Officials from the Mexican city of Tijuana stated that people did not make the most of restrictions being lifted along the 2,000-mile (3,200-km) border to avoid getting caught in traffic.
"In the morning, there was no line," Tijuana resident Claudia Hernandez said as she prepared to enter the United States to go shopping ahead of the Thanksgiving holiday next week.
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"Next week, we'll see the massive lines that always form."
Javier Delgado, a Tijuana transport official, said there was nearly 35% less traffic than expected on the city's border with San Diego, one of the busiest border crossings in the world.
Hundreds of cars formed lines suddenly on Sunday stretching back kilometers from Tijuana, fueling fears the reopening could become a problem, yet traffic advanced steadily.
In the Mexican city of Ciudad Juarez on the other side of the border from El Paso, Texas, just 20 people lined up early on Monday before crossing to embrace family on the other side.
"We thought they were going to tell us again they had decided not to open it," said Lorena Hernandez, smiling broadly after they were reunited in El Paso for the first time since March 2020. "I said: If they don't reopen, I'm going to take a plane."
Yet differing rules over COVID-19 vaccines may hold up other family reunions, and the prospect of some restrictions easing has also led some migrants to try their luck to seek U.S. asylum - posing a test for the Biden administration.
Some vaccinated Mexicans could not enter the United States immediately if they received vaccines in Mexico not yet approved by the World Health Organization, such as China's CanSino and Russia's Sputnik V.
"I never imagined that because I got the CanSino vaccine, I wouldn't be able to cross," lamented Donato Suarez, a driver at a private university in Tijuana. The latter is trying to visit relatives in the U.S. he has not seen for nearly two years.
"We even had plans to do something when the border reopened," he added, noting that nearly 300 people where he works are in the same predicament. "We'll have to wait."