Eighteen months after the Legislature passed a controversial increase in the state gas tax to fund transportation projects, the Minnesota Department of Transportation is projecting that it is still facing a $50 billion funding gap over the next 20 years.

A new 20-year plan issued by MnDOT identifies more than $65 billion in transportation needs around the state but predicts that it will be able to spend only $15 billion to address them -- even with the influx of funds from the federal stimulus program.

"I think people felt that these increases would essentially take care of our needs," Tom Sorel, Minnesota's transportation commissioner, said Monday. But with so much of the highway system past its prime -- more than half of all state-owned bridges are more than 30 years old -- the needs are outstripping the state's resources.

The likely result, according to the plan, is that three-quarters of the $15 billion will be targeted at preserving existing roads and bridges, so Minnesotans shouldn't expect to see a lot of new state highways. And some of the lanes we're driving on now will get bumpier.

In February 2008, the Legislature voted to raise the state's gas tax by 8.5 cents, with the increase fully phased in in 2012, and raised other fees, including many license-plate renewals. But the money goes only so far when you're talking tens of billions of dollars. A one-cent increase in the gas tax yields an additional $30 million a year for Minnesota's state and federal highways.

At the time of the gas tax vote, MnDOT tried to manage public expectations about how far the money would go, saying much of it would need to be spent on preserving existing roads and bridges.

"All the bridges built during the '50s and '60s -- the baby boomer bridges -- are nearing the end of their useful life," the plan says, while the amount of state-owned pavement in poor condition is expected to nearly triple, from 600 miles to 1,600 by 2018.

The new plan is more about policies than specific road work, Sorel said. "This isn't a list of projects for the next 20 years," he said, noting that "there's always been a gap" between the state's transportation needs and the funding to satisfy them.

But he said the state won't skimp on crucial functions such as bridge inspections. "We're not going to have an unsafe transportation system for our people." MnDOT is putting some of the gas tax and other revenues toward replacing or rehabbing 133 bridges in the next decade.

Planning for 20 years from now is tricky business, and the report acknowledges as much. Construction materials are relatively cheap now because of the recession, but that savings is not expected to last. A new federal transportation bill has yet to be hammered out. Gas tax revenues are shaky because of a drop in driving and ever-increasing fuel efficiency.

Sorel sees innovation as a big part of narrowing the gap between needs and means, from new methods of fixing pavement more cheaply and durably to ways of fitting more cars onto existing highways in the metro area. He pointed to the managed lanes being built on Interstate 35W in the southern half of the metro area and said that technology exists to allow cars to communicate with roads and one another, meaning they could travel closer together at higher speeds.

He also said Minnesota is looking into studying a system under which drivers would pay a fee per mile driven rather than through the gas tax. Such a system was tested in Oregon a few years back, and the University of Iowa is organizing a federally sponsored study in five cities around the country.

The 20-year plan acknowledges that the state's transportation funding gap is unlikely to ever be filled, and Sorel said that ways of determining those needs are likely to change over time, shifting from standards such as vehicle-miles traveled to perhaps the amount of freight moved or as-yet-undefined "quality of life" measurements.

"We can have excellent pavements, excellent bridges, but if we're not improving people's quality of life in some way, then are we doing all the right things?" he said.

Jim Foti • 612-673-4491