PITTSBURGH - The Pennsylvania Turnpike Commission's level of debt is "potentially unsustainable" because projected toll increases could lead motorists to find free alternatives, the state Auditor General's Office said in an audit released Tuesday.

Auditor General Eugene DePasquale said at a news conference that his auditors found that the turnpike's financial health has deteriorated substantially since 2007, when the legislature began requiring the agency to make payments of $450 million a year to PennDot. The commission has had to borrow a substantial part of that money, and its annual debt payments are about $600 million of its $980 million annual budget.

The audit found the commission's net position - assets minus liabilities - has "spiraled" from $1.76 billion in 2007 to negative $4.11 billion in 2015. This has occurred despite the commission's raising tolls nine years in a row.

"In order to meet its debt payments, the commission is expected to increase tolls each year through 2044, which is burdensome for travelers," the audit said. "In addition, it is necessary for traffic volume on the turnpike to also continue to increase.

"However, if enough motorists decide that tolls are too high and utilize alternate toll-free routes, the commission's revenue will not increase as projected. Therefore, the commission risks being unable to raise sufficient revenue to cover its debt payments."

In July, the commission ordered a complete review of all capital expenses to make sure projects are needed and affordable. A report is expected this month. The commission also is lobbying the legislature to eliminate the $450 million payment to PennDot sooner than 2023, when it is scheduled to drop to $50 million a year.

DePasquale also noted that the turnpike will lose nearly $20 million this year in unpaid tolls from motorists who drive through the EZ-Pass lane without an operating account. Their license plates are recorded and they are mailed a bill, but in fiscal year 2016, $19.96 million in tolls and fees for nonpayment were written off as uncollectible.

DePasquale recommended the legislature give the commission the power to revoke registrations for vehicles whose owners haven't paid and set up agreements with other states for out-of-state drivers who don't pay.