ATLANTA - Starting today, Americans flying home from Mexico, Canada and the Caribbean must show their passports to get back into the country.

Only about a quarter of U.S. citizens hold valid passports. Most Americans have been accustomed to traveling to neighboring countries with just a driver's license or birth certificate, which were long sufficient to get through airport customs on the trip home.

But Congress adopted the new passport regulations in 2004 to secure the borders against terrorists.

For now, the rules affect only air travelers. Land and sea travelers will not have to show passports until at least January 2008. Air travelers who cannot produce a passport will be interviewed by customs agents, who will decide whether to let them into the country.

Travelers at Hartsfield-Jackson Atlanta International Airport and other airports yesterday said they had no complaints about the requirement.

"I'd rather be going through a security check than possibly being blown out of the air because of lack of security measures," said John Golden of Columbus, Ga., who was headed to Cancun, Mexico.

As of today, Canadian, Mexican and Bermudan air travelers, as well as U.S. citizens flying home from those countries or the Caribbean, must display their passports to enter the United States.

The only valid substitutes for a passport will be a NEXUS Air card, used by some American and Canadian frequent fliers; identification as a U.S. Coast Guard merchant mariner; and the green card carried by legal permanent residents. Active members of the U.S. military are exempt.

AAA spokeswoman Teresa Hildebrand said: "We're not seeing a panic from travelers, because we've been pretty diligent in telling them for over a year that they need a passport. It's written on any piece of paper we have going out."

Internet travel sites such as Expedia.com have posted warnings "in bold with exclamation point," said company spokeswoman Erin Krause, adding that agents followed up with e-mail to customers traveling to the affected destinations.

Canadian consulate officials in the United States reported fielding hundreds of calls a day, mostly from the almost 100,000 Canadian "snowbirds" who spend the winter in Florida or Arizona and feared they might be unable to fly back without passports, said Lawrence Barker, president of the Canadian Snowbird Association. (They can, Barker said.)

The State Department issued a record 12.1 million passports in 2006 and expects to issue 16 million more this year to meet the increased demand.

Mexican consulates are seeing a demand for passports three times higher than usual in some offices. In San Francisco yesterday, the line of people applying for passports at the Mexican consulate stretched around the block.

Read more from the State Department about the new passport regulations via http://go.philly.com/passportsEndText