BUFFALO, N.Y. -- U.S. and Canadian citizens whose documents wouldn't measure up to new travel requirements taking effect next week are being given written instructions and a warning at U.S. ports of entry.

Beginning Jan. 31, a driver's license will not be enough to get past U.S. Customs and Border Protection agents. Instead, the Western Hemisphere Travel Initiative will require travelers 19 and older to produce a passport, NEXUS card or other secure ID at the Canadian or Mexican border. A driver's license coupled with a birth certificate or other ID will be OK until next June.

The rules are outlined on "tear sheets" being handed out at the Peace Bridge in Buffalo and other ports of entry.

"This tear sheet will be given to those travelers who are not in compliance and we will be asking them to get in compliance as quickly as possible," Robert Jacksta, the border agency's deputy assistant commissioner for field operations, said at the Buffalo crossing Thursday.

"We recognize this is a major cultural change for the traveling public," Jacksta said.

Until now, residents of border cities like Buffalo and Niagara Falls routinely traveled back and forth to Canada to shop or dine by showing a driver's license and telling an inspector where they were born and where they were going.

But "in our post 911 environment," Jacksta said, "oral declarations are simply not enough to ensure the security of this country."

The biggest effect of the change will be at the Canadian border since it applies to both Canadians and Americans. That is not sitting well with congressional critics representing Northern border states who fear disruptions in all-important trade and tourism partnerships.

"These ill-conceived and unnecessarily cumbersome travel requirements will have a deleterious effect on our nation's weakening economy and will adversely affect the economies of the border communities," U.S. Rep. Louise Slaughter, D-N.Y. and 33 colleagues said Thursday in a letter to Homeland Security Secretary Michael Chertoff.

The group asked Chertoff to hold off on any changes until the WHTI, adopted in 2004 in reaction to the Sept. 11 attacks, is fully implemented in June 2009. In June, Chertoff delayed the law's passport requirement for land and sea crossings until next summer. Congress then pushed it back to 2009.

"Your imminent actions not only violate the intent of the United States Congress, but threaten our nation's economic security by implementing a new, resource intensive system without ensuring a smooth transition," the group wrote.

The letter cited a Government Accountability Office report that found CBP was severely understaffed and would be unable to handle the influx of documents without significantly increasing wait times at crossings.

Chertoff said last week that longer lines at the border in the early days of the new policy are inevitable. On Thursday, Jacksta said agents will make some exceptions in the beginning to avoid sending everyone without the right paperwork for a potentially lengthy secondary inspection. Those travelers will be given the tear sheet and instructed on the new rules.

"A U.S. citizen will never be refused entry into the United States," Jacksta said.

Slaughter's concerns were echoed by the Binational Tourism Alliance, whose executive director, Arlene White, predicted "more delays at the border, more confusion and frustration by the traveling public and more job losses in the area."