It's been almost a month since President Obama sent Congress his budget for the next fiscal year, and lawmakers are growing aggravated over one of his plans to build more highways, roads and bridges in the U.S.

What do they want to know? How it's going to be paid for.

"Bipartisan financing for Transportation Trust Fund," was the wording in the Obama budget, which called for $328 billion in funding over ten years for infrastructure.

But there were no specifics on how it would be paid for. And there still aren't any specifics almost four weeks later.

"We have to know where the money is going to come from," insisted Sen. Jay Rockefeller (D-WV), who pressed Secretary of Transportation Ray LaHood for answers at a hearing of the Senate Commerce Committee.

"It's a big chunk that's unfunded," Rockefeller said.

The only detail that LaHood would offer was to rule out the most common way of funding highway and road projects - saying the President wanted no increase in federal gasoline taxes.

"He is not in favor of raising the gas tax when unemployment is at 8.9%," LaHood told Senators.

"There are some people in this country that can little afford to buy a gallon of gasoline, let alone one that's been increased by an increase in the gas tax," LaHood said.

But while LaHood ruled out higher gasoline taxes, he had no answer on where the White House was coming up with $328 billion, let alone $328 billion in "bipartisan" financing for transportation projects.

"A sustainable source of funding is the real question here," said Sen. Kay Bailey Hutchison (R-TX), who joined in a bipartisan chiding of LaHood for the lack of detail from the White House.

Any kind of extra fee or tax is going to get intense scrutiny in the Congress this year, no matter how popular the mission might be - and transportation funding remains high up on the wish list for both parties.

The last time the gasoline tax was raised was in 1993, as part of a budget deal struck by President Clinton with the Congress.

18 years later, every $1 of gasoline tax revenue certainly buys a lot less in terms of construction, one reason why there are advocates in Congress of a gas tax hike.

So, if there isn't going to be a gas tax increase - where does the White House get the money to fund $328 billion in dedicated money for transportation projects?

Your guess is as good as mine right now.