HARRISBURG (AP) - A state audit released Tuesday warns that plans for the Pennsylvania Turnpike to raise tolls repeatedly in the coming years may not be sustainable, leaving the 550-mile road network at financial risk.

The study by the state auditor general's office also said the turnpike commission has recently experienced an increase in the number of people who are not paying tolls, and warned the problem will continue unless the agency gets more power to take action against toll payment violators.State law, the report said, "does not give the commission the authority to suspend the registration of Pennsylvania motorists' vehicles or otherwise impose a penalty on vehicle owners who fail to pay their tolls."Sean Logan, chairman of the Pennsylvania Turnpike Commission, said the turnpike's debt, which is fueled largely by the payments to PennDOT, is a growing concern."We look forward to working with the auditor as well as the Legislature to secure passage of meaningful tolling enforcement legislation that will allow all tolling agencies in the state to make sure motorists pay their fair share," he said. He noted that in about a month turnpike staff is expected to complete a comprehensive study of turnpike capital spending.The survey said drivers are avoiding tolls by exiting through an E-ZPass lane without an E-ZPass account; because E-ZPass transponders are failing to be picked up, perhaps because of low batteries; and because some drivers don't maintain enough money in their E-ZPass account to cover the cost of their fare.The turnpike uses a collection agency to track down people who owe then for unpaid fares, along related costs and fees, but those efforts are of limited effectiveness. The report said the number of fare violations that generated invoices increased from 455,000 to 726,000 between 2011 and 2015, while turnpike vehicle usage rose only slightly, from 189 million to 192 million.The report suggested the turnpike authority be granted the power to suspend vehicle registrations and to set graduated fines for repeat offenders. It also proposed publishing a list of "egregious" toll scofflaws, establishing reciprocal enforcement programs with other states and possibly letting toll collectors accept credit and debit cards.The report faulted the turnpike for what it said was an insufficient response to previous recommendations that it clamp down on letting its own employees, contractors and others ride for free for personal, nonbusiness trips.The audit said the turnpike has given about 2,100 badges and 900 transponders to its workers, providing them with about $1.2 million in free trips between June 2014 and February 2016. The agency also provided more than $4 million worth of toll-free travel to some 5,000 consultants, contractors and state government officials.The auditor general's office recommended the Legislature eliminate or reduce the hundreds of millions of dollars the turnpike is paying to the state Transportation Department under a 2007 state law.Tolls are currently projected to increase every year through 2044. The auditor general's office said a passenger vehicle's 270-mile trip on the main stem from Valley Forge to Pittsburgh, using E-ZPass, will go up from $23 today to $67 by 2044. A five-axle truck currently pays $124 for that route when using cash, but the same trip will cost $356 in 2044, the study said."We believe that at some point, the average turnpike traveler will be deterred by the increased cost and seek alternatives," the auditor general's office concluded.