The Lake Pontchartrain Causeway is a pair of parallel concrete trestle bridges that carry Causeway Boulevard about 38.4 km (roughly 23.9 miles) across Lake Pontchartrain in southeastern Louisiana, from Metairie on the south shore to Mandeville on the north shore. The first span opened on August 30, 1956 and the second on May 10, 1969, and Guinness World Records lists the Causeway as the longest continuous bridge over water in the world. It matters because it turned a lake that once took a ferry ride or a long detour into a daily commute, pulling St. Tammany Parish into the New Orleans metropolitan area.
Two Trestles, 9,500 Piles and a Lake Barely 4 Metres Deep
The Causeway is not one bridge but two, side by side, each carrying two lanes in a single direction. The design is a low-level trestle with a bascule span in the middle: a long run of short, identical spans laid on closely spaced supports rather than a few long spans hung from towers. The supports are concrete piles driven into the lake floor, and across the two structures there are about 9,500 of them. Seen from the air the result looks like a pair of ruled pencil lines drawn across open water, because there is nothing tall to break the line except the drawbridge 13 km from the north shore.
The lake explains why a trestle works here. Lake Pontchartrain covers about 1,600 km² (630 sq mi) and measures roughly 64 km from west to east and 39 km from south to north, yet its average depth is only 3.7 to 4.3 metres (12 to 14 ft). Shallow water means short piles and cheap foundations repeated thousands of times, instead of deep caissons. The deck sits low too: the fixed spans give about 4.6 m (15 ft) of clearance above the water, enough for small boats and fishing craft, while taller vessels must use the navigation channel.
Because the shoreline is so flat, the bridge rises directly from the developed lakefront at Metairie, in Jefferson Parish, and lands on the Mandeville side in St. Tammany Parish, with only short approach roads at each end. The original 1956 contract included the bridge, three approach roads on the north end and a long stretch of road on the south end, all for $46 million.
The Bascule Drawbridge 13 km From Mandeville
A low trestle blocks anything taller than a pleasure boat, so the Causeway needs one moving part. About 13 km (8 miles) south of the north shore, a bascule bridge spans the navigation channel. A bascule is a counterweighted leaf that pivots upward like the arm of a balance scale; the name comes from the French word for that kind of scale. Because the counterweight balances the span throughout its swing, it opens quickly and needs relatively little power, and once raised it leaves unlimited headroom for the vessel passing through.
The weakness of any low bridge is the barge that misses the channel. On June 16, 1964, barges tore a gap in the original span and a bus plunged into the lake, killing six people. The accident happened before the second span existed, when the 1956 bridge carried traffic in both directions on two lanes. When the parallel span opened in 1969, at a cost of $30 million, each bridge could carry traffic one way, separating the two flows.
Turnarounds, a Southbound Toll and the Causeway Commission
Along its length the Causeway has turnarounds, places where vehicles can leave the travel lanes and reverse direction. On a bridge with no exits for nearly 40 km they give police and breakdown crews a way to turn back without driving the full length of the lake. They were also the most exposed part of the structure when Hurricane Katrina struck.
The bridge pays for itself through tolls, and the way they are collected is part of the design. Until 1999 drivers paid in each direction. To relieve congestion at the south shore, collection on the northbound span was dropped, and since May 1999 a single toll has been charged on the north shore for southbound traffic. That toll rose in 2017 to fund safety improvements, from $3.00 cash to $5.00, and again in 2023 to $6.00 cash, or $3.40 with a toll tag.
The body that runs all this is the Greater New Orleans Expressway Commission, usually called the Causeway Commission. Headquartered in Metairie, it has five commissioners drawn from the two parishes the bridge connects, Jefferson and St. Tammany, and it is responsible for maintenance, construction and enforcing safety laws on the spans. It traces back to 1948, when Ernest M. Loëb Jr. pushed the idea of a lake crossing and the Louisiana Legislature created the commission. The Louisiana Bridge Company was formed to build the bridge and appointed James E. Walters Sr. to direct the project.
Traffic on the Causeway was put at about 43,000 vehicles a day in a 2006 report in The Times-Picayune. In 2013 the American Society of Civil Engineers designated the bridge a National Historic Civil Engineering Landmark.
How the Record Was Split With Jiaozhou Bay in 2011
For decades Guinness World Records listed the Causeway simply as the longest bridge over water. In July 2011 it gave that title to the Jiaozhou Bay Bridge in Qingdao, China, measuring 42.6 km including land bridges at each end and an undersea tunnel. Causeway supporters objected that only about 38.28 km of their bridge sits over water, against roughly 25.9 km for the longest continuous segment in Qingdao. Guinness settled the argument by creating two categories: the Causeway holds the record for the longest bridge over water (continuous), and Jiaozhou Bay for the longest by aggregate length.
Within Louisiana the Causeway is one of several highway bridges at least 8 km (5 miles) long. The Manchac Swamp Bridge on Interstate 55, opened in 1979, is close behind and is the longest bridge on the Interstate Highway System, while the Atchafalaya Basin Bridge on Interstate 10 crosses the swamp between Baton Rouge and Lafayette. The older crossings at the eastern end of the lake are far shorter: the Maestri Bridge on US 11, which opened on February 18, 1928 as the first permanent crossing of Lake Pontchartrain, runs 7.69 km.
Hurricane Katrina, August 29, 2005
A long low bridge across a shallow lake ought to be vulnerable to hurricanes, and a storm surge driven into Lake Pontchartrain can overwhelm the levees protecting New Orleans. When Hurricane Katrina came ashore on August 29, 2005, the surge was much higher at the eastern end of the lake, near the 8.9 km I-10 Twin Span Bridge between New Orleans East and Slidell, which was wrecked. Under the Causeway the surge was lower, and damage was mostly limited to the turnarounds. The foundations held, and the Causeway has never suffered major structural damage from a hurricane, which is unusual for a causeway.
That survival made the bridge a lifeline. With the Twin Span out of action, recovery crews who were based inland to the north used the Causeway to reach New Orleans. It reopened first to emergency traffic and then to the public on September 19, 2005, with tolls suspended until mid-October. The replacement Twin Span bridges did not open until July 9, 2009 for the first span and April 7, 2010 for the second.
From Marigny's Ferry to a Commuter Belt: St. Tammany Since 1956
The idea of bridging the lake goes back to Bernard de Marigny, who laid out Mandeville in 1834 as a resort for New Orleanians and started a ferry service that ran into the mid-1930s. From the mid-19th century daily steamboats carried New Orleanians across the water, and in the 1920s one plan proposed building artificial islands linked by bridges, financed by selling home sites on them. Before 1956 a driver from the north shore had to go round: east via the Maestri Bridge or the Rigolets Bridge near Slidell, or west on US 51 through Manchac.
The Causeway cut up to 50 minutes from the drive into New Orleans, and the effect on St. Tammany Parish shows clearly in the census. The parish had 26,988 residents in 1950, when it was sparsely populated and almost wholly rural. Growth around Mandeville, Covington and Madisonville gathered pace only in the late 1960s, around the time the second span opened, and the parish passed 100,000 by 1980 and 200,000 in the years after Katrina, when many families moved north of the lake.
By the 2020 census St. Tammany had 264,570 people, the fourth-largest population of any Louisiana parish, nearly ten times its 1950 total. Mandeville, the town at the north end of the bridge, grew from 1,368 residents in 1950 to 13,192 in 2020. At the south end, Metairie counted 143,507 people in 2020, making it the largest community in Jefferson Parish and the fifth-largest census-designated place in the United States. The bridge ties these two halves of the metro area together across water only three or four metres deep.
The Causeway is the product of a particular landscape: a broad, shallow estuary on the delta plain, connected to the Gulf of Mexico through the Rigolets and Chef Menteur Pass into Lake Borgne, with flat shores on both sides. The same conditions shaped the swamp bridges on I-10 and I-55 and much of the state's road network, which our guide to Louisiana's geography traces from the Mississippi delta to the Florida Parishes, and you can see how the Causeway compares with the great sea and river crossings of Asia and Europe in our list of the longest bridges in the world.