Last night’s address to Congress by President Trump was devoid of detail on infrastructure investment. But in justifying his desire to harness $1 trillion of public and private funds for “new roads, bridges, tunnels, airports and railways,” the president used two lines of bad economic reasoning that are sadly prevalent in public debate on this issue.

First was to invoke the building of the interstate highway system. “The time has come,” Trump declared, “for a new program of national rebuilding.” The implication: the interstate highway system was good for the economy, so we should invest more in roads today - a common rhetorical technique, but one which confuses average with marginal.

Previous economic research has indeed found that the construction of the interstate highway system substantially boosted productivity for industries associated with road use. But the same research finds those benefits to be largely one-offs, meaning this analysis does nothing to inform us about new decisions. In fact, more recent work has found that too many new highways have been built between 1983 and 2003, and that marginal extensions to the highway system tend not to increase social welfare, because the cost savings of reducing travel times are small relative to incomes and prices.

In other words, building a highway system can boost growth. Building a second highway system? Not so much. Rather than appealing to grand projects based on historical experience, all new government projects should stand up on their own merits – ideally having high benefit to cost ratios and being things that would not be undertaken by the private sector.

The second mistake was to highlight “creating millions of new jobs” as an aim or positive of any infrastructure spending. When the government is investing to build something, it should aim to do so most efficiently. “Jobs” in this sense are a cost, not a benefit, and ones “created” only come through the diversion of resources and opportunities in other parts of the economy.

Upon visiting an Asian country in the 1960s, Milton Friedman is frequently quoted as reacting to the absence of heavy machinery in a canal build by asking why the project was being undertaken by men with shovels. Upon being told it was a “jobs program,” he is said to have remarked: “Oh, I see. I thought you were trying to build a canal. If you really want to create jobs, then by all means give these men spoons, not shovels.”

If one is concerned with improving the economic growth potential of the economy, then you would base both the selection of projects and the means of undertaking them according to that objective. Sadly, when governments are involved, other ambitions (be it stimulating particular regions, appeasing certain interests, obtaining political prestige, or facilitating observable jobs) tend to interfere with the stated aim. The constant talk of the benefits of wise, productive investment is an ambition, rather than something we should expect.