People kicking off fall vacations in Mexico this weekend won’t be able to use the nation’s busiest border crossing to get there.

The southbound lanes of the San Ysidro crossing connecting San Diego to Tijuana closed at 3 a.m. Saturday and will remain closed until noon Monday for work on a $741 million expansion project.

The Mexican side of the crossing, used by people traveling into the United States, will remain open.

U.S. officials have been warning travelers for days to avoid driving to Baja California until the crossing reopens. Several large “road closed” signs were placed along Interstate 5 early Saturday morning as traffic was diverted off the freeway. Those who do will be detoured east to the much smaller Otay Mesa crossing, and a massive traffic jam could be the result.

“Don’t even think about going across in a vehicle,” Jason M-B Wells, executive director of the San Ysidro Chamber of Commerce, said earlier this week. “It’s going to be a standstill.”

The closure is to allow for the removal of a large metal canopy spanning the southbound lanes into Mexico.

The work is believed to represent the largest renovation of a crossing along the nearly 2,000-mile (3,218-kilometer) U.S.-Mexico border. It has been in the works for years to ease congestion and boost cross-border commerce.

When the crossing reopens Monday, it will contain three southbound lanes while California’s Interstate 5 is realigned to feed into the renovated arrival point. U.S. officials expect traffic flows to go back to normal by Thanksgiving, when four of the five lanes at the Western Hemisphere’s largest land crossing will be open.

Once complete in summer of 2019, there will be 10 southbound lanes — twice the current number — to handle the estimated 40,000 vehicles that pass through San Ysidro on weekends and 50,000 on weekdays. The expansion also adds eight more lanes from Mexico to the United States.

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This article was written by Julie Watson from The Associated Press and was legally licensed through the NewsCred publisher network. Please direct all licensing questions to [email protected].

Photo Credit: In this September 19, 2017 photo, construction continues on a new curve along California's Interstate 5 has it approaches the borer with Tijuana, Mexico. The San Diego to to Tijuana border crossing in the U.S. is closing this weekend to allow for the removal of a large metal canopy spanning over the southbound lanes into Mexico. Gregory Bull / Associated Press