A bipartisan group of senators involved in a new round of infrastructure spending discussions raised the idea of tying the federal gas tax to inflation as one way to help pay for their $1 trillion proposal — a suggestion that could see the first changes to the tax since 1993.

The group, composed of moderate Republicans and Democrats, is assembling a package to fund roads, bridges, pipes and Internet connections, people familiar with their negotiations told The Washington Post.

Here’s how indexing the gas tax to inflation could fit in.

— What does the federal government do with money it raises through gas taxes? How much are we talking about?

The government historically has used gas taxes to pay for investments in highways and transit systems through the Highway Trust Fund. The tax was last increased in 1993 and set at 18.4 cents per gallon. Diesel is taxed at 24.4 cents. Together, the taxes brought in about $36.5 billion in 2019, according to the Congressional Budget Office. Other taxes on trucking raised another $6.9 billion. In 2019, 13 percent of the money was allocated to transit and the rest went to highways.

How much individual drivers pay varies depending on how much they drive and their vehicle’s fuel efficiency. A driver behind the wheel for 12,000 miles a year in a car that gets 22 miles per gallon would pay about $100 a year. The Post built a calculator that lets drivers estimate their costs.

States also tax gasoline. The amounts vary, but the average is 30 cents per gallon, according to the U.S. Energy Information Administration.

— Why are lawmakers talking about indexing the tax to inflation?

The purchasing power of the gas tax has been reduced dramatically since 1993 as the costs of building roads has increased and vehicles have become more fuel efficient. Indexing the tax, meaning it would rise along with the cost of living, would help revenue keep up with spending. It’s an approach many states already take.

Sen. Shelley Moore Capito, R-W.Va., a leader on the Environment and Public Works Committee, recently told a meeting of road building industry leaders that indexing the federal gas tax would bring in an extra $2 billion a year.

— That doesn’t sound like a lot in the context of a $1 trillion spending proposal.

It’s not. The gas tax hasn’t brought in enough money to cover spending on roads and transit since 2008, so Congress has had to fill the gap with other money. Even if spending only increased in line with inflation, the CBO forecasts widening gaps between money coming in and going out.

The proposals lawmakers are advancing include significantly greater spending increases. The CBO estimated that adding another 15 cents to the gas tax in 2022, then indexing it to inflation going forward, would raise $291 billion between 2023 and 2031. But because of the projected shortfalls, just $95 billion would be available for new spending.

Jeff Davis, a fellow at the Eno Center for Transportation, a policy analysis group, estimates the tax would need to be more than doubled in the coming years to cover the costs of a $547 billion transportation bill advancing in the House.

— Is anyone in Washington proposing those kinds of big increases?

Not really, no. Some industry groups have expressed support for increasing the tax by a few cents, but lawmakers have shown little interest. The Biden administration has been clear it would consider a raise to be a violation of its pledge not to increase taxes on families making less than $400,000 a year.

That opposition led Rep. Earl Blumenauer, D-Ore., long a staunch supporter of raising the gas tax, to conclude it was time to move on.

“Let’s just not beat our heads against the wall for something that is not going to happen,” Blumenauer said. “We ought to start now accelerating the transition to a different system.”

— What other options are there?

One that has gained traction among transportation experts is charging drivers based on how far they travel by imposing a per-mile fee. That sort of system would allow the government to collect revenue from electric vehicles and haul in more money from efficient hybrids. But right now, according to the CBO, there are so few electric vehicles on the roads that imposing such a fee would bring in a negligible amount of extra money. A flat $100 annual fee on electric vehicles would have brought in about $150 million in 2019, the CBO estimates.

There are other potential issues: A fee could reward drivers of the most polluting vehicles and, like the gas tax, would fall heaviest on the poorest drivers.

Another alternative would be to impose a mileage fee only on big trucks, which could be more straightforward to implement because truck travel is already closely monitored. That idea is opposed by the trucking industry.

Biden’s proposal looks beyond fees on travel entirely. He has called for increasing corporate taxes to pay for infrastructure proposals, something Republicans don’t support.