IN THE suburbs of Des Moines, Iowa, yellow diggers slowly knock chunks out of a tired-looking bridge. Beneath lies Interstate 80, one of America’s economic arteries, which traces the route of the Lincoln highway—the first road to cross America—from New York to San Francisco. The bridge impedes traffic both on the highway—it is too low—and over it—it is too narrow. Its replacement will be a third wider, making room for cycle lanes and more traffic, and six inches higher, allowing bigger lorries to pass beneath. The diggers are out across Iowa, which embarked on a five-year programme to upgrade its bridges and highways in January.

Elsewhere, however, the diggers sit idle. America’s government invests much too little in infrastructure. A study in 2013 by the American Society of Civil Engineers claimed that additional spending of $1.6 trillion, in 2010 dollars, is needed by 2020 to bring the quality of the country’s infrastructure up from “poor” to “good”. Roads are a particular problem: over the past decade, America’s roads have fallen from seventh to fourteenth in the World Economic Forum’s rankings of the quality of infrastructure. Part of the problem is that the federal tax on petrol (gasoline), which provides most of the funding for federal spending on roads, has been stuck at 18.4 cents per gallon since 1993. Over that period the price of construction materials and the wages of construction workers have both risen by more than 75%.

Happily, the federal government is responsible only for about a quarter of spending on highways. States are keener to raise local taxes on petrol: 12 did so in the year to January. Iowa’s rise was the sharpest. Since last year Iowans have paid 31 cents per gallon to the state, up from 21 cents per gallon—a rate set in 1989. Most states tax by the gallon and so have also benefited from the falling oil price, which has boosted petrol sales. Nationally, they are up by 3% on a year ago.

Since 2013 state and local governments’ net investment (ie, after depreciation) is up by about a quarter. In contrast, federal net investment, which was barely positive in 2013, is now negative, meaning that Congress is not even spending enough to maintain what it has. In the aftermath of the financial crisis, states cut investment to balance their budgets, while the federal government upped its spending as part of a stimulus program. Now the reverse is true: state and local investment adds to growth, while falling federal investment is a drag on it (see chart).