You often find people talking about our economic difficulties as if they were complicated and mysterious, with no obvious solution. As the economist Dean Baker recently pointed out, nothing could be further from the truth. The basic story of what went wrong is, in fact, almost absurdly simple: We had an immense housing bubble, and, when the bubble burst, it left a huge hole in spending. Everything else is footnotes.

And the appropriate policy response was simple, too: Fill that hole in demand. In particular, the aftermath of the bursting bubble was (and still is) a very good time to invest in infrastructure. In prosperous times, public spending on roads, bridges and so on competes with the private sector for resources. Since 2008, however, our economy has been awash in unemployed workers (especially construction workers) and capital with no place to go (which is why government borrowing costs are at historic lows). Putting those idle resources to work building useful stuff should have been a no-brainer.

But what actually happened was exactly the opposite: an unprecedented plunge in infrastructure spending. Adjusted for inflation and population growth, public expenditures on construction have fallen more than 20 percent since early 2008. In policy terms, this represents an almost surreally awful wrong turn; we’ve managed to weaken the economy in the short run even as we undermine its prospects for the long run. Well played!

And it’s about to get even worse. The federal highway trust fund, which pays for a large part of American road construction and maintenance, is almost exhausted. Unless Congress agrees to top up the fund somehow, road work all across the country will have to be scaled back just a few weeks from now. If this were to happen, it would quickly cost us hundreds of thousands of jobs, which might derail the employment recovery that finally seems to be gaining steam. And it would also reduce long-run economic potential.