Shopping-mall owners have struggled this year with a darkening economy, slowing consumer spending and store closings by retailers. But they face another problem that may persist long after the economy bounces back: a decade of overbuilding.

Developers have built one billion square feet of retail space in the 54 largest U.S. markets since the start of 2000, 25 percent more than what they built during the same period of the 1990s, according to Property & Portfolio Research Inc. of Boston. U.S. retail space now amounts to 38 square feet for every person in those 54 markets, up from 29 square feet in 1983, the firm says.

Consider a six-mile stretch of highway north of Dallas, where three developers are racing to finish four huge shopping centers with a combined three million square feet of space. Not only will they compete with each other, but there are three existing malls within a 10-mile radius.

"There just aren't enough tenants to go around for three projects," concedes Gar Herring, president of shopping center developer MGHerring Group of Dallas, which is building the largest of the centers.

Similar scenes are playing out across the country. DeBartolo Development indefinitely postponed construction of 700,000 square feet of retail space in Mesa, Ariz., due to weak demand. Green Street Advisors, a real-estate research firm, says 13 strip shopping centers under development have been canceled this year and 90 others have been delayed by the seven shopping-center developers it monitors.

Of course, retail landlords struggle and store vacancies rise in every economic downturn. But this time, experts say, the overbuilding means that high occupancy rates at malls and strip centers may not return for years.

For retailers, the glut can have an upside: cheaper rents, shorter lease terms and fatter allowances from landlords for outfitting stores. This year, the rents in new lease signings are 10.4 percent lower on average than the asking price, down from the 9.3 percent discount of two years ago, says market researcher Reis Inc. of New York.

Shopping-center owners with a hefty focus on development, including Regency Centers Corp. of Jacksonville, Fla., and Weingarten Realty Investors of Houston, are compensating for the construction slowdown by trying to raise rents and sell older centers. Others, such as Kimco Realty Corp. of New Hyde Park, N.Y., have shifted much of their development abroad. Brian Smith, Regency's chief investment officer, said the real-estate investment trust has canceled some development projects, continued more cautiously with others and turned partly to upgrading existing centers.