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.