There are 21 NFL franchises that play in stadiums that have been built or renovated over the past 25 years using tax-free public borrowing.

The building boom was intended for teams to increase revenues from luxury seating, naming rights and concessions as those items are exempt from revenue sharing.

The venues have helped double the value of sports franchises since 2000, according to W.R. Hambrecht & Co., a financial services firm.

Congress tried in 1986 to bar cities and states from building stadiums with tax breaks originally set up to help local governments cut their borrowing costs for building roads, sewers and schools.

During the past decade, studies by Grant Long; Robert Baade of Lake Forest College near Chicago; Victor Matheson, an economist at College of the Holy Cross in Worcester, Massachusetts; and others have found that stadiums are poor municipal investments.

The Giants and Jets built their stadium together with private financing.

Bob Packwood, the former Senate Finance chairman, calls MetLife Stadium “the most expensive stadium built without public money” and said the private financing shows “it can be done.”