A toll booth attendant hands a driver their toll receipt and change. Emile Wamsteker | Bloomberg | Getty Images

Improved fuel efficiency is one of the easiest and practical solutions society can implement to combat climate change, but its unintended consequences are large: potholes and shaky bridges all across the U.S. map. That's because more efficient cars means barren fuel-tax coffers, which traditionally have funded bridge and road infrastructure. The federal gas tax of 18.4 cents a gallon hasn't budged in two decades, and that hasn't helped those coffers, either; indexed for inflation, it would be at about 30 cents per gallon today. The effect is that such infrastructure has nearly 40 percent less funding than in 1993. Given the lack of funding, it's little wonder, then, that the American Society of Civil Engineers gives America's roads a D and bridges a C in its 2013 report card. (Read more: ASCE: Nearly $4 trillion to fix U.S. roads by 2020) Oregon—where 1 in 6 bridges is functionally obsolete and 65 percent of roads are in poor to mediocre condition—is doing something about it. And other states are watching closely.

Instead of taxing gas, Oregon is conducting an experiment with taxing road usage, or vehicle-miles-traveled (VMT). (Read more: Why California's high-speed rail is off track)

On its face it makes sense. If you drive more miles, you should pay a little more. David Zahn FuelQuest's vice president of marketing

In 2015, 5,000 volunteers will start paying 1.5 cents per mile instead of the state's 31-cents-a-gallon tax—that is, they'll be reimbursed the tax at the pump. (The total gas tax in Oregon, including the federal portion, is 49.4 cents.) "On its face it makes sense," says David Zahn of FuelQuest, a software services company for fuel management. "If you drive more miles, you should pay a little more."

Trey Baker of the Texas A&M Transportation Institute was on his way to Portland on Tuesday for the Road Usage Charge Summit about VMT. He said the Oregon VMT is a new way to address the infrastructure funding problems without being dependent on the purchase of gasoline.

"It started 12 years ago with the road-usage-fee task force," said James Whitty, one of the leaders of the state's program. "The aim was to figure out what Oregon's future road funding should look like, as we saw cars becoming more fuel-efficient, leaving less money for roads. And that has come to pass."

Overinflated

Zahn and Baker expressed admiration for Oregon's willingness to try something different, but they both saw a fundamental flaw: VMT is one more driver tax that is not indexed to inflation. Such indexing is, of course, a political bugaboo.

"When you replace one non-indexed tax with another, does that solve anything?" asked Zahn.

"VMT doesn't do anything to fix that political problem," Baker said. "And you're probably going to end up in the same place with the same problem if you don't raise the tax over time." Whitty sees it as a separate issue. "There are two deflating factors in the gas tax: inflation and fuel-efficiency. The per-mile charge addresses one of them," he said.

(Read more: The dirtiest battle in clean energy heats up)