A roof collapsed at the San Ysidro border crossing between Mexico and the United States on Wednesday, piling debris on lines of cars, injuring at least 17 travelers and construction workers and closing lanes to northbound traffic. The world’s busiest border crossing -- linking Tijuana, Mexico with San Diego, California -- has been undergoing a $600 million upgrade since February that aims to more than double its capacity. Scaffolding supporting the roof collapsed across several lanes at 10:45 a.m. local time, the U.S. Customs and Border Protection agency said. It was not clear whether the collapse was linked to the expansion work.