On a big spring-break night, Inertia Tours would take about 1,500 college students across the Texas border into Mexico for a pub crawl in Matamoros.

Chad Hart, president of the Austin, Texas-based tour agency, said the crawl was always a popular draw for students to sign up for his company's South Padre Island vacation package.

Even so, Inertia canceled all border crossings this year. While a spike in violent crime in Mexico played a part, the main reason for ending the trips was the introduction of new federal rules requiring anyone returning to the United States from Mexico or any other foreign country to show a passport or a birth certificate.

"Above everything else, it's the change in law from requiring a valid U.S. state-issued ID to a certified copy of a birth certificate or passport to cross the border on foot," Hart said. "We didn't feel our travelers would come to South Padre with that documentation in hand."

In the past, a U.S. driver's license or even a verbal declaration of citizenship was sufficient for those casually crossing the borders.

But since Jan. 31, when the State Department's new Western Hemisphere Travel Initiative took effect, declarations and driver's licenses are no longer enough to prove citizenship for those crossing into the United States from Mexico or Canada. Travelers now are required to bring a passport, a certified copy of a birth certificate or naturalization papers to enter.

Until June 2009, travelers without passports will get information about the new rules and allowed to enter. Once the grace period ends, the leniency does, too.

"We're not going to deny entry to U.S. citizens," said Vincent Bond, public information officer for U.S. Customs and Border Protection in San Diego.

Although they may have upended some spring-break plans, the new requirements have been implemented nearly seamlessly in the first six weeks of the "educational phase," said Ron Smith, public information officer for U.S. Customs and Border Protection in Detroit.

In Detroit, which shares a border with Canada, the word about the new rule apparently has gotten out. During one 24-hour period this month, about 99 percent of those crossing into the United States from Windsor, Ontario, had the proper papers, Smith said.

Any decline in traffic there seems to have been caused more by the decline in the value of the U.S. dollar versus the Canadian dollar than the need for a passport, Smith said.

Border posts in Texas and California have also seen high compliance. In San Diego, Bond said, 90 percent or more of the crossers have presented the required ID.

Even so, the document hassle is still keeping some students home.

"A good friend of mine is flying down to San Diego from Wisconsin for his spring break, and I'd love to take him down to Mexico. However, he doesn't have a passport," said Matthew Tong, a University of California-San Diego student.