The route connects all six parish seats as it travels through the thick pine forests of western Louisiana on a wide four-lane right-of-way.

The northern half of the route serves as the only major north–south corridor between Interstate 49 (I-49) to the east and the Toledo Bend Reservoir along the state line to the west.

Between the metropolitan areas at each terminus, US 171 passes through the smaller cities of DeRidder, Leesville, and Mansfield, as well as the towns of Rosepine, Many, Zwolle, and Stonewall.

Improvements over the years have included the paving of the entire route during the early 1930s, the elimination of many at-grade railroad crossings during the remainder of the decade, and the widening of the highway to four lanes through its urban areas during the 1960s and 1970s.

Four-laning of the entire route was accomplished between 1990 and 2010 via the state's massive TIMED program with the goal of improving safety and promoting economic development along the corridor.

The route has long been associated with the U.S. Armed Forces due to its proximity to at least four military bases active during the World Wars, the lone survivor of which is Fort Johnson near Leesville.

After immediately crossing over the BNSF and Union Pacific (UP) railroad lines, N. Martin Luther King passes through a partial cloverleaf interchange with I-10 at exit 33, connecting with Beaumont, Texas to the west and Lafayette to the east.

Upon exiting Lake Charles, US 171 traverses a strip of wetlands with twin-span bridges over two navigable waterways, English Bayou and the Calcasieu River.

[5][6][7] North of Moss Bluff, a rural area known as Gillis introduces a mixture of open farmland, thick forest, and scattered residential development that characterizes the next 34 miles (55 km) to the city of DeRidder.

During this stretch, US 171 enters Beauregard Parish and engages in a folded diamond interchange with US 190 and LA 12 that allows a grade separation with the Union Pacific Railroad line.

US 171 proceeds due north from Ragley co-signed with US 190 through the community of Longville, located at the eastern terminus of LA 110, and enters the outskirts of DeRidder.

The highway then crosses the Kansas City Southern Railway (KCS) line into the downtown area, where it passes several historic buildings over the next few blocks.

[5][7][8] DeRidder is separated by the small town of Rosepine by a short distance, during which an intersection with LA 3226 allows southbound traffic to bypass the city en route to US 190 west.

At a point known as Pickering, located 7.7 miles (12.4 km) north of Rosepine, US 171 intersects LA 10, the first of several connections to the United States Army installation at Fort Johnson.

Served by the highway's only business route, Zwolle is bypassed by mainline US 171, which travels around its east and north sides, intersecting LA 120 in the process.

US 84 overlaps US 171 for a short distance until reaching Polk Street, while the bypassed original mainline route follows the continuation of Washington Avenue signed as US 84 Bus.

It alternates between a rural and urban principal arterial over the course of its route, as determined by the Louisiana Department of Transportation and Development (La DOTD).

[18] Decades earlier, two markers were installed on US 171 identifying it as a Blue Star Memorial Highway as "a tribute to the Armed Forces that have defended the United States of America."

Minor deviations, mostly near the highway's southern end, included a diagonal cutoff that bypassed a section line road zigzag running northwest from Gillis[d] and the straightening of the route through Longville.

That year, $3.2 million was allocated for this purpose by a Works Progress Administration program and was earmarked by the Louisiana Highway Commission for 28 separate projects.

[35] All have since been replaced or supplemented with newer four-lane crossings, except the DeRidder and Grand Cane structures, which have been eliminated due to abandonment of the railroad lines.

In 1959, the final alignment change in Shreveport had US 171 transition from Mansfield Road onto Hearne Avenue to reach its northern terminus, giving the highway its first section of four-lane pavement.

This was roughly concurrent with the opening of the I-20 interchange on Hearne Avenue, connecting US 171 with the new expressway through Downtown Shreveport and improving access to the US 80 corridor east of the city.

[47] A more substantial project completed in 1969 overhauled the two most significant water crossings on US 171, the English Bayou and Calcasieu River bridges between Lake Charles and Moss Bluff.

[56][57] This last segment involved the creation of a one-way pair using an improved 6th Street to carry southbound traffic and the construction of a parallel railroad underpass.

[58][59] Meanwhile in Shreveport, the highway was widened southward from the Caddo–DeSoto parish line through Stonewall in 1981, joining with the four lanes extending from there to LA 5 in Gloster constructed in the late 1970s.

The widening of these mostly rural stretches was eventually accomplished as a major component of the Transportation Infrastructure Model for Economic Development program.

The route heads west into town on North Main Street alongside the Kansas City Southern Railway (KCS) line.

In the center of town, signage directs motorists south on Obrie Street (LA 475) toward North Toledo Bend State Park.

At an intersection with Willow Street and Old Pleasant Hill Road, the route begins an S-curve that terminates at a second junction with mainline US 171 at the north end of town.