WASHINGTON — Transportation Secretary Elaine Chao last week raised the prospect that needed infrastructure improvements may be funded to some extent by imposing tolls on more of the nation’s roads and bridges.

It was unclear whether she intends to expand tolling on the U.S. interstate system.

The concept of granting massive tax credits to lure private infrastructure investment was the centerpiece of President Trump’s campaign promise to raise $1 trillion for roads and bridges.

“To launch our national rebuilding, I will be asking Congress to approve legislation that produces a $1 trillion investment in infrastructure of the United States,” Trump said in Tuesday’s address to a join session of Congress.

He said the money would be raised “through both public and private capital, creating millions of new jobs.”

Beyond repeated calls to draw private investment, however, the White House had not reiterated its campaign promise to raise at least of portion of the money through new tolls. Even if investors were attracted by the 82 percent tax credit Trump hopes to offer, they would expect an additional return on their money, and when it comes to roads and bridges, that cash would come from imposing new tolls.

“The federal government cannot assume the cost for all of it,” Chao told Fox News after Trump’s address, reiterating a point she made during her Senate confirmation hearing. She told Fox host Sean Hannity that “new and innovative ways” were necessary to find funding.

She said the president had “exciting and novel ideas about how to finance” what independent authorities estimate is the need for $3.7 trillion to meet infrastructure needs by 2020.

“Public private partnerships are a very important part of a new way of financing our roads and bridges,” she said.

Asked whether that would require new tolls, Chao responded “that is certainly one example of how that would work.”

“I have to say that there are some people who may not support toll roads,” Chao said, “but we have to take a look at all of these financing mechanisms, because once again, the needs of our infrastructure are so great that the federal government cannot and should not be the only source of funding to repair our bridges, our roads and our energy grids.”

The challenge in luring private investment to build roads and bridges is that vast portions of the country outside of high-traffic volume urban hubs would not produce the toll revenue desired by investors.

There has been strong bipartisan opposition in Congress to imposing tolls on the interstates, beyond those like the New Jersey and Pennsylvania turnpikes that were grandfathered in when they became part of the network.

Chao has been largely unavailable to reporters, and she was whisked away after addressing a convention of transportation officials this past week.

The Congressional Budget Office said in 2015 that just 26 private-investment projects were completed or underway.

Overreliance on private funding has been called into question by Sen. John Barrasso, R-Wyo.

“Funding solutions that involve public-private partnerships, as have been discussed by administration officials, may be innovative solutions for crumbling inner cities, but do not work for rural areas,” Barrasso said last month.