WASHINGTON (CNN) -- Headed to Canada, Mexico, Bermuda or the Caribbean? Here's a tip: Take your passport to avoid a headache coming home.
Americans and Canadians entering the United States will have to show a passport or other identification.
Beginning Monday, the slow squeeze of border security will tighten further when a rule takes effect requiring U.S. and Canadian citizens to present passports -- or a very limited number of other travel documents -- when entering the United States at land and sea borders.
Air travelers have been required to carry travel documents since January 2007. But the number of people crossing land borders is far greater, and the June 1 deadline has been viewed with some trepidation, especially in Canadian border communities where cross-border travel by citizens of both countries historically has required minimal documentation.
Compliance with the new rules was high early Monday, according to Kelly Ivahnenko, a spokeswoman for U.S. Customs and Border Protection in Washington.
"We're not seeing any backups or unusual wait times at this point," Ivahnenko said.
"We are taking a practical and flexible approach so if for whatever reason the traveler doesn't have the proper documents, we're taking a look at those on a case-by-case basis. No one is being denied entry."
The same rules will be in effect for U.S. residents attempting to re-enter the country at the busier southern border. Existing document requirements will remain in place for Mexican nationals wanting to enter the United States.
At the border crossing in Laredo, Texas, 20 to 25 percent of those crossing the border are U.S. citizens, according to Rick Pauza, a U.S. Customs and Border Protection spokesman in Laredo. Of those affected by the new rules, about 80 percent are complying with the documentation requirements, Pauza said.
"We haven't really seen any hiccups or anything," Pauza said. U.S. citizens who do not have the proper documentation are being provided with an information sheet telling them how to go about getting an approved form of identification. The majority of those who are non-compliant still are being processed and allowed entry at the primary inspection point.
The rule was originally scheduled to take effect more than a year ago, but Congress delayed it amid complaints that people weren't prepared and that trade, tourism and commerce with Canada -- the United States' biggest trading partner -- would be hurt.
Those misgivings have now partly subsided. The Department of Homeland Security says 80 to 90 percent of routine border crossers have the required documents, and new technology has been installed at major border crossings to speed up passenger and pedestrian identification. Further, the U.S. government is promising "soft" enforcement for the indefinite future.
But business and tourism groups along the U.S.-Canada border aren't entirely at ease, saying this and other border restrictions are hurting regional commerce.
Hardest hit, they say, are retail businesses and restaurants on both sides of the border as some shoppers opt to stay in their home countries rather than get travel documents. Tourist destinations also could be hit as families calculate the cost of getting passports for the entire brood.
In addition, trans-national convention businesses could be hurt, as could Detroit, Michigan's ailing auto business if the program misfires and traffic clogs international bridges and tunnels, the conduits for auto parts manufactured and assembled on both sides of the border.
"We're going to watch it. We're going to watch it very closely," said Roger Dow, president of the U.S. Travel Association.
He said the new rule -- known as the Western Hemisphere Travel Initiative -- went smoothly for air travel but faces bigger challenges at land borders.
"I guarantee you there's someone in Kansas or someone in Ottawa who's never even heard what WHTI is, who is used to going on their summer vacation with their driver's license," he said. "So we're watching it."
The year-long delay has given many border crossers time to get travel documents. About 92 million Americans now have passports, 585,000 have enrolled in three "trusted traveler" programs whose cards also serve as border documentation, and 130,000 have "enhanced driver's licenses," which are accepted as well.
That means about 30 percent of Americans have approved travel documents.
Parrin Beatty, president of the Canadian Chamber of Commerce, said the effect of border security measures has been cumulative.
"What you see is a series of new charges, new inspections, new delays, new forms that have to be filled out," Beatty said. "All of these taken together just make the border stickier, thicker and more costly. And that works very much against us."
But U.S. border officials say new electronic passport readers, purchased as part of the $350 million travel initiative, should expedite international traffic.
The top 39 land border crossings -- through which pass 96 percent of the traffic -- are now equipped with electronic proximity readers, said Jayson Ahern, acting commissioner of U.S. Customs and Border Protection. The radio-frequency identification cards enable border agents' computers to display information even before the car pulls up to the border booths.
That "will save time -- seconds," Ahern said. "When you take seconds and you multiply it times the millions of people that cross our borders, that adds up to real time and real efficiencies."
Ahern said the RFID chip and technology does not contain or transmit any personally identifiable information; it only sends a unique number. That ensures people's privacy, he said.
What will happen to U.S. citizens who attempt to re-enter the country without proper travel documents?
Ahern acknowledges that U.S. citizens can't and won't be kept out of the country.
"Certainly, a bona fide U.S. citizen can not be denied entry back into their own country. But we may need to spend additional time to verify that that person is a bona fide U.S. citizen," he said.
"Each case will stand on its own set of facts. It will be a determination by our officers, who have been trained and prepared for this. Keep in mind, they've been dealing with this for a number of years."
CNN's Marnie Hunter contributed to this report.
All About Travel and Tourism • Border Control and Customs