MADISON - The cost of major road projects in Wisconsin doubled between the time they were planned and built, a sweeping audit of the state Department of Transportation revealed Thursday.

It cost $1.5 billion to build 19 major projects between 2006 and 2015 - $772.5 million more than originally estimated, the Legislative Audit Bureau found. Even though the state was spending money on most of those projects for 18 years or more, the DOT didn't take into account the considerable effects that inflation and changes to project design would have on those costs over time.

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Another 16 projects that were ongoing as of August 2016 saw similar spikes in their costs. Originally expected to cost $2.7 billion, they are now slated to come in at $5.8 billion, the audit found.

"They're not estimating them properly," state Sen. Rob Cowles, R-Allouez, said on radio station WHBY-AM. "The Legislature is more likely, along with the governor, to approve (a project) if they are told it's going to cost much less than it actually does."

The skyrocketing costs will influence the debate over whether more money should be put toward roads, giving both sides arguments.

Those seeking more money for roads, including Assembly Speaker Robin Vos, R-Rochester, can point out the audit suggests the shortfall in state transportation funding is more dire than previously thought since costs have essentially been low-balled. Those reluctant to raise taxes for highways, including Gov. Scott Walker, will argue the DOT is not giving lawmakers honest estimates or managing its budget effectively.

"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 a statement from Tom Evenson, a spokesman for Walker. "We welcome the opportunity to deliver services taxpayers expect at a price they can afford."

But Evenson did not explain why the Republican governor's DOT had given estimates that were so off-base for years.

Even considering the DOT's current cost estimates, officials face a $1 billion gap over the next two years between the amount of money they expect to take in and the projects that are planned.

The fight over transportation comes as the Federal Highway Administration has warned Wisconsin officials they should hold off on starting major projects until they complete stalled ones already planned. The state must get approval from the federal agency for many of its projects because it provides about a quarter of its road funding.

The audit contained several findings that suggest Wisconsin highways compare poorly with other states - and that the condition of Wisconsin highways has worsened significantly in recent years.

The proportion of state highways rated in good condition decreased from 53.5 percent in 2010 to 41 percent in 2015 under the department's rating system, the audit found.

Using a different index employed by the Federal Highway Administration, the audit found 32.2 percent of Wisconsin state highways are in "good condition," far less than any Midwestern state. The next closest was Iowa, which had 55.3 percent of highways in good condition.

The Wisconsin State Journal contributed to this report.