U.S. border officials on Monday shut down all vehicular traffic flowing north from Mexico into California at the country’s busiest border crossing and diverted staff to help bulk up the infrastructure and security there, in anticipation of the arrival of thousands of migrants traveling in caravans from Central America.

The San Ysidro port of entry, which processes 20,000 pedestrians and 70,000 vehicles moving from Tijuana to San Diego each day, closed all passenger and commercial lanes and one of its two pedestrian crossings.