A rerouting of tens of thousands of vehicles a day begins Thursday at the busiest crossing on the U.S.-Mexico border.

Officials say the changes are part of a major expansion that aims to keep lines manageable while allowing for heightened inspections for contraband.

The reconfigured San Ysidro border crossing between San Diego and Tijuana, Mexico, promises to give inspectors from both countries more time and space to search for illegal goods without disrupting motorists.

A week of trial runs eased fears that a sharp turn will create big delays for Mexico-bound motorists.

Mexican President Felipe Calderon drove through an inspection booth Wednesday night and said travelers can expect a smooth trip.