GRANTS PASS, Ore. — Oregon has a plan for retrofitting the hundreds of bridges that are likely to fail in a major earthquake or are getting too old to support heavy trucks.

It just doesn’t have a way to pay for it all, and prospects for finding a solution are dim.

The federal gas tax hasn’t been raised in 21 years, and Oregon’s share has been declining for years.

The state gas tax, a major funding source for transportation infrastructure projects, hasn’t generated enough to repair the state’s roads and bridges as cars have gotten better mileage or quit using gas at all, and a new system of charging motorists by the mile rather than the gallon is still in the experimental stage.

Leaders in the Oregon Legislature agree something must be done, but there is strong disagreement over how.

The result has been that while per capita state and federal highway funding in Oregon was up 16 percent from 2003-2008, it dropped 11 percent from 2008-2013, according to figures reviewed by The Associated Press.

“We’re looking at a pretty big cliff” with no serious discussions in sight, Oregon Rep. Peter DeFazio, top Democrat on the U.S. House Transportation and Infrastructure Committee, said of the funding picture.

Earthquakes were not considered a major threat in Oregon until the 1980s, and bridges built before that weren’t designed to withstand them. Scientists think the region is due for a massive quake, and officials want to make safety improvements to get ready.

“Areas of the state will be cut off for years by highway,” said Oregon Department of Transportation Assistant Director for Communications Travis Brouwer. “You’ll be relying on helicopters.”

The Oregon Highways Seismic Plus Report issued last October calls for replacing 138 bridges, and retrofitting 580 over the next 50 years in four phases at an overall cost of $5 billion. Phase one focuses on U.S. Highway 97 on the east side of the Cascades, Interstate 5 through the Willamette Valley, and two routes connecting them.

Even without the threat of earthquake, 900 of Oregon’s 2,700 bridges need fixing in the next 20 years, but the projected cost is $230 million a year, nearly five times the $50 million available, Brouwer added. Without more money, by 2035 about 350 will have weight restrictions.

Paying for road work the usual way, through increases in the gas tax and DMV fees, has raised strong objections from Republicans, unless Democrats drop plans for legislation to combat greenhouse gases with a low-carbon fuel standard, which has passed the Senate.

That low-carbon fuel standard is likely to raise gas prices, making it very difficult to ask Oregonians to pay even more to raise money to fix roads and bridges, said Rep. Cliff Bentz, an Ontario Republican.

Meanwhile, another pilot program to demonstrate the workability of a pay-by-the-mile tax applying to high-mileage vehicles that don’t pay their share via the gas tax kicks off July 1, with 5,000 volunteers. An earlier attempt based on GPS trackers was scrapped over privacy complaints.