If a U.S. government shutdown persists, it might disrupt travel plans, potentially causing extended airport wait times, flight delays, and even cancellations.

The shutdown commenced Wednesday after President Donald Trump and Congress couldn’t agree on a deal to maintain funding for government services and operations. This results in a significant number of employees crucial for the functioning of U.S. airports and air travel working without pay until the government resumes.

Geoff Freeman, president and CEO of the U.S. Travel Association, warned, “The longer a shutdown continues, the more likely we are to experience longer TSA lines, flight delays and cancellations, neglected national parks, and unnecessary postponements in updating travel infrastructure.”

Here’s what to know about the shutdown and its potential impacts on your travel plans.

Flying during a shutdown

Travelers can still fly despite the lapse in funding, and early on in the shutdown, they might not even notice any issues at the airport.

Airport security checkpoints and control towers will continue to be staffed, with around 13,200 air traffic controllers and over 61,000 Transportation Security Administration employees expected to keep working.

However, as the days pass without pay for these workers, travelers may begin to encounter longer security lines and flight disruptions, according to Jeffrey Price, an aviation professor at Metropolitan State University of Denver.

“The system does become a little bit more brittle, and the longer this goes, the more the traveler is going to notice it,” Price said.

That’s what happened in 2018 and 2019, when Trump led the country into its longest shutdown ever for 35 days during his first term.

Roughly three weeks into that shutdown, some unpaid security screeners began calling in sick, and air traffic controllers initiated a lawsuit against the government seeking their paychecks. Miami International Airport had to temporarily close one of its terminals as TSA officers were reporting sick at double the airport’s regular rate.

The latest shutdown is unfolding at a time when both the TSA and the Federal Aviation Administration are already facing staffing shortages, including a shortage of about 3,000 air traffic controllers.

If the system can’t handle the number of flights that are scheduled, the FAA will slow down landings and take offs and passengers will see more delays and cancellations.

Safety at airports and in the air

Nick Daniels, president of the union that represents air traffic controllers, said a shutdown weakens the aviation safety system by adding stress on controllers and taking away many of the workers who support them and maintain the outdated system they rely on.

“They’re out there working right now with critical staffing — the lowest staffing we’ve had in decades of only 10,800, where there should be 14,633. And on top of that, they’re working with unreliable equipment,” Daniels said.

During the 35-day shutdown during Trump’s first term, controllers in the busiest U.S. air traffic facilities reported working as much as 60 hours per week, and an increasing number of TSA agents also quit their jobs.

“This is more than just an inconvenience to the traveler,” Price said of the shutdown. “This is definitely going to bring up safety and security issues, if it really starts to go into the long term.”

Travel by train

Amtrak says its passengers trains will continue running. It operates more than 300 trains daily in 46 states, the D.C. area and three Canadian provinces.

In a statement, the country’s rail operator said that passengers planning to travel on its trains “in the coming days and weeks can be assured that Amtrak will remain open for business.”

International travel into the U.S.

Ports of entry into the country are expected to stay open for international travelers, according to the Department of Homeland Security’s contingency plan.

The department estimated that about 63,000 workers at Customs and Border Protection would still report to work. They include employees who are responsible for protecting the country’s borders and monitoring traffic coming into the U.S. at official border crossings, like airports and land crossings from Canada and Mexico.

Passports and visas

The U.S. Citizenship and Immigration Services, the agency responsible for overseeing the naturalization process, is primarily funded by application fees, meaning a lapse in funding at the federal government has minimal impacts on most passport and visa processing.

Agency spokesperson Matthew Tragesser said in a statement, however, that the shutdown does temporarily shutter the agency’s E-Verify program, a free online system that employers can use to confirm their new employees are authorized to work in the U.S.

Museums and national parks

Smithsonian museums, research centers and the National Zoo were scheduled to stay open to the public through at least Oct. 6. Further updates will be posted on the Smithsonian website.

The Smithsonian is the world’s largest museum complex, with 17 museums and its zoo located in the Washington, D.C. area, as well as two museums in New York City.

National Parks will remain mostly open during the shutdown.

The National Park Service’s contingency plan says park roads, lookouts, trails and open-air memorials will “generally remain accessible to visitors.” But parks without ”accessible areas” will be closed, and additional sites could shutter if damage is done to park resources or garbage builds up during the shutdown.

The park service oversees 400 sites, including large national parks such as Yellowstone and Grand Canyon, national battlefields and national monuments like the Statue of Liberty.

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Associated Press transportation reporter Josh Funk contributed from Omaha, Nebraska.