The invention of the automobile led to the creation of this Bay bridge in 1947. The bridge's two-lane original 4.3 mile long span (which today carries eastbound traffic) cost $45 million and was, at the time, the world's longest continuous over-water steel structure. Construction of the bridge's $148-million second span, which currently carries westbound traffic across the Bay, was completed in 1973. Over 26 million vehicles travel the bridge annually.