The Highway Trust Fund will need an additional $85 billion in U.S. general fund transfers over the next six years just to keep operating despite recent events that have shown current spending is not enough, a Transportation Department official told a Senate panel Thursday.

The Interstate 5 Skagit River Bridge collapse in Washington state on May 23 has prompted a renewed discussion of American infrastructure needs, DOT undersecretary for policy Polly Trottenberg told members Senate Appropriations Committee's subcommittee on transportation, housing and urban development, and related agencies at a hearing on infrastructure.

The General Accountability Office found in a report issued Thursday that, of the 607,380 bridges on the nation's roadways in 2012, 1 in 4 was classified as deficient.

"To me, the fundamental question is not so much whether our transportation infrastructure is crumbling and even collapsing," Trottenberg said. "Rather, what is the right level of public investment in our nation's transportation system to ensure continued safety, foster economic growth, and increase mobility choices for our citizens and businesses?"

The Highway Trust Fund is a pool of federal money stocked primarily by gasoline taxes, but it has been dwindling steadily in recent years as the purchasing power of the dollar has fallen and more fuel-efficient vehicles have begun to fill the nation's roadways. Federal disbursements to state and local governments from the trust fund are sometimes back bonds called grant anticipation revenue vehicles, or Garvees, yet the fund was in danger of going broke until legislation enacted last year began shuffling general fund money over to it to make up for the shortfall.

Trottenberg said that this could be a temporary fix only, as the amounts needed going forward are simply too large to find without new sources of revenue.

"This is clearly fiscally and politically unsustainable," she said.

The full committee is responsible for allocating federal spending once it is authorized, and the subcommittee wanted to hear more about how much investment the U.S. transportation system would need going forward.

Sen. Susan Collins, R-Maine, said the government would likely struggle to meet transportation funding needs without some major change.

"The Federal Highway Administration estimates that it must invest more than $100 billion in all levels of the government each year just to maintain our highways and bridges over the next 15 years," Collins said. "Improving the system to meet future demands will require $170 billion annually. This will prove extremely difficult given that the revenues collected for the Highway Trust Fund already do not support the current level of highway funding."

Sen. John Boozman, R-Ark., said he wanted to see projects given the federal green light more quickly, and acknowledged that cutting current funding would be very problematic.

"With the bonding and this and that, states are relying on current funding," he said.

Subcommittee chairman Sen. Patty Murray, D-Wash., worried that the once non-controversial task of allocating transportation money has become poisoned by partisanship in Congress.

"For more than a generation, there was bipartisan agreement on the need for smart infrastructure investment," Murray said. "Sometimes I worry that bipartisan consensus is now eroding."

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