President Trump’s infrastructure plan is turning out to be a mirage. He had talked about a $1 trillion, 10-year effort. But the White House now proposes allocating only $200 billion, which would come from cutting aid to states and localities and giving it to Wall Street investors as tax credits, which it hopes will attract $800 billion in investment for big projects that would turn a profit through tolls and user fees. As an opening act, for example, Mr. Trump is pushing privatization of the nation’s air traffic control system, which could jack up the price of air travel for passengers.

But most of the nation’s unmet infrastructure needs involve smaller projects to operate, maintain and upgrade — not only highways, but also water, sewer and other systems that are of no interest to private investors. In Ohio, where Mr. Trump went on Wednesday to deliver a campaign-style speech about his plan, more than 1,500 highway projects to be completed over four years have an average cost of only $9.2 million, according to research by the Center for American Progress. That’s far too little to attract huge investment funds that are the presumed recipients of the tax credits.

Since 1995, 14 of 36 privately financed highway projects across the nation have been completed, with mixed results, according to a 2015 Congressional Budget Office report. The C.B.O. found that private investments did not increase the amount spent or reduce costs — two supposed goals. It simply substituted for money that could otherwise have been raised through low-cost municipal bonds.

As to whether private financing resulted in more reliably completed and maintained projects, the C.B.O. found that it sometimes could be arranged more quickly than public financing, which allowed some projects to be completed sooner. But three of the projects went bankrupt and one required a public buyout of the private partners.