Summey on I-526 (copy)

Charleston County Council Chairman Elliott Summey talks to reporters Thursday, Jan. 10, on Statehouse grounds after the State Transportation Infrastructure Bank approved a new contract to extend Interstate 526. Rep. Leon Stavrinakis, D-Charleston, and Sen. Sandy Senn, R-Charleston, also talked about their push for the project. Seanna Adcox/Staff

We don’t know exactly how much it will cost to build the rest of I-526 across Johns and James islands, although we know a years-old estimate puts the price tag around $725 million.

We don’t know exactly how Charleston County will cover its portion of that cost — about $300 million — or the costs of potential budget overruns and inevitable legal fights.

We don’t know when — or if — the permitting process will allow the long-awaited project to move forward.

We don’t know what other vital infrastructure needs in the Charleston area might suffer if the county siphons millions of dollars in half-cent transportation sales tax funds away from projects voters explicitly approved.

We don’t know the long-term impact that extending a freeway across rapidly developing Johns Island will have on growth there or on the even more rural islands beyond.

We don’t know how channeling so much money and effort into a single 7-mile stretch of road will distort the Charleston region’s longstanding need to diversify its transportation options and reduce its car dependency.

But we do know that on Thursday, the state Transportation Infrastructure Bank and Charleston County Council voted to proceed with a nebulous plan to build an expensive, unnecessary highway and leave county taxpayers on the hook for any unexpected costs.

Worse, County Council enthusiastically agreed to use the half-cent sales tax voters approved in 2016 as a guarantee for its share of the funding should no other sources be readily apparent, which seems increasingly likely.

Voters agreed to the tax via a referendum that explicitly did not include 526 but mentioned other important improvements like adding a flyover at Highway 17 and Main Road, expanding capacity on overburdened Highway 41 and adding pedestrian facilities on James Island, where at least three people were killed walking across roads in the past two years.

The half-cent tax is also intended to generate $270 million in local funds for a planned bus rapid transit system connecting Charleston and Summerville.

Others have suggested that the half-cent sales tax could be used to fund flood prevention and mitigation infrastructure, which would certainly be worth considering. After all, even the best transportation network is worthless if the region is underwater.

The specifics of the 21-page agreement between the SIB and Charleston County were not available to the public on Thursday until both parties had already voted to approve it. Officials cited contractual concerns.

Such secrecy is unacceptable when a tremendous amount of public money is at stake. At $300 million, the 526 completion would be the most expensive Charleston County investment in a single capital project in history.

That 526 is slowly grinding forward is not a surprise. Nor, unfortunately, is Charleston County Council’s cavalier attitude toward spending an indefinite amount of taxpayer money on such a project.

Not surprising, perhaps. But disappointing.