Stimulating private investment in infrastructure projects, as President-elect Donald Trump has proposed, can cover only a fraction of the costs of solving America’s transportation problems, Transportation Secretary Anthony Foxx warned as he prepares to leave office. Financing schemes called public-private partnerships, which involve both government dollars and private capital, are useful and can address about to 10 percent to 20 percent of America’s transportation “deficit,” Fox said in an interview this week with The Associated Press.