The day before Thanksgiving is traditionally one of the busiest travel days of the year. This year could see even more delays at airports because of spreading protests over new security-screening technology.

Several grass-roots movements, including National Opt-Out Day, have arisen in the past week, inviting people to opt out of the relatively speedy but controversial digital scan in favor of the slower “enhanced" pat-down, the only alternative offered by the Transportation Security Administration (TSA).

"People are getting groped in America’s airports,” says Chris Calabrese, legislative counsel for the American Civil Liberties Union. The apparent goal of the protests is to show that both procedures – the digital scans and the pat-downs – are too invasive, he adds.

Objections to the digital scanners – known as advanced imaging technology, or AIT scanners – have been growing for months. The equipment uses X-ray technology or millimeter-scale waves to generate an image of the body’s outer several centimeters, effectively allowing TSA employees to look under passengers' clothes without ever touching them. The generated images are relatively anonymous, but they leave little to the imagination.

When the machines were introduced at London’s Heathrow Airport earlier this year, the first sexual harassment suit resulted within two months, reported the BBC. A young woman spoke to the police after the guard commented on her breasts after seeing her scan. The angry and embarrassed woman said she felt traumatized by the incident.

But the machines enhance security in the post-9/11 world, administration officials say, and procedures are in place to avoid instances such as the Heathrow one.

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"AIT machines are safe, efficient, and protect passenger privacy," wrote Janet Napolitano, head of the Department of Homeland Security, in a Nov. 15 op-ed in USA Today. "And the weapons and other dangerous and prohibited items we've found during AIT screenings have illustrated their security value time and again. Rigorous privacy safeguards are also in place to protect the traveling public.... The officer assisting the passenger never sees the image, and the officer viewing the image never interacts with the passenger."

Secretary Napolitano's next sentence, however, touched off an Internet furor: "The imaging technology that we use cannot store, export, print, or transmit images."

That statement is contradicted by the Electronic Privacy Information Center (EPIC), a Washington privacy group, which alleges that it has evidence that the machines can indeed store the images. One device used in a Florida courtroom had more than 35,000 images stored in its memory, the US Marshals Service acknowledged in a response to a Freedom of Information Act request from EPIC. While TSA employees are instructed not to use the machines to store images, EPIC obtained copies of TSA's purchase orders specifying that the machines be able to save and export image data when in "Test Mode."

By Tuesday morning, users of the Twitter social-media site were asking whether President Obama would allow his wife and girls to go through the AIT scanner.

Actually, the scanner is the less invasive option. The enhanced pat-down, on the other hand, requires screeners to use their palms and fingers to make physical contact with every surface of a passenger’s body.

"I stood there, an American citizen, a mom traveling with a baby with special needs formula, sexually assaulted by a government official," writes blogger Erin, who does not publish her last name on her blogs to protect her family's anonymity. "I began shaking and felt completely violated, abused and assaulted by the TSA agent. I shook for several hours, and woke up the next day shaking."

When American John Tyner refused to go through the AIT or have his genitals touched in the pat-down, he was threatened with a civil suit and a $10,000 fine.

During initial tests of the machines, in a few airports, TSA found that more than 98 percent of passengers chose AIT over the pat-down.

“I think National Opt Out Day is an ineffective and poorly timed protest of the current TSA policies in place,” writes Steven Frischling, an airline consultant who blogs about the airline industry. “In the United States you are free to voice your opinion,... but you do not have the right to refuse current airport security procedures and still board a flight.”

But if thousands of passengers decide to protest the technology by opting for the slower pat-down at busy airports before Thanksgiving, it could cause big delays.

“If going through the pat-down significantly increases the amount of time that it takes to get through the checkpoint process, then [Opt-Out Day] is going to slow down an already congested process,” says David Castelveter, spokesman for airline industry group Air Transport Association (ATA). “If people choose to do this – and I emphasize the if – you stand the risk of not getting to your gate in time to make your flight. Secondly, airplanes are going to be full…. If you miss your flight because you opted out or others opted out, the odds of being re-accommodated in a timely fashion lessen.”

“Nearly 2 million will fly that day,” estimates Mr. Castelveter, who recommends that passengers at high-traffic airports in places such as New York, Los Angeles, and Chicago allow several hours to get to their gates.

“At the end of the day, what’s important to me is flying safe,” he says. “I want to get on the airplane and know I’m as safe as possible flying, so if that means going through the AIT machines or getting the pat-downs – if I’ve chosen to fly, those are the choices I’ve made.”

Privacy groups, however, are hoping that the public outcry combined with the threat of delays will get the TSA to change course.

“These agencies do respond to public pressure,” says Mr. Calabrese. “TSA backed down from a very similar proposal in 2004,” when a short-lived policy involved searching for explosives against women’s torsos after Chechen women managed to sneak explosives onto a plane in Russia. “Hopefully, similar good sense would prevail here,” he says.