Across the country, lengthy security lines have plagued travelers heading out on spring getaways.

We’ve seen three-hour backups at airports in Houston and Atlanta, and backups snaking through the terminal in cities ranging from New Orleans to New York.

Travelers have faced mass uncertainty about how early to arrive for their flight. Airlines have had to decide whether to hold flights or depart without passengers stuck in line. And the Trump administration on Monday elected to send Immigration and Customs Enforcement officers to terminals.

Suffice to say, it’s been chaotic.

But at nearly two dozen airports, things are a lot calmer.

For instance, San Francisco International Airport (SFO) on Friday turned heads when it posted on social media that it’s been totally immune to the disruptions. “While we’ve seen and heard about the long security checkpoint lines over the last few weeks, SFO is NOT experiencing this issue,” the airport wrote.

There’s a reason.

Some airports use private security companies

SFO’s security checkpoints aren’t technically staffed by the Transportation Security Administration.

The airport is the largest hub in the country to use contracted-out private companies to staff TSA checkpoints — though it’s not alone. Nationally, there are 20 airports — mostly medium-size and smaller — that lean on private screeners via the TSA’s Screening Partnership Program.

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As part of that setup, these airports rely on private companies to operate checkpoints. Those contractors have to abide by all TSA protocols and tactics (so don’t expect to sneak a full-size shampoo bottle through; the 3-1-1 liquids rule still applies). And because those workers aren’t federal employees, they’re not affected by the ongoing partial government shutdown at the U.S. Department of Homeland Security.

That means they’re continuing to get paid and to show up for work — and as a result, the checkpoint wait times remain largely normal.

Related: Why having your Known Traveler Number is vital right now

20 airports that use the TSA’s Screening Partnership Program

Here’s the full list of airports that, along with SFO, use private security personnel. As you’ll see, the list includes airports located at a number of popular vacation (and spring break) destinations.

• Atlantic City International Airport (ACY) in New Jersey
• Charles M. Schulz Sonoma County Airport (STS) in Northern California
• Dawson Community Airport (GDV) in Montana
• Great Falls International Airport (GTF) in Montana
• Glacier Park International Airport (FCA) in Montana
• Frederick Douglass Greater Rochester International Airport (ROC) in New York
• Havre City-County Airport (HVR) in Montana
• Kansas City International Airport (MCI) in Missouri
• L. M. Clayton Airport (OLF) in Montana
• Orlando Sanford International Airport (SFB) in Florida
• Portsmouth International Airport (PSM) in New Hampshire
• Punta Gorda Airport (PGD) in Florida
• Roswell Air Center (ROW) in New Mexico
• San Francisco International Airport (SFO)
• Sarasota Bradenton International Airport (SRQ) in Florida
• Sidney-Richland Regional Airport (SDY) in Montana
• Sioux Falls Regional Airport (FSD) in South Dakota
• Tupelo Regional Airport (TUP) in Mississippi
• Wokal Field-Glasgow Valley County Airport (GGW) in Montana
• Yellowstone Airport (WYS) in Montana

What this means for travelers during the partial shutdown

What does this mean as the partial government shutdown carries on?

If you’re flying out of one of these airports in the coming days and weeks, you may be able to get to the airport right at your normal, expected time — no need to arrive four or five hours early, as some of the worst-affected airports have advised.

Related: 3-hour security queues? Closed airports? What to know about TSA and the shutdown

If you’re planning a last-minute trip, you may want to consider flying out of one of these airports instead of one of the many more serviced by the TSA — if feasible, of course. This could save you a headache before your flight.

Finally, if you have flights booked out of a nearby airport that doesn’t use private security, check whether you can change them. If your airline services one of these 20 airports — and if you booked a trip that allows you to change your origin or destination city — you might be better off flying out of one of those airports, even if it costs a little more.