An audit of the state Department of Transportation released Thursday found 16 ongoing major highway projects have increased in cost by about $3 billion since they were approved by the state Legislature.

The report, written by the nonpartisan Legislative Audit Bureau, also found 19 major highway projects completed from 2006 through 2015 cost about $1.5 billion, an increase of $772.5 million from the department's estimates when the projects were approved.

The cost overages occurred because the department failed to account for inflation in its project cost estimates, the audit said.

"The Legislative Audit Bureau report provides a roadmap to improved efficiency and transparency at the DOT," said Dave Ross, secretary of the Department of Transportation, in a prepared statement. "As the new secretary, I couldn't ask for more timely help."

The audit comes less than two weeks before Gov. Scott Walker is expected to present his 2017-2019 biennial budget proposal, kicking off months of intense budget deliberations in the state Capitol.

In addition to outlining the department's failure to account for inflation in its project cost estimates, the audit found the department has not complied with its own administrative rules for selecting construction projects or documented project selection rationale.

The audit includes several recommendations for changes to the Department of Transportation’s fund management, project planning and highway maintenance. It also outlines possible legislative action to change department practices.

Those recommendations include changing how the department calculates costs for work completed by state workers, rather than contractors. The audit also recommends the department maintain a database that tracks why and how much projects go over their estimated cost.

The Department of Transportation budget has been a hot topic in recent months, as a memo from the nonpartisan Legislative Fiscal Bureau released last July estimated the department could face a nearly $1 billion shortfall in the next state budget, if funding remains the same.

Some members of the Republican majority in the Capitol, including Assembly Speaker Robin Vos, R-Rochester, and Joint Finance Committee Co-Chair Rep. John Nygren, R-Marinette, have advocated for the consideration of a gas tax or vehicle registration fee to close that gap.

Walker, however, opposes any tax or fee increase without a corresponding cut elsewhere in the budget.

"The bottom line is we shouldn’t even be thinking about raising the gas tax or fees until we find every last cost savings at the DOT, and the audit shows we can find more savings," said Tom Evenson, spokesman for the governor, on Thursday. "We welcome the opportunity to deliver services taxpayers expect at a price they can afford."

Revenue estimates released last week showed the state bringing in more money through mid-2019 than initially estimated, which has spurred talk among Republican legislators of a $300 million tax cut and corresponding increase to pay for road projects.

Walker will introduce his budget proposal, which will include his proposals for the Department of Transportation, on Feb. 8.