NEW YORK — (CNN) Think U.S. security lines are bad now?

Wait until everyone starts taking summer vacations.

More than 231 million passengers are expected to fly on U.S. airlines this summer, including more than 30 million travelers on international flights, according to Airlines for America, a U.S. airline trade group.

That’s a 4% increase from last summer’s all-time high of 222 million passengers.

That could translate into hourslong security wait times getting even longer at the nation’s airports, and even more travelers missing their scheduled flights.

Thousands have already missed their flights.

“Due to the length of the lines, tens of thousands of customers have missed their flights and thousands of checked bags have been delayed in TSA resolutions rooms due to low staffing,” American Airlines spokesman Ross Feinstein said.

American airports and airlines have been predicting this perfect storm due to increased passenger volume, decreased TSA screener staffing and a TSA security system that slows down every time a screening machine shows potentially prohibited items.

“It has been a challenging spring with fliers waiting in lines that take more than 60 to 90 minutes to get through security,” said Sharon Pinkerton, Airlines for America’s senior vice president of legislative and regulatory affairs, in a statement.

“We encourage TSA to quickly hire and train new staff to help alleviate this problem, and we also encourage more travelers to enroll in TSA PreCheck as we move into another record-setting travel season.”

The TSA has 42,525 front-line staffers this year, a 10% drop since fiscal year 2013, according to the agency.

The staffing numbers are what the TSA says it can afford based on congressional appropriations.

Congress has allowed the Department of Homeland Security, which oversees the TSA, to shift funds to pay existing TSA officers overtime and hire new security officers.

The TSA hopes to have 768 new security officers in place as early as mid-June, Department of Homeland Security Secretary Jeh Johnson said last week.

Airlines for America launched the hashtag #ihatethewait for travelers to share their security wait times.

The group also encouraged travelers to sign up for TSA Pre-Check or Customs and Border Protection’s Global Entry — which includes Pre-Check — to get expedited screening at the airport.

Some 2.5 million travelers have signed up for the TSA program, far short of the 25 million the TSA would like to enroll, according to The New York Times.

Reports of long lines at many of the nation’s airports seem to have inspired more people to consider the program.

Pre-Check applications increased from 8,400 per day in April to more than 11,000 per day in May, according to the TSA.

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