DALLAS — Ask anyone old enough to remember travel before Sept. 11, 2001, and you're likely to get a gauzy recollection of what flying was like.

There was security screening, but it wasn't anywhere near as intrusive. There were no long checkpoint lines. Passengers and their families could walk right to the gate together, postponing goodbye hugs until the last possible moment. Overall, an airport experience meant far less stress.

That all ended when four hijacked planes crashed into the World Trade Center towers, the Pentagon and a field in Pennsylvania.

The worst terror attack on American soil led to increased and sometimes tension-filled security measures in airports across the world, aimed at preventing a repeat of that awful day. The cataclysm has also contributed to other changes large and small that have reshaped the airline industry — and, for consumers, made air travel more stressful than ever.

Two months after the attacks, President George W. Bush signed legislation creating the Transportation Security Administration, a force of federal airport screeners that replaced the private companies that airlines were hiring to handle security. The law required that all checked bags be screened, cockpit doors be reinforced, and more federal air marshals be put on flights.

There has not been another 9/11. Nothing even close. But after that day, flying changed forever.

Here's how it unfolded.Security measures evolved with new threats, and so travelers were asked to take off belts and remove some items from bags for scanning. Things that clearly could be wielded as weapons, like the box-cutters used by the 9/11 hijackers, were banned. After “shoe bomber” Richard Reid's attempt to take down a flight from Paris to Miami in late 2001, footwear started coming off at security checkpoints.Each new requirement seemed to make checkpoint lines longer, forcing passengers to arrive at the airport earlier if they wanted to make their flights. To many travelers, other rules were more mystifying, such as limits on liquids because the wrong ones could possibly be used to concoct a bomb.“It's a much bigger hassle than it was before 9/11 — much bigger — but we have gotten used to it,” Ronald Briggs said as he and his wife, Jeanne, waited at Dallas/Fort Worth International Airport for a flight to London last month. The north Texas retirees, who traveled frequently before the pandemic, said they are more worried about COVID-19 than terrorism.“The point about taking shoes off because of one incident on a plane seems somewhat on the extreme side,” Ronald Briggs said, “but the PreCheck works pretty smoothly, and I've learned to use a plastic belt so I don't have to take it off.”The long lines created by post-attack measures gave rise to the PreCheck and Global Entry “trusted-traveler programs” in which people who pay a fee and provide certain information about themselves pass through checkpoints without removing shoes and jackets or taking laptops out of their bag.But that convenience has come at a cost: privacy.On its application and in brief interviews, PreCheck asks people about basic information like work history and where they have lived, and they give a fingerprint and agree to a criminal-records check. Privacy advocates are particularly concerned about ideas that TSA has floated to also examine social media postings (the agency's top official says that has been dropped), press reports about people, location data and information from data brokers including how applicants spend their money.“It's far from clear that that has any relationship to aviation security,” says Jay Stanley, a privacy expert at the American Civil Liberties Union.

Despite the trauma that led to its creation, and the intense desire to avoid another 9/11, the TSA itself has frequently been the subject of questions about its methods, ideas and effectiveness.Flight attendants and air marshals were outraged when the agency proposed in 2013 to let passengers carry folding pocket knives and other long-banned items on planes again. The agency dropped the idea. And after another outcry, the TSA removed full-body scanners that produced realistic-looking images that some travelers compared to virtual strip searches. They were replaced by other machines that caused fewer privacy and health objections. Pat-downs of travelers are a constant complaint.In 2015, a published report said TSA officers failed 95% of the time to detect weapons or explosive material carried by undercover inspectors. Members of Congress who received a classified briefing raised their concerns to Pekoske, with one lawmaker saying that TSA “is broken badly.”Critics, including former TSA officers, have derided the agency as “security theater” that gives a false impression of safeguarding the traveling public. Pekoske dismisses that notion by pointing to the huge number of guns seized at airport checkpoints — more than 3,200 last year, 83% of them loaded — instead of making it onto planes.Pekoske also ticked off other TSA tasks, including vetting passengers, screening checked bags with 3-D technology, inspecting cargo and putting federal air marshals on flights.“There is an awful lot there that people don't see,” Pekoske says. “Rest assured: This is not security theater. It's real security.”Many independent experts agree with Pekoske's assessment, though they usually see areas where the TSA must improve.“TSA is an effective deterrent against most attacks,” says Jeffrey Price, who teaches aviation security at Metropolitan State University of Denver and has co-authored books on the subject. “If it's security theater, like some critics say, it's pretty good security theater because since 9/11 we haven't had a successful attack against aviation.”This summer, an average of nearly 2 million people per day have flowed through TSA checkpoints. On weekends and holidays they can be teeming with stressed-out travelers. During the middle of the week, even at big airports like DFW, they are less crowded; they hum rather than roar. Most travelers accept any inconvenience as the price of security in an uncertain world.