It won't be quite as easy to travel by car to Mexico or Canada beginning June 1.
On that date, all adult U.S. citizens will need a passport to cross the border, U.S. officials say.
Children 16 years old and younger will still be able to use their U.S. birth certificates as proof of citizenship.
Officials say getting Americans to use passports will improve border security.
The requirement is part of a 2004 law passed by Congress called the Western Hemisphere Travel Initiative.
"We are going to know who is coming into our country and it's going to be faster," said Angelica De Cima, a spokeswoman for the U.S. Customs and Border Protection in San Diego.
U.S. citizens now can cross the border using a birth certificate coupled with a government-issued identity card that has a photo, such as a driver's license.
Last year, the federal government eliminated a long-standing practice of allowing U.S. citizens to declare their citizenship verbally.
Starting in January 2008, the law required Americans visiting Canada, Mexico, Central and South America, the Caribbean and Bermuda by sea or air to carry passports.
The new regulation affects Americans crossing the border by land.
Only about 28 percent of the U.S. population, or about 85 million people, had a passport as of February 2008, according to State Department statistics.
But the number of applications processed by the department has increased in recent years, from 12 million in 2006 to 18 million in 2007.
But there are holdouts, among them Maggie Gonzalez of Escondido.
She still doesn't have a passport.
"It's going to be a big problem for us," said Gonzalez, who runs a small business helping Spanish-speakers translate and fill out documents. "There are many people (who) don't have the money to pay for the passports."
Passports cost $100 per adult and $85 per child under 16 years old, plus the cost of passport photos.
It takes the U.S. State Department an average of four to six weeks to process an application.
Expedited passports cost an additional $60 and take an average of two to three weeks, according to the department's Web site.
U.S. Customs and Border Protection also offers a passport card, which is cheaper -- $45 per adult and $35 per child -- but it is only good for land travel, not air and sea travel abroad.
At least one prominent Democrat in Congress, Rep. Louise Slaughter of New York, and the chairwoman of the House rules committee, has suggested postponing the new rules for one year.
Rep. Brian Bilbray, R-Solana Beach, who is chairman of the congressional Immigration Reform Caucus, said he does not favor extending the deadline.
The caucus is a predominantly Republican group of lawmakers that favors stricter immigration enforcement measures.
"The passport and other Western Hemisphere Travel Initiative documents are the most secure forms of identification for border crossings, land or otherwise," said Fritz Chaleff, a spokesman for the congressman.
Obama administration officials also say they oppose delaying the rules.
Department of Homeland Security Secretary Janet Napolitano told business leaders at a conference in Washington, D.C, on Tuesday that travelers who plan to visit Mexico should get a passport.
"We're in the process of conducting as aggressive outreach as possible, recognizing that whenever there is a deadline, people assume that it's not really a deadline," she said. "And, quite frankly, procrastination is a very human trait. Nonetheless, the deadline is real."
Napolitano said that 80 percent of the people who regularly cross borders by car have passports or other documents that comply with the new regulation, such as frequent-traveler cards known as Nexus, Sentri and Fast.
The Western Hemisphere Travel Initiative was adopted by Congress as a result of the recommendations issued by the 9/11 Commission, which investigated the 2001 terror attacks.
It was designed to "get control" of the borders by verifying the citizenship and identity of everyone entering the U.S. by land, sea or air from Canada, Mexico and the Caribbean.
U.S. military cards will be accepted only when traveling on official orders.
Otherwise, military personnel must carry a passport just like everyone else.
De Cima said Americans will not be denied entry if they do not carry the documents, but they may be delayed while authorities verify the individual's citizenship.
Contact staff writer Edward Sifuentes at (760) 740-3511 or esifuentes@nctimes.com.
For more information, visit www.travel.state.gov.
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