Busy roads and crowded skies are as much a part of the holiday season as turkey and pumpkin pie.

And as travel industry chiefs hope to avoid the sort of meltdowns that have marred previous Thanksgivings, they are also urging the public not to resort to abuse when dealing with inevitable flight delays.

“If you’re flying, please be nice to your flight crew. They are there for your safety,” Mike Whitaker, the administrator of the Federal Aviation Administration, said on X. “The FAA has zero tolerance for unruly behavior.”

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Introduced in 2021, a zero tolerance policy promises legal action including possible criminal prosecution against any passenger who “assaults, threatens, intimidates, or interferes with airline crew members.”

But the FAA say airline staff have still reported a “disturbing increase” in threatening or violent behavior recently. And this week may prove a test of patience for the traveling public and of the aviation industry’s capacity.

At least 38,000 flights were expected to take off across the country Thursday, while around 55 million people are driving to see friends and relatives over the holiday period.

Flightaware.com reported that there were 312 cancellations and more than 6,000 delayed flights by 5 a.m. ET Thursday morning, following 18,000 throughout Wednesday. The site’s “Misery Map“ of live delays showed that Los Angeles International Airport was handling the most delays.

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The spike in travel numbers will continue through the week with some 2.9 million expected to make their return trips by air on Sunday, the Transport Security Administration said, which would beat an all-time passenger number record set on June 30 this year.

In total, the TSA said it expected to screen 30 million passengers between Nov. 17 and Nov. 28.

“We expect this holiday season to be our busiest ever. In 2023, we have already seen seven of the top 10 busiest travel days in TSA’s history,” said TSA Administrator David Pekoske.

“We are ready for the anticipated volumes and are working closely with our airline and airport partners to make sure we are prepared for this busy holiday travel season,” he added.

The FAA said that the main cause of flight delays was weather, causing 73.3% of delay minutes this year.

The National Weather Service said that heavy snow could disrupt travel plans in the norther and central Rockies and High Plains on Thursday, with cold air from the Arctic sweeping over much of the northern parts of the country.

Much of the rest of the country will, mercifully, remain dry, the NWS said.