the department has not yet agreed to take it on. If it doesn’t, the county is on the hook for $11.6 million already spent on the project.

Local elected officials at Friday’s news conference said they were trying to demonstrate to the DOT that the project had local support so the department would agree to take it on.

Beach said he thinks the DOT’s estimate to extend I-526 probably is accurate, and that department staff members likely are better equipped than Summey to make estimates on road projects.

And the cost could be even higher than $556 million, he said. For instance, many project supporters would like to see enhancements to the current parkway-style “Alternative G,” which is the DOT’s preferred alternative, he said.

One of those enhancements, an overpass at Folly Road on James Island, would drive the cost up to $600 million, Beach said.

Reach Diane Knich at 937-5491 or on Twitter @dianeknich.