Interstate 55 in Louisiana in the context of Ponchatoula, Louisiana

The Manchac Swamp Bridge is a twin concrete trestle bridge near Manchac in the U.S. state of Louisiana. It carries Interstate 55 and U.S. Route 51 over the Manchac Swamp in Louisiana and represents a third of the highway's approximately 66 miles (106 km) in Louisiana.

With a total length of 22.80 miles (36.69 km), it is one of the longest bridges in the world over water, and is the longest bridge on the Interstate Highway System, and some claim it is the longest toll-free road bridge in the world.

U.S. Highway 51 (US 51) is a part of the United States Numbered Highway System that spans 1,277 miles (2,055 km) from LaPlace, Louisiana to a point north of Hurley, Wisconsin. Within the state of Louisiana, the highway travels 69.12 miles (111.24 km) from the national southern terminus at US 61 in LaPlace to the Mississippi state line north of Kentwood.

After heading north through LaPlace to a junction with Interstate 10 (I-10) west of New Orleans, the highway joins I-55 on a 22-mile-long (35 km) twin-span viaduct across the Manchac Swamp. The bridge, which passes between Lakes Pontchartrain and Maurepas, is the second-longest bridge over water in the United States and serves as a vital hurricane evacuation route for the New Orleans metropolitan area.