Consisting of parallel, 24-mile-long spans, the Lake Pontchartrain Causeway is the world's longest overwater highway bridge and one of the oldest prestressed concrete bridges built in the U.S. Engineers loaded the bridge pieces on barges and towed them to construction points on the lake. The twin bridges are supported by more than 9,500 hollow-cylinder pilings approximately 55 inches in diameter; the largest piles ever driven at the time of the first span's construction in 1955. Since the second span opened to the public in 1969, a drawbridge has been added, along with a tracking device to detect approaching barges during foggy conditions.more
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