A preponderance of evidence suggests that Keynes was right. But as the German experience illustrates, progress is possible without settling that question. The Germans are investing in infrastructure not to provide short-term economic stimulus, but because those investments promise high returns. Yet their undeniable side effect has been to bolster employment substantially in the short run.

Not all German public investments have met expectations. Berlin’s new consolidated airport, for example, has suffered multiple delays and cost overruns, and parts of the city’s recently constructed central rail station are to have major repairs. But private investment projects suffer occasional setbacks, too, and no one argues that businesses should stop investing on that account.

The Germans didn’t become bogged down in debate over stimulus policy, and they didn’t explicitly portray their infrastructure push as stimulus. But that didn’t hamper their strategy’s remarkable effectiveness at putting people to work. The unemployment rate in Germany, at 5.3 percent and falling, is now substantially lower than in the United States, where it ticked up to 7.6 percent last month. (By contrast, in March 2007, before the financial crisis, the rate in Germany was 9.2 percent, about five percentage points higher than in the United States.)

A prudent investment is one whose future returns exceed its costs — including interest cost if the money is borrowed. Opportunities meeting that standard abound in the infrastructure domain. According to the American Society of Civil Engineers, the nation has a backlog of some $3.6 trillion in overdue infrastructure maintenance. No one in Congress seriously proposes that we just abandon our crumbling roads and bridges, and everyone agrees that the repair cost will grow sharply the longer we wait.

The case for accelerated infrastructure investment becomes more compelling with our economy still in the doldrums. That’s because many of the needed workers and machines are now idle. If we wait, we’ll need to bid them away from other tasks. Also because of the sluggish economy, the materials required for the work are now relatively cheap. If we wait, they will become more expensive. And long-term interest rates for the money to pay for the work continue to hover near record lows. They, too, will be higher if we wait.