ALBANY - On average, it costs a nickel a mile in tolls for passenger cars to use the Thruway.

While that might be easy on the wallet, it is nowhere near enough to support the rebuilding of a 507-mile superhighway system now in its seventh decade.

That was the message from four former state Thruway Authority officials who spoke Monday at the Hearst Media Center on how much it could cost to bring the highway into the 21st century and how to find the money to do it.

The numbers sounded daunting. If the entire Thruway, along with its 134 interchanges and more than 800 bridges, were to be rebuilt all at once, the bill would be $15 billion, said Michael Shamma, who was the Thruway's chief engineer until 2015.

Shamma, who started as a state Department of Transportation engineer in 1988, said the Thruway's current condition can be understood in thirds - a third is in good shape, a third is in fair condition, and a third is poor.

Spreading out needed rebuilding work over years would take at least $400 million annually, he said, significantly more than the $285 million usually in the Thruway budget.

Maria Lehman, a former acting executive director and chief operating officer, was blunt: "This is not a midlife crisis on the Thruway. It is an old age crisis."

She pointed to the example of the Pennsylvania Turnpike, which opened in 1940. After years of incremental upgrades and "diminishing returns," said Lehman, the state decided to reconstruct the turnpike entirely, starting in 1999 and finishing in 2004.

Lehman, who is also an engineer, said the state might have to consider spending between $500 million and $1 billion a year to restore the Thruway "during our lifetimes."

Finding the money will not be easy, she said. "Everyone wants infrastructure like the Thruway. But no one wants to pay for it."

Lehman was a longtime Thruway Authority staffer and resigned as acting executive director in 2016 after Gov. Andrew Cuomo criticized the pace of the Tappan Zee Bridge rebuilding.

Taxes and a potential toll increase are the most likely way to find the funds, said Thomas Madison, executive director from 2011 to 2015, when Lehman took over.

Tolls were unchanged from 1988 to 2005, and since then have increased twice, in 2005 and 2009.

"People expect to pay a little more for using a premium highway," said Madison.

Michael Fleischer, the executive director before Madison, from 2003 to 2011, said the toll for a passenger vehicle is about a five cents a mile, which is among the lowest in the nation for passenger vehicles. Rates for commercial trucks were average for toll roads nationwide, he said.

She said the state can still borrow money at bargain-basement interest rates "so now is the time to go for it."

Long-term interest rates have been slowly rising, and Lehman warned that the tariffs imposed on imported steel and aluminum by the Trump administration could boost the cost of rebuilding.

The event was sponsored by Rebuild New York Now, a construction-trades coalition that support infrastructure projects.