Transportation Secretary Sean Duffy warns a ‘substantial’ number of Americans won’t be able to travel for Thanksgiving

• Transportation Secretary Sean Duffy says travel could slow to a trickle over Thanksgiving.
• Thousands have flights are being canceled as air traffic controllers go unpaid during the shutdown.
• The number of Americans unable to make flights home for the holiday will be "substantial," he said.

If you're traveling for Thanksgiving, it might be wise to have a backup plan.

Transportation Secretary Sean Duffy said on CNN's "State of the Union" on Sunday that the number of Americans who won't be able to be with their families for the holidays will be "substantial" if the government shutdown continues.

The shutdown is straining US air travel as air traffic controllers, who are paid with federal funds and have largely gone without a paycheck for over a month, are increasingly not showing up for work, forcing the Federal Aviation Administration to ground thousands of flights.

Duffy said the controllers he spoke to said they're able to miss one paycheck, but two would be difficult.

"Virtually none of them can miss two paychecks," Duffy said. "So they're going to be confronted with the idea of, as you mentioned, going to get a side job, a second job to make ends meet," he said.

Airline pilots and flight attendants have been stepping in to help, sending free meals to air traffic controllers at airports nationwide.

Airlines have already canceled thousands of flights this weekend, according to data from Cirium, an aviation analytics firm. Those numbers are likely to only increase as the political stalemate over the government shutdown persists.