In his first State of the Union address, President Donald Trump called for a $1.5 trillion infrastructure initiative to upgrade America's aging transportation networks.

But the bridge from wishes to reality begins with finding the necessary funds, a topic he did not address in the Tuesday night speech.

Built for the most part between the 1950s and 1970s, US highways, bridges, railroads, airports and other infrastructure are showing their age. Even Trump said they were "crumbling."

A third of US bridges, or 226,837, and a third of highway overpasses, 17,726, are in need of repair, according to the American Road & Transportation Builders Association (ARTBA).

And the association says that is just ’ the tip of the iceberg."

The American Society of Civil Engineers first sounded the alarm about decaying infrastructure decades ago. In 1998, it gave US infrastructure a "D" grade and put the total cost of investing in modernization at $2 trillion over 10 years.

I n 2007, a bridge over the Mississippi River in Minneapolis, Minnesota collapsed killing 13 people, and federal

investigators found the structure was not designed to handle the increased use it was seeing as a major commuter route into the city.

Former President Barack Obama pushed for years for Congress to fund a rebuilding program to address the problems and create jobs but Republicans blocked the effort almost entirely.

In his address, Trump called on Congress to act but gave few details. He claimed that winning regulatory approvals to build a road could take as much as a decade, whereas New York's 102-story Empire State Building was completed in about a year.

"This is not a plan, this is a number," said lacob Kirkegaard, senior fellow at the Peterson Institute for International Economics.

He noted that the Trump administration has promised for months to release a detailed proposal.

So far, the White House says it favors a public-private partnership. Trump's budget proposal last year included $200 billion for infrastructure, in the hopes of generating a total of a $ 1 trillion from private investors as well as state and local governments.

"I think this is a completely unrealistic aspiration," Kirkegaard told AFP.

"An aspiration is not a plan."

Lawmakers seem unlikely to approve funding, especially given that massive tax cuts approved in December already have ballooned budget deficit forecasts.

Other, less politically palatable options could involve charging tolls on roads and bridges -- something many US states already do - or raising the federal gasoline tax, which has long been used for road projects but has not changed since 1993.

The choice of what to build and where also will have to break with the tradition of "pork barrel" spending which funds infrastructure projects to help political allies, eager to unveil a new road or bridge in their districts.

Different types of infrastructure projects have distinct funding models. Airports are handled by private investors, and the US Interstate Highway System was built by the federal government although its upkeep is mainly a responsibility for states, while county governments are responsible for smaller local roads.

The nearly 48,000-mile (77,249-kilometer) Interstate Highway System literally moves the US economy. It carries 75 percent of the nation’s heavy truck traffic,' according to ARTBA.