The roughly $1 trillion infrastructure package just passed by Congress will modestly help the economy in the short run while priming the country for slightly stronger growth in coming decades, economists say.

The law includes roughly $550 billion in additional spending on top of what the government already planned to spend over the next decade on infrastructure. About a fifth of the new spending is earmarked for building and repairing roads. Smaller components will be used to improve broadband networks, subway systems, passenger...

The roughly $1 trillion infrastructure package just passed by Congress will modestly help the economy in the short run while priming the country for slightly stronger growth in coming decades, economists say.

The law includes roughly $550 billion in additional spending on top of what the government already planned to spend over the next decade on infrastructure. About a fifth of the new spending is earmarked for building and repairing roads. Smaller components will be used to improve broadband networks, subway systems, passenger railways, airports, ports and power facilities.

Economists generally agree that federal spending on infrastructure helps the country become more efficient because it affords workers greater mobility and improves transportation of goods. As a result, U.S. productivity rises, enabling the economy to grow faster and improving Americans’ living standards.

“The infrastructure bill should increase labor productivity in the long term in the exact ways the American economy needs,” said Adie Tomer, a transportation expert at the Brookings Institution, a center-left think tank.

Economist Mark Zandi of Moody’s Analytics estimates that by 2031, the law will have improved labor-productivity growth by 0.03 percentage point a year. As a result, the economy could sustainably grow at 1.93% a year, instead of the Congressional Budget Office’s estimate of 1.9%, he said.

Those benefits often take years to materialize, partly because of planning and procedural hurdles. Analysts expect the money to start flowing in the second half of 2022, though the biggest chunks will be spent in 2024 and beyond.

Moreover, the new spending approved by the law is small compared with the economy, representing about half a percentage point of U.S. gross domestic product over the next decade, according to University of Pennsylvania economist Kent Smetters.

Moody’s estimates the law will raise U.S. output by 0.17% or $34 billion by year-end and by 0.5% by year-end 2026, with the effect then fading as spending winds down. By late 2031, Moody’s expects GDP to be only $39 billion or 0.12% larger than if the bill hadn’t been passed. Average annual growth would be 0.02 percentage point faster through 2031.

“I’d say it’s a slam-dunk positive—a small positive because the numbers are small,” Mr. Zandi said, contrasting it with the infrastructure spending in the period after World War II, when the federal government built the interstate highway system.

A hiring sign in New York City is indicative of U.S. economic growth despite concerns about the Delta variant of the coronavirus.

Photo: Spencer Platt/Getty Images

Moody’s projects the law will create a cumulative 566,000 jobs by 2026, raising total U.S. employment by a third of a percentage point, a modest boost on top of the 6.4 million increase it already projected. Many of the jobs will be temporary, so by 2031 the net boost to employment will have shrunk back to 75,000 jobs, Moody’s projects.

Economists say the law isn’t likely to increase inflationary pressures. Most pandemic-related bottlenecks will be sorted out by the time spending begins, and activity will be spread over a decade or more, long enough to absorb the additional spending without straining the economy’s capacity.

In a few years, the economy is expected to be at or near full employment. Mr. Smetters believes that instead of creating many new jobs, the plan will largely pull workers from other construction projects or industries. “It’s one thing if you’re in this really deep recession where you have a lot of people who really want to come back to work and can’t find jobs,” Mr. Smetters said. “That actually is not the current situation. People who aren’t coming back—it’s often they don’t want to come back.”

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He said another reason the economic effects will be muted is that most of the law will be paid for by government borrowing, nudging up interest rates and crowding out investment in projects with quicker turnaround times.

“We basically find the increase in productivity from infrastructure at this scale, when you combine it with new financing methods, it’s pretty much a wash,” Mr. Smetters said.

The Congressional Budget Office estimated in 2016 that for every dollar of capital investment by the federal government, annual private-sector output rises about 5 cents.

There is a risk, however, that the government will pay for projects that might not have a big payoff, said Marc Scribner,

a transportation expert at the Reason Foundation, a libertarian think tank. For example, in a broad economic-recovery package in 2009, the Obama administration and Congress included funding for high-speed rail projects around the U.S., including one designed to run most of the length of California. That project has been undermined by higher-than-expected costs and delays, and has been scaled back.

“This is a uniquely bad time to be placing bets on those types of things,” Mr. Scribner said. “We don’t know if mass transit is ever going to recover pre-pandemic travel volumes. We don’t know if morning rush is going to return. There are a lot of unknowns.”

Write to Josh Mitchell at joshua.mitchell@wsj.com