Similar to Interstate 2 in Texas and Interstate 4 in Florida, Interstate 12 (I-12) is an interstate highway located entirely within the state of Louisiana. These (and other highways) are designated as interstate highways (although they are intrastate) because they were built to interstate standards.
I-12 runs west-east/east-west and parallels the older US 190 corridor and traverses the North Shore of Lake Pontchartrain in southeastern Louisiana. At only 86.65 miles, I-12 is very short for a mainline interstate; however, it is comparable to the country’s longest auxiliary interstates. It is also one of the shortest mainline interstates to terminate at the same route (I-10) at both ends.
In addition to serving the various communities located along the North Shore, I-12 serves as a long bypass of New Orleans and is heavily used as an alternative route for through traffic on I-10. While I-10 curves southward to pass through the New Orleans city limits, I-12 takes a more direct route that reduces the distance between Baton Rouge and Slidell by about 22 miles.
At its western terminus, I-12 begins three miles east of downtown Baton Rouge at an interchange with I-10 known as the I-10/I-12 split. As noted above, I-10 heads to the southeast toward the New Orleans metropolitan area, while I-12 proceeds eastward toward the North Shore of Lake Pontchartrain.
Moving eastward, I-12 reaches the Amite River, where it enters Livingston Parish and the city of Denham Springs. I-12 skirts the southern edge of Denham Springs; east of the town, I-12 skirts the southern edge of Walker. Past Walker are smaller communities in the eastern half of the parish.
Interstate 12 then crosses the Natalbany River into Tangipahoa Parish. There is a cloverleaf interchange with I-55 at the southeast corner of Hammond, the largest city in Tangipahoa Parish. A major north-south route, I-55 runs toward Jackson, Mississippi to the north and New Orleans to the south.
Shortly after crossing into St. Tammany Parish, I-12 takes a southeastern curve and begins to parallel the North Shore of Lake Pontchartrain. The highway then crosses the Tchefuncte River. Two miles east of the bridge over the Tchefuncte River, there is an interchange with US 190 between Covington and Mandeville. This section of US 190 serves as St. Tammany Parish’s busiest north-south thoroughfare, connecting Covington and surrounding areas with the Lake Pontchartrain Causeway, a 24-mile-long bridge leading to New Orleans.
I-12 then enters the northern edge of Slidell. A couple of miles later, I-12 reaches its eastern end at an interchange with I-10 and I-59 at the northeast corner of Slidell. From this interchange, I-10 heads southwest toward New Orleans and east toward Bay St. Louis, Mississippi, while I-59 heads north toward Hattiesburg, Mississippi.
While other very short interstates have been added to the Interstate Highway System (IHS) because of population growth in the decades after the system was designed, I-12 was added to the IHS on October 17, 1957. Originally, I-12 was proposed to run from I-10 in Baton Rouge to I-59 north of Slidell. By the mid-1960s, the routes had been realigned to their current routes; I-12 and I-59 both end at I-10 near Slidell. The route was opened to traffic in sections between January 1967 and June 1976.
Hurricane Katrina inflicted major damage on the I-10 Twin Span Bridge across Lake Pontchartrain. I-12 temporarily functioned as Interstate 10 between Baton Rouge and Slidell. On October 14, 2005, the eastbound span of I-10 over Lake Pontchartrain reopened to two-way traffic. About three months later (January 6, 2006), the westbound span of I-10 reopened (with speed, weight and size restrictions), which relieved a great deal of the congestion that was clogging I-12 from the Lake Pontchartrain Causeway to its junction with I-10 and I-59.