The Manchac Swamp Bridge is a twin concrete trestle bridge[1] in the U.S. state of Louisiana. With a total length of 22.80 miles (36.69 km)[citation needed] it is one of the longest bridges in the world over water. Some claim it is the longest toll-free road bridge in the world.[citation needed] The bridge carries Interstate 55 over the Manchac Swamp in Louisiana, and represents one-third of the highway"s approximately 66 miles in Louisiana. Opening in 1979,[1] its piles were driven 250 feet (76 m) beneath the swamp and with the cost of the span being $7 million per mile to complete.