But a 2012 analysis commissioned by ERC did provide some initial estimates. And project documents made clear that the non-compete was seen as a guarantee that the company would be "made whole."

Expanding the Hampton Roads Bridge-Tunnel from four to eight lanes would cut ERC's toll revenue by about 5 percent in a single year, the analysis found. If comparable losses continued through 2069, company revenue could, at least in theory, shrink by roughly $300 million. But that probably overstates the impact.

Since the actual Hampton Roads tunnel project contemplates going from four to six lanes - not four to eight - the losses would be smaller. How much smaller is unclear. The partnership has to prove actual losses based on conditions on the ground once the expanded Hampton Roads tunnel opens, the company emphasized.

Of course, the Hampton Roads tunnel is only one of several projects covered by the clause. And if the rest of those potential improvements were made, the state's tab could reach $774 million, according to a second analysis provided to ERC's outside investors.

"Compensation for competing facilities is a common risk allocation feature" in such deals, the company said in a Dec. 20 statement, and allows for handling developments "which cannot be predicted."