SAN FRANCISCO — San Francisco and Santa Clara are California’s only two counties that have enacted orders requiring travelers to quarantine on arriving in their jurisdictions. But unlike in New York state, it is unclear whether authorities are doing much to publicize or enforce the order.

Kristen Green, a radiologic technologist at Kaiser Permanente, said she didn’t notice any signs or fliers as she left San Francisco International Airport’s Terminal 3 on Wednesday, after arriving on a flight from Denver.

“It’s possible there were signs,” she said. “I can’t say I was paying a whole lot of attention.”

She had gone to Colorado to say goodbye to her father, who died of cancer.

On Wednesday, the Transportation Security Administration recorded the nation’s highest single day of travel since the pandemic began, with the TSA screening 1,191,123 people passing through airports. That is higher than the record set on the Sunday after Thanksgiving, when 1,176,091 travelers were screened.

It comes amid heightened concern about the surging coronavirus, including a more transmissible form of the virus detected in Britain.

In many parts of the world, nations are shutting down borders and instituting new travel restrictions. In New York City, Mayor Bill de Blasio announced that law enforcement officials will be deployed across the city to enforce its 14-day quarantine for anyone flying into New York City from the United Kingdom.

United Airlines is now requiring passengers boarding flights originating in the U.K. to show proof of a negative COVID-19 test taken no later than 72 hours prior to departure, said Annabelle Cottee, United Airlines spokeswoman.

There is one daily nonstop flight from the London Heathrow to San Francisco. That flight will be suspended starting Jan. 1, a decision that was made by the flight’s carrier, United Airlines, before news of the variant form of the virus had been reported.

Last week, health officials in San Francisco and Santa Clara counties also announced mandatory quarantine measures for any travelers coming into the region from outside of the Bay Area — 10 days for San Francisco, 14 days for Santa Clara. The quarantine is enforceable by law in San Francisco; lack of compliance is a misdemeanor punishable by fine or imprisonment.

However, Bryan Dahl, a spokesman for San Francisco’s health department, said the city was not yet enforcing the order.

“We do not want to use a criminal justice approach for a public health challenge,” he said. “This is about first educating the public so there is compliance.”

Santa Clara’s order does not specify any enforcement steps.

On Wednesday afternoon, SFO appeared nearly empty of holiday travelers.

Although it’s usually a madhouse even during non-holiday weeks, a visitor was able to circle by car through the arrivals and departures drop-offs without having to stop for congested traffic, or even the occasional pedestrian.

“Compared to Denver, this is like a ghost town,” said Green, who was standing on the curb outside the airport Wednesday. She said she was aware of the quarantine only because of her involvement with the healthcare industry.

According to SFO spokesman Doug Yakel, 17,304 travelers went through SFO’s TSA checkpoints Wednesday — roughly 20% of the passengers traveling through this same time last year.

Things “might be different for SFO,” said Yakel, who said numbers have trended downward since Thanksgiving.

“During Thanksgiving, we saw some of the heaviest days of activity since the pandemic began, but even that was 75% lower than pre-pandemic levels,” he said. “Traffic this week is running about 15% lower than what we saw during Thanksgiving.”

Yakel said that paper fliers notifying incoming travelers of the quarantine were being given out at the exit of every concourse.

In addition, he said, public address announcements were being broadcast every 30 minutes in all of the terminals, and signs — digital and “static” — were in abundance throughout the terminals.

Things are slower at SFO than at LAX, where there currently is no travel order.

TSA spokesman Daniel Velez said that although travel volume was up 58.26% at LAX compared with Wednesday last week, it was down 59.3% compared with the same day last year.

In San Francisco, volume was up 39.5% compared with last week but down 75.5% compared with the same day last year.

“We are thankful for the public’s cooperation during this health crisis,” Dahl said. “Most people have done a good job looking out for their neighbors and following the health orders, and we expect businesses and residents will continue to do that with this order.”