Atlanta ? When Antonio Hernandez Jr. planned a trip to Mexico with his parents, he was sure to get the passports two weeks ahead of time.

Hernandez wanted to be certain the three were ready to comply with new travel rules that take effect today requiring Americans flying to neighboring countries to show a passport to get back into the United States.

“This is a way to protect ourselves and give many who travel another form of ID,” Hernandez said Monday at Hartsfield-Jackson Atlanta International Airport. Travelers had to pack passports for trips to Mexico, Canada and the Caribbean.

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

The new regulations requiring passports were adopted by Congress in 2004 to secure the borders against terrorists.

Travelers in Atlanta and other airports 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,” John Golden of Columbus, Ga., who was headed to Cancun, Mexico.

Starting 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.

People line up outside a passport office in downtown Ottawa, Ontario, on Friday. Starting today, air travelers who are citizens of Canada, Mexico and Bermuda, as well as U.S. citizens returning home, must with few exceptions display 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.

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.

“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,” said AAA spokeswoman Teresa Hildebrand.

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-mails to customers traveling to the affected destinations.