WASHINGTON, D.C. ? Holiday travelers who left before Christmas may have a different airport experience if they fly home after New Year’s Day.

Their checked bags likely will be searched for explosives, although the method — machine, human hands or dogs — will vary by airport. And at more than 40 airports, travelers with only carry-on bags no longer can go straight to the gate. They’ll have to make a detour to the ticket counter or a kiosk to get a boarding pass.

The changes are bound to create problems, said Michael Boyd, a Denver-based airline consultant. He offered this advice: Don’t check anything, and get there early.

“It could be total chaos,” he said.

Enhanced security at airports isn’t new for frequent air travelers. They know that they’ll have to show a government-issued ID several times before reaching the gate. Coats, and sometimes shoes, must be taken off and run through the same machines that check carry-on bags. Travelers may be randomly selected for a second, and even a third, search.

The new security is overseen by the Transportation Security Administration, created after the 9-11 attacks to protect travelers from terrorists. In the past year, the agency has hired more than 50,000 people — distinguished by their white shirts and yellow embroidered badges — to screen passengers and baggage at 424 commercial airports.

Now the TSA is in the midst of adding another layer of security: screening all checked bags for explosives. It’s an enormous undertaking — an estimated 1.5 billion bags get checked at U.S. airports every year.

Small airports can easily meet the requirement that all bags be screened because they can use labor-intensive methods such as searching by hand and using a wand that detects explosives residue on the outside of bags.

Larger airports need more efficient, SUV-sized bomb-detection machines. They’ve been in short supply, though, and it can take months for older airports to shore up floors to carry them, build power stations to run them and construct ramps, conveyor belts and guardrails to incorporate them in baggage-handling systems.

Congress originally stipulated that every bag be screened starting Wednesday. But last month lawmakers agreed to extend the Jan. 1 deadline after airport managers complained the TSA had waited until this summer to begin ordering, delivering and installing the bomb-detection machines — too late to meet the cut-off date.

‘A madhouse’

The TSA is working feverishly to get as many of the machines in place as possible by New Year’s Day, often in temporary locations such as lobbies.

“It’s a madhouse,” said Jerry Orr, aviation director at Charlotte-Douglas International Airport in North Carolina, where 16 big bomb-detection machines are being delivered. “I’m hoping that we won’t delay any airplanes.”

TSA spokesman Robert Johnson said the agency was pleased by the progress, though he acknowledged the situation at some airports was hectic.

Charlotte is temporarily setting up its machines to stand alone, which means TSA screeners will have to put bags into machines by hand, a cumbersome process. It will take months for the airport to incorporate the bomb-screening machines with the automated baggage system.

There’s also the problem of “false positives” — a machine recording an explosive or weapon when none exists. Some machines register such readings nearly one-third of the time. In those cases, a TSA screener must open the bag and search it by hand.

“The machines are simply too unreliable to efficiently process the number of people and bags going through the system,” said Kenneth Quinn, former general counsel for the Federal Aviation Administration.

Johnson says officials are confident that by New Year’s Day every bag will be checked by a machine, by hand, by wands that detect explosives, by bomb-sniffing dogs — or matched to a passenger prior to takeoff.

Not foolproof

David Stempler, president of the Air Travelers Assn., said matching bags to passengers — a system in place since February — is easier for airports but not as effective as physically inspecting a suitcase for bombs, though it is easier to do. “It assumes you have a nonsuicidal bomber,” he said.

Some airports will manage to screen all baggage Wednesday. Boston’s Logan International Airport began construction this summer and spent $146 million to build an automated screening system — including 2.8 miles of new conveyor belts — behind the ticket counter. Logan officials say air travelers won’t notice any difference from the old system that didn’t screen bags.

But at Ronald Reagan Washington National Airport, travelers must carry their checked bags from the ticket counter to the bomb-detection station in the lobby, where a TSA screener will put it through a machine. The passenger then walks around to the other side where the machine spits out the baggage after scanning it. A screener then hand searches bags that generate false positives. Then the passenger checks the bag.

That can involve standing in three lines. One day last week, with only eight people in line, it took 15 minutes to complete the process.

“I do find it confusing,” said Jolie Palensky, 33, a statistician from Alexandria, Va., who was traveling from Washington to Minnesota.

Passengers may be confused, as well, by differing policies at different airports. But some don’t mind the hassles.

“I fully support and like to work in a safe and secure environment,” said Jack Koller, a 29-year-old government worker from Washington who waited 15 minutes for a screener to search his baggage at Reagan National. “I’ll make the necessary sacrifices to make it happen.”