Here is a look at what has changed, and what some experts think about it.
The federal response
The Transportation Security Administration, created by Congress after Sept. 11, has taken over airport security.
The agency recently released a list of technology improvements, which have included:
Replacing all existing X-ray machines and walk-through metal detectors.
Buying extra hand-held metal detectors.
Redesigning checkpoints and installing explosive trace detectors.
Installing equipment to ensure that all checked baggage is screened for explosives; before Sept. 11, only 5 percent of checked baggage was screened.
Developing additional checkpoint and air cargo screening equipment.
In addition to making these upgrades, the TSA oversees such security measures as a federal air marshal program, bomb-sniffing dogs and self-defense training for flight crew members.
"We have layers of security to ensure the security of the traveling public and the nation's transportation system," Lara Uselding, a TSA spokeswoman, said in an e-mail. "Each one of these layers alone is capable of stopping a terrorist attack. In combination their security value is multiplied, creating a much stronger, (more) formidable system."
A local perspective
The TSA has done much to improve airport security here and elsewhere, said John Schalliol, executive director of South Bend Regional Airport.
"I guess my initial reaction would be that we are safer now because we have an inspection system in place, random in nature, that would foil any potential attack by wrong-doers," he said.
The TSA's contributions don't stop there, he said, adding that the agency also has a deterrent effect on would-be terrorists.
"If I were considering doing something," he said, "I would certainly have second thoughts because of what's been going on."
A critic's view
Meanwhile, Joe Schwieterman, a transportation expert at DePaul University in Chicago, gives a mixed review to the air security measures since Sept. 11.
For instance, if it were up to him to grade things, he'd give the TSA a B. The air marshal program would earn a D for taxpayer accountability but a B for overall performance, he said.
Growing evidence indicates that officials made a mistake by tossing out the idea of private security at airports, he said, adding, "We rolled the dice by creating a huge bureaucracy."
Although the TSA isn't doing "all that bad of a job," he said, the United States could have benefited more from a federally managed system that used only "blue chip" companies to handle airport security.
A self-described critic of the federal air marshal program, he said the arrangement "probably wouldn't pass a rigorous cost-benefit analysis." Although it has benefits, he noted that it focuses on hijacking, which is just one of the terrorist threats the United States now faces.
But Schwieterman said he finds some screening technology improvements impressive, and he lauded the Registered Traveler program. The program, which is being developed by the TSA and private companies, allows travelers who pass background checks to bypass the longer security lines that regular travelers use.
One version of the program is running in Orlando, where some 27,000 people have signed up, the New York Times reported recently. More than 20 other airports have expressed interest in the program, according to the TSA's Uselding.
But the local airport is not interested, Schalliol said. It doesn't have the space for a separate lane, doesn't face the sort of demand for such a program that a larger airport might and doesn't want to annoy customers who'd have to stand in longer lines while others zipped to the front, he said.
Schwieterman described the program as a "long-awaited improvement that should've happened sooner."
Changing and adapting
Five years after Sept. 11, the need to adapt to terrorists' changing tactics is as great as ever, Schwieterman said.
He cited the alleged plot that British officials foiled last month as a case in point, noting that those behind it apparently planned to use liquid explosives to blow up as many as 10 flights.
Schalliol agrees about the need for flexible thinking, and he sees the TSA's response to the alleged plot as an example of it. Afterward, the agency announced passengers were not allowed to take liquids or gels onto planes.
In this case, Schalliol said, the response matched the threat. He contrasted the episode with one that happened not long after Sept. 11, when there was a ban on parking vehicles within a certain distance of terminal buildings.