WASHINGTON (Reuters) - Airport security lines can annoy passengers, but there is no evidence that they make flying any safer, U.S. researchers reported on Thursday.

A team at the Harvard School of Public Health could not find any studies showing whether the time-consuming process of X-raying carry-on luggage prevents hijackings or attacks.

They also found no evidence to suggest that making passengers take off their shoes and confiscating small items prevented any incidents.

The U.S. Transportation Security Administration told research teams requesting information their need for quick new security measures trumped the usefulness of evaluating them, Eleni Linos, Elizabeth Linos, and Graham Colditz reported in the British Medical Journal.

"We noticed that new airport screening protocols were implemented immediately after news reports of terror threats," they wrote.

"Even without clear evidence of the accuracy of testing, the Transportation Security Administration defended its measures by reporting that more than 13 million prohibited items were intercepted in one year," the researchers added. "Most of these illegal items were lighters."

The researchers said it would be interesting to apply medical standards to airport security. Screening programs for illnesses like cancer are usually not broadly instituted unless they have been shown to work.

"We'd like airport security screening to be of value. As passengers and members of the public we'd like to know the evidence and the reasoning behind these measures," Linos said in a telephone interview.

"Can you hide anything in your shoes that you cannot hide in your underwear?" they asked.

TSA spokesman Christopher White said the agency has not had a chance to read the article.

"If anyone has questions about whether our efforts have been fruitful over the past five years -- come on," White said in a telephone interview.

"While we can't publicize everything that we've done, every event, we can say definitively that our efforts over the last five years have not been for nothing," White added.

With $5.6 billion spent globally on airport protection each year, the public should be encouraged to query some screening requirements -- such as forcing passengers to remove their shoes, the researchers said.

White said the agency has pictures of shoe bombs on its Web site at (www.tsa.gov/) and welcomes people to examine them. "We encourage a legitimate public dialogue. We want passengers to understand why we do what we do," he said.

(Reporting by Maggie Fox, editing by Eric Walsh)