In June Mike Braun, the governor of Indiana, signed a law giving the state the authority to raise tolls on all of its existing interstate highways. No state has previously tried that, says Robert Poole, a transportation expert at the Reason Foundation, a libertarian think-tank.

For now, drivers pay to access just 6,300 miles of America’s 160,000 or so miles of highway. But, says Mr Poole, the share may be about to grow rapidly. Assuming the federal government goes along with it, Indiana’s experiment could lead to toll roads proliferating across the United States.

The reason why federal law banned states from collecting tolls on interstates was that drivers had already paid for the roads by means of a tax on petrol (or gasoline). In 1956, when the Federal-Aid Highway Act passed, for every gallon drivers bought, three cents (or $0.36 today) went into a “federal highway trust fund”. The fund raised enough money to build out the federal highway system, enabling the transformation of America into a hypermobile, motorised society.

The problem is that the model no longer works. Over the decades, the cost of maintaining roads and highways has risen, even as cars have become more fuel-efficient. And raising gas taxes, even just in line with inflation, is generally considered to be political suicide. The last time Congress did it was in 1993. The result is a giant deficit. In fiscal 2024, the federal government spent $27bn more on maintaining roads than it collected in tax. At the state and local levels, fuel taxes covered barely a quarter of road spending.

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By deterring some drivers, tolls also limit congestion. Some of the most successful projects have been “express” lanes added to highways where drivers pay variable rates to skip traffic. California State Route 91, a famously clogged highway in Los Angeles, has a toll lane with high peak prices and where the cost to enter it can adjust as often as every three minutes, according to traffic levels. These sorts of projects are expanding.