The highway crosses into Tangipahoa Parish after a short distance and passes through an interchange with I-55 (exit 40) just west of Independence.

Over the next 12 miles (19 km), the highway zigzags along section line roads but maintains a general eastward trajectory.

LA 40 makes a zigzag past Lee Road Junior High School then turns northeast for about five miles (8.0 km).

Near the end of its route, the highway makes a final zigzag via LA 21 in the unincorporated community of Bush.

It then continues a short distance further to a T-intersection with LA 41, which turns north onto a divided-four lane alignment heading toward the city of Bogalusa.

[13][14] The portion of LA 40 running one mile (1.6 km) south from Loranger was technically part of State Route 229, which was supposed to turn east across Chappapella Creek but was never completed.

With the 1955 renumbering, the state highway department initially categorized all routes into three classes: "A" (primary), "B" (secondary), and "C" (farm-to-market).

[19][20] The original path of LA 40 here exists as two dead-end local roads on either side of the interstate called Fontana Lane and Old Highway 40.

[21] La DOTD is currently engaged in a program that aims to transfer about 5,000 miles (8,000 km) of state-owned roadways to local governments over the next several years.

[22] Under this plan of "right-sizing" the state highway system, two portions of LA 40 are proposed for deletion as they no longer meet a significant interurban travel function.