Hauling the mail has long been an important source of extra revenue for airlines. But that revenue has been sliced in half since Sept. 11, 2001, Cox News Service reports.

New security restrictions prohibit airliners from carrying U.S. mail packages weighing more than 1 pound until a security screening method is in place. Trucking companies are helping to pick up the slack.

Since the new restrictions took effect, the U.S. Postal Service has sprouted truck hubs in Atlanta, Dallas, Los Angeles and San Francisco.

Ailing airlines are lobbying to end the restriction. A U.S. House bill includes a provision that would toss out the restriction, but the bill remains pending, and it is unclear when the provision would take effect.

In the meantime, big airliners are losing about $500 million per year, according to the news service. It is unclear whether, even when the Transportation Security Administration settles on inspection procedures for mail, airlines will get all the business back.

"That is volume that has left the commercial air industry and will never come back," says Paul Vogel, Postal Service vice president. "Trucks are always less expensive."