Elaine Chao testifies before a Senate Commerce Science and Transportation Committee confirmation hearing on her nomination to be transportation secretary on Capitol Hill in Washington, U.S., January 11, 2017

As the Trump administration touts its as-yet-unannounced $1 trillion infrastructure program — most recently talked up by Transportation Secretary Elaine Chao this week — private equity firms are readying their pocketbooks.

They are now raising $30.5 billion in funding for 43 new funds targeting North American infrastructure, according to Preqin, an industry data service. That money will be in addition to $68 billion in "dry powder" that funds have on hand but have yet to invest.

The United States is still considered an "emerging market" for public-private partnerships, referred to colloquially as "P3s," according to PWC. States and municipalities have access to cheap funding, and investors are often reticent to have their money tied up in long-term projects. But the pipeline is growing for the "right kind of project with certain criteria," according to Kylee Anastasi, director of capital projects and infrastructure at PwC.

The criteria are strict but pretty straightforward: Private equity funds and companies want to see returns greater than 10 percent, and they want their money tied up in projects for fewer than 10 years, since they need to return that money to investors. They also seek projects that guarantee a revenue stream while their money is committed — which is one reason toll roads have been popular targets.

"The greatest challenge in the public-private partnership is there's only so many projects that easily lend themselves to the kind of toll revenue that pays back private investors," said Gene Sperling, a former economic adviser to President Barack Obama.

Sperling served during Obama's release of the 2009 Recovery and Reinvestment Act, an infrastructure program that sought to inject $800 billion into the U.S. economy during the financial crisis, but which some studies later suggested did not have a statistically significant effect on the economy.