The busiest border crossing in the United States will close this weekend to the more than 40,000 cars that pass through it daily to Mexico.

   The closure between San Diego and Tijuana for work on a $741 million expansion project presents a monumental headache for border businesses, workers, tourists.

   The expansion is believed to be the largest renovation of a crossing along the nearly 2,000-mile-long U.S.-Mexico border. It has been in the works for years to ease congestion and boost cross-border commerce.

   U.S. officials are warning people to avoid driving to Baja California from 3 a.m. Saturday until noon Monday. They want to ease what's feared will be a massive traffic jam on the U.S. side as Mexico-bound cars are detoured to the much smaller Otay Mesa crossing to the east.