Economic policy is subject to fads and fashions. The most recent economic-policy fad is public infrastructure. Its advocates include progressives on the left and populists on the right, and it is an undefined hallmark of President Trump’s economic program. They all tell us to take the chains off fiscal austerity and spend — spend a lot — on public works. They allege that this elixir will cure many, if not all, of our economic ills. Let’s take a look at their arguments and evidence.