Without more money — at least $12 billion over the next 20 years — Minnesota’s 12,000 miles of roads and bridges will continue to deteriorate, traffic jams will worsen and the effects will be far-ranging, according to an updated assessment of transit needs by the Minnesota Department of Transportation, or MnDOT.

Despite plans to spend $18 billion over the next two decades on transportation projects, $30 billion is required to meet the needs identified in the 20-year Minnesota State Highway Investment Plan, known as MnSHIP, released this week by the agency.

And those needs come at a time when several factors are conspiring to drive down the state revenue sources that are constitutionally dedicated to fund bridges and highways.

The plan, which is updated every four years, will help frame the debate for transportation funding in the upcoming session of the Legislature, which convenes Feb. 25. Bonding and transportation issues are expected to dominate the agenda.

“We’re very concerned when we look at the money that’s going to be available and the needs that we have, and are going to have,” said Margaret Donahoe, executive director of the Minnesota Transportation Alliance, a coalition of groups that advocate for transit projects.

“It’s really just a maintenance and preservation plan — and it won’t even do a good job of that.”

The plan acknowledges that, based on revenues, MnDOT won’t meet all of its goals over the course of the next two decades. “The number of roads and bridges in poor condition will more than double and perhaps even triple within 20 years,” the plan says.

It lays out dozens of highway and bridge projects across the state through the year 2019, and it projects what kinds of transportation investments will be made through 2033.

Because of its age, the state’s highway system will need more capital investment and maintenance in the years ahead. And the decisions get more difficult as the plan progresses.

“This is a very resource-constrained plan,” said Kevin Gutknecht, spokesman for MnDOT. “Whatever we decide to spend has to match the revenues that we’re projecting.”

Good transportation is vital to commerce, in addition to safety and convenience, he added. “We have to have a good transportation system, or the economy is going to have problems.”

The challenge is daunting.

The plan notes that half the state’s highway pavement is more than 50 years old, and 35 percent of state highway bridges are more than 35 years old.

Minnesota ranks 38th nationally in the condition of its interstate highway pavement and ninth in state highway bridge condition.

Besides bonding and federal aid, the state has three dedicated sources of transportation revenue: the state gasoline tax, license tab fees and the sales tax on vehicle purchases.

Several factors, according to the plan, are putting pressure on those revenues:

• Minnesotans, like others across America, are driving more fuel-efficient vehicles, including hybrids and electric vehicles that don’t even use gasoline. That helps the environment but hurts gasoline tax revenues.

• People are driving less. And although the population is growing, gasoline sales will be reduced as people age and change behaviors.

• New vehicle sales have slowed. Consumers are keeping their cars longer, cutting into tab fee revenues.

• Construction costs, including those for oil-based materials like those used in paving projects, are increasing at a faster pace than revenues.

On top of that, a landmark highway bill passed by Congress this summer, called MAP-21 (Moving Ahead for Progress in the 21st Century) makes changes in how federal highway dollars are doled out to the states and sets performance standards for state highway agencies in areas such as road safety, congestion and condition.

“We need to get back to making the effort to doing the work that needs to be done on our roads and bridges,” said state Rep. Ron Erhardt, DFL-Edina, chairman of the Transportation Policy Committee. “Right now, it’s just patch and repair. We have a lot of things that we need to be doing, but we’re just not keeping abreast with it.”

Both Erhardt and Donahoe said lawmakers likely will be looking at new ways to generate revenue for transportation. In response to declining gasoline tax revenues, Erhardt said, the Oregon Legislature passed a bill this year creating a mileage-based user fee based on how much a driver uses the roads. Though Minnesota is far from having the infrastructure to do that, it could supplement or replace the gasoline tax.

There also might be ways to tie the state gasoline tax — currently 28.6 cents — to the overall fuel purchase, rather than having it based on the per-gallon price. Unlike inflation, the set amount does not increase.

Unlike other taxes, revenues dedicated for good roads are generally supported by voters, Donahoe added.

“Transportation really was an issue that didn’t get done in 2013. In 2014, we know the Legislature will be focusing on infrastructure” as it focuses on the bonding bill, she said.