~ A Homeland Security spokeswoman said people have known for months that the new rules were coming

DETROIT -- Some businesses and lawmakers are worried that new identification requirements going into effect Thursday at the borders with Canada and Mexico could create widespread confusion and delays and discourage some people from making the trip.

Americans and Canadians will be required to show some document, such as a passport or a birth certificate, beyond a driver's license to prove citizenship. In the past, some people entering the U.S. from Canada or Mexico simply had to declare their nationality.

Ron Smith, a spokesman for U.S. Customs and Border Protection in Detroit, which has the busiest northern border crossing, said the agency will provide a grace period for travelers without the extra ID and will hand out fliers explaining the changes.

"The first couple of days, weeks maybe, could add a few seconds to the inspection process. But once people become aware of these requirements, and we're getting the word out to them ... those minor delays should disappear," Smith said.

He said few people have crossed the northern border by just declaring their nationality since the 2001 terrorist attacks, because officers have always had the discretion to request documents.

Earlier this week, 19 senators said in a letter to the Department of Homeland Security that commerce will be stifled and lives disrupted if federal officials proceed with the new requirements. A Homeland Security spokeswoman said people have known for months that the new rules were coming.

Casino Windsor, across the Detroit River from the United States, is buying radio ads to inform its American customers that they can still visit without passports. Casino spokeswoman Holly Ward said any changes to border requirements risk driving away business.

But David Nolan, assistant manager at a sporting goods store in Bellingham, Wash., said he wasn't concerned about the changes.

"I don't see it affecting us that much because people are already carrying that ID," Nolan said, adding that 45 percent of the store's business is from Canadians, and "no one has actually talked about it."

At the San Diego border crossing, the nation's busiest, the new rules have fueled concerns about longer waiting times.

Oscar Franco of San Diego, who visits Tijuana, Mexico, several times a week to see family, worries that his 4- and 6-year-old children won't be able to cross on their birth certificates alone. (The new rules don't apply to people 18 years and younger.)

"I have two little ones, and I don't know if they need a photo ID, or any other kind of ID," said, Franco, 45, after waiting 90 minutes in his car.

An electronic sign at the crossing warns motorists of the requirements. Since December, inspectors have been giving reminder slips to U.S. citizens.

Last week, Mexican tourism officials in Tijuana introduced a "Get Your Passport" campaign that gives U.S. passport holders discounts at restaurants and shops.

Quincy Epperson of San Diego said he will stop going to Mexico until he replaces his birth certificate. He has an old photocopy but worries it won't pass muster with inspectors.

"I don't want to take the chance," said Epperson, 31, as he returned Tuesday from one of his weekly visits to Tijuana's sports betting parlors. If he wins his Super Bowl bet on the Giants, Epperson will send a friend to Tijuana collect his money.

People who regularly cross the border at Rouses Point, N.Y., appeared to know about the changes. While not everyone has the needed paperwork, they said they were ready to comply -- particularly Canadians who cross to save nearly $1 a gallon for gas.

"We shop out here, we buy gas out here. I buy all my groceries," said Don Bedard, 70, of St. Paul, Quebec, who was gassing up in Alburgh, Vt.

In Texas, El Paso City Councilman Steve Ortega thinks the rules will reduce American travel in far West Texas, at least at first.

"Over the long term, you will see the numbers start to rise up again once people get used to the new requirements," he said.

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Associated Press writers Alicia A. Caldwell in El Paso, Texas; Wilson Ring in Alburg, Vt.; and Elliott Spagat in San Diego; and Tim Klass in Seattle contributed to this report.