Today, the inconveniences - and indignities - are familiar to every air passenger. And train depots like 30th Street Station are patrolled by armed guards and police dogs, while even subway stops are subject to random VIPR ("Visible Intermodal Prevention and Response") sweeps, with federal officers swabbing chemicals on bags to test for explosives, beneath posters that implore commuters, "If you see something, say something."
An army of 50,000 transportation security officers has been deployed around the country, and since 9/11, the Transportation Security Administration has spent $57 billion on aviation security.
So far, it's worked.
No plane or train in the United States has been successfully attacked since 9/11. Every day, federal screeners thwart attempts big and small to breach security at airports and other transportation hubs.
In the last four years, the TSA has seized 3,400 guns and more than 400,000 "incendiary items" from airline passengers.
Just last week, the TSA reported catching 18 guns, six "artfully concealed prohibited items," and seven passengers who were arrested after investigations of "suspicious behavior or fraudulent travel documents."
But at what cost?
Critics say the government's techniques invade privacy - sometimes in humiliating ways - erode individual rights, and threaten the health of travelers and screeners alike from scanning radiation.
And the trend, said Jay Stanley, a senior policy analyst at the American Civil Liberties Union, is toward more intrusions.
"When a frightening thing happens, like the Christmas bomber, the media and politicians jump all over it, clamoring for government to 'do something,' and the big security bureaucracies like TSA put their gears in motion," Stanley said. "It sort of develops a momentum of its own."
"They're often fighting the last war, putting in programs that would have stopped the last threat."
Stanley criticized the government's growing efforts to gather ever more data about citizens and to compile secret "no-fly" lists, in the name of security.
"Who's a dangerous person? What is that based on? Does a person get to challenge that? This runs contrary to our oldest traditions of being able to confront our accusers.
"Hopefully, we're not going to turn into a garrison state where we worry about nothing but security," Stanley said.
In a lawsuit filed last year challenging the TSA's use of full-body scanners, the Electronic Privacy Information Center said the agency "has initiated the most sweeping, the most invasive, and the most unaccountable, suspicionless search of American travelers in history."
A federal appeals court in Washington in July ruled that the TSA must seek public comment on the scanners, but could continue its practices while it gathers public reaction.
In the future, the TSA says it wants to reduce the hassle for most travelers while concentrating on the most serious threats.
"The vast majority of the 628 million annual air travelers present little to no risk of committing an act of terrorism," TSA Administrator John Pistole said recently.
"We should screen smarter and appropriately focus on those who do present the greatest risk, thereby improving security and the travel experience for everyone else."
So security officials are looking for more palatable ways to screen passengers.
The intrusive full-body scans that show a naked portrait of a passenger are being replaced with scans that show generic outlines of a human figure. The new scan software will highlight areas of the body where "anomalies" are found.
One TSA plan in the works would establish a new database of names of passengers who have been deemed low-risk. Starting this fall, some "trusted travelers" who volunteer certain identifying information ahead of time will get a boarding pass with their information embedded in a bar code, and will go to a fast lane for "expedited screening" at airports in Detroit, Miami, and Dallas-Fort Worth.
Other airports and airlines will be added, if the test program succeeds.
In another test program already under way at Boston's Logan Airport, TSA "behavior detection officers" at gate checkpoints are talking to every passenger to try to detect evasions or fear. Passengers deemed suspicious after a brief conversation are subjected to a pat-down search and having their carry-on luggage examined.
That program will also be expanded to other airports, if the 60-day Boston test, which started Aug. 15, is deemed successful, said Ann Davis, a TSA spokeswoman.
While air travel will continue to get the most attention from security officials, railroads and mass-transit systems also worry the Department of Homeland Security.
Terrorist attacks in Madrid, Spain, in 2004 and in London in 2005 starkly demonstrated the vulnerability of trains, subways, and buses.
Pistole says American mass transit and passenger rail systems have been the subject of "numerous plots" since 9/11. The relatively open access of trains and subways makes them inviting targets.
The response so far has been to patrol train stations, require some rail passengers to show identification, and to conduct unannounced security sweeps to search bags for explosives.
In the last year, 25 VIPR teams conducted more than 8,000 sweeps, including 3,700 in mass transit and rail stations. The Obama administration is seeking funding for 12 more VIPR teams in its budget for next year.
Pistole acknowledges that despite its best efforts, the TSA won't be able to make travel completely safe.
"We are not in the risk-elimination business. We never will be, and don't let anyone tell you we can be," he said. "The only way to eliminate risk is for everyone to stop traveling.
"That is not a real solution, and it's just not going to happen."
To view videos from the stories of people whose lives were affected by Sept. 11, 2001, go to www.philly.com/9-11
Contact staff writer Paul Nussbaum at 215-854-4587 or firstname.lastname@example.org.