As 15 million people in the United States head to airports this holiday season — slightly fewer than last year — some travelers will find welcome changes to security screening procedures.

The Transportation Security Administration has expanded its PreCheck trusted traveler program to 35 airports, allowing members who have been deemed low risk to keep shoes, jackets and belts on. Children 12 and under and passengers 75 and older also receive expedited screening at any checkpoint; pilots, flight attendants, members of the military and people with top-secret security clearances qualify at some airports.

John S. Pistole, administrator of the TSA, said in an interview that the agency’s priority this year had been to move toward a risk-based approach to screening, recognizing that the vast majority of travelers are not potential terrorists.

"When the agency was set up, it was focused almost exclusively on the security mission and not as much on the passenger experience," Pistole said. "It became an adversarial relationship, so what we’re trying to do through all these initiatives is change that paradigm and make this a partnership."

Even with these changes, the agency is under pressure to refine its strategy further in 2013. Its operations have been scrutinized by independent researchers, travel industry committees and government officials charged with oversight, and their ideas for reform are coalescing around a consistent theme.

"I use this acronym SEE," said Stephen M. Lord, director of homeland security and justice issues for the Government Accountability Office, the investigative arm of Congress that has issued many lengthy reports about the TSA. "They need to make the process more selective, more effective and more efficient."

More selective means "shrinking the haystack and really focusing on the dangerous people," Lord said. While PreCheck and other expedited screening options are a step in that direction, only 7 percent of passengers qualify for these programs, a number Pistole said the agency was working to expand.

One option being tested is to use dogs that sniff for explosives in tandem with behavior detection officers to divert more people to PreCheck lanes. That process was used at Indianapolis International Airport the day before Thanksgiving, allowing nearly a third of passengers to have expedited screening.

Regarding effectiveness, Lord said, the TSA needs to improve the technology it relies on — primarily expensive body scanners that may not detect explosives reliably. Although the test results he has seen are classified, lawmakers briefed on them have called them disappointing. The agency has acknowledged problems with the slow pace of its X-ray body scanners, and removed many of the machines from larger airports in favor of millimeter wave scanners.

Finally, becoming more efficient means addressing the time passengers spend waiting to get through security — a factor that the TSA does not measure consistently or make public, but one that is of growing concern to the travel industry as passenger volume has stagnated.

"You can’t focus exclusively on security," Lord said. "You’ve got to be mindful of customer service."

It is difficult to assess how travelers truly feel about airport security, especially at a time when full planes, higher ticket prices and a shaky economy are also affecting how often people fly.

Still, a GAO report released in November found that the TSA did a poor job of tracking and handling customer complaints, a process Lord described as allowing officials to "essentially investigate themselves." A separate GAO study called for better performance assessments, particularly as a way to gauge whether airports that use private companies to handle screening, under federal supervision, score higher than airports that use TSA employees.

Pistole said the agency was working to improve its relationship with passengers. It is training officers and supervisors to defuse escalating situations at checkpoints, appointing customer service representatives at some airports and creating a process to channel complaints that are not resolved locally to an ombudsman.

The TSA has also resurrected the Aviation Security Advisory Committee that was disbanded years ago. A subcommittee of that group is working on customer service recommendations that the TSA "won’t love," said its chairman, Geoff Freeman, chief operating officer of the U.S. Travel Association.

Another member of the subcommittee, Charlie Leocha, director of the Consumer Travel Alliance, shared his own ideas for changes recently in testimony before the House Subcommittee on Aviation. Among his suggestions were revising the forbidden items list to focus on explosives, not pointy objects that even maximum-security prisons fail to intercept, and going back to metal detectors for primary screening of passengers.

"We have two giant differences between now and the situation we had back in 9/11," Leocha said. "We’ve got cockpits which are hardened and we’ve got watch lists that now screen every passenger. Today, there’s less of a reason for invasive searches at the airport, and yet TSA continues to expand their operation."

Some of the agency’s efforts to expand have met with resistance, however. A program to buy scanners to verify the authenticity of passengers’ boarding passes and IDs was postponed after members of Congress questioned the program’s $100 million expense. And both the GAO and the Homeland Security Department’s inspector general are investigating a behavior detection program being tested at Logan Airport in Boston, in response to accusations of racial profiling and skepticism about the underlying science.

Pistole said he recognized that some security measures were "not perfect," but he defended the effectiveness of the agency’s multilayered strategy.

"Reasonable people can disagree as to what’s the precise solution to the current threat," he said. "The whole idea is to take these good security protocols and layer them in a way that gives us the best opportunity to detect a putative terrorist."

While that approach has become a TSA mantra, a report by researchers at the RAND Corp. challenged the concept, questioning whether the benefits of the country’s current system of aviation security outweigh the costs — some of which are not calculated in the agency’s $8 billion annual budget.

"As you add more layers of security, you gradually get increasing amounts of irritation to the public that has to go through them," said Brian A. Jackson, director of RAND’s safety and justice program.

Jackson said that these layers could interfere with one another. Employees may pay less attention to their duties because they know the system is redundant, he said. Passengers annoyed by pat-downs display agitation that screeners might mistake for terrorist intent, and equipment that generates false alarms can distract employees from real threats.

"More layers is not necessarily better," he said. "It is usually more expensive."

© 2012 The New York Times Company