U.S. Highway 51 (US 51) is a part of the United States Numbered Highway System that spans from LaPlace, Louisiana to a point north of Hurley, Wisconsin. Within the state of Louisiana, the highway travels 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 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.
With Ponchatoula served by the highway's only designated business route, US 51 splits from the interstate in the neighboring city of Hammond. For the remainder of its journey in southeastern Louisiana, the route closely parallels I-55, which carries the bulk of through traffic toward Jackson, Mississippi and points north. US 51 serves local traffic through a string of communities in Tangipahoa Parish situated along the Canadian National Railway line, including the towns of Independence, Amite City, and Kentwood.
Prior to 1951, US 51 began in downtown New Orleans and followed the route of US 61 to its current southern terminus in LaPlace. For a time in the late 1920s to mid-1930s, however, it was intended to follow a scenic route along the shore of Lake Pontchartrain known as the New Orleans–Hammond Lakeshore Highway. Construction difficulties and budget limitations led to the abandonment of the project before the portion through the swamps of St. Charles Parish could be completed.
Route description
LaPlace to Hammond
From the south, US 51 begins at an intersection with US 61 (West Airline Highway) in LaPlace, an unincorporated community located on the east bank of the Mississippi River in St. John the Baptist Parish. US 61 parallels the route of I-10 between New Orleans and Baton Rouge. US 51 heads northeast as an undivided four-lane highway with a center turning lane through a growing residential and commercial corridor. After , the route reaches the edge of town and passes through a diamond interchange with I-10 at exit 209. This interchange is also used to feed traffic between I-10 to the west and I-55 to the north as there is no direct connection. With a median replacing the center lane, US 51 proceeds a short distance to an intersection with Frenier Road, where signage directs through traffic to turn west and follow a ramp onto the elevated I-55 at exit 1. For the next , US 51 travels concurrently with the elevated four-lane interstate through the thickly wooded swamp lying between Lake Pontchartrain and Lake Maurepas. This twin-span concrete trestle is known as the Manchac Swamp Bridge and is the second-longest bridge over water in the United States. along the way, a tight diamond interchange at Ruddock connects with the grade-level pre-interstate alignment, which serves as a frontage road and closely parallels the entire stretch.
After , the highway elevates higher to cross Pass Manchac, a waterway that connects the two lakes and forms the boundary between St. John the Baptist Parish and Tangipahoa Parish. The area flanking the pass, generally called Manchac, contains a cluster of boat houses and fishing camps situated along the frontage road. This small community is also known as Galva on the south side and Akers on the north side. The highway proceeds through the swamps of southern Tangipahoa Parish for another before returning to grade. At this point, an exit primarily serving southbound traffic connects to the north end of the frontage road. Soon afterward, I-55/US 51 curves to the northwest at exit 23 while US 51 Bus. begins and splits off straight ahead into the small city of Ponchatoula. The interstate proceeds around the west side of town, an area of scattered suburban development within the thick pine forest. A cloverleaf interchange at exit 26 connects to LA 22, the primary east–west highway through Ponchatoula. US 51 departs from I-55 at the following exit and enters the neighboring city of Hammond, the largest city along its route.