The southern segment of US 31 connects the cities of Mobile, Montgomery, Birmingham, and Decatur in Alabama, and Nashville in Tennessee.

North of Brewton, US 31 assumes a northeast–southwest trajectory, passing through rural areas and small towns in Escambia and Conecuh Counties.

Although US 31 parallels Interstate 65 throughout Alabama, the two routes do not directly junction each other until they reach Pintlala in southern Montgomery County, which is 164 miles north of Mobile.

North of Prattville, US 31 passes through rural areas of Autauga and Chilton Counties, primarily along two-lane roadways.

US 31 continues to be routed along multi-lane streets and highways as it continues north of Birmingham, meeting the eastern terminus of I-22 just south of Fultondale, and passing through the small town, along with Gardendale, Morris and Kimberly, also closely paralleling I-65.

Two miles north of Warrior, US 31 and I-65 begin a three-mile concurrency between the interstate route's Exits 284 and 287.

North of Garden City, and passing through Cullman, Hartselle, Decatur and Athens, it is routed along multi-lane and often divided roadways.

US 31 also picks up the path of SR 6 here, in addition to the historical significance of being the former warpath of the Confederate forces during 1863 and 1864, during the American Civil War.

Upon leaving Columbia, US 31 develops a median and a speed limit not too dissimilar from that on I-65, which is still only a few miles to the east of the highway.

On the way up to Spring Hill, US 31 passes straight through a small, corner town little-known to be called Neapolis.

No more than two miles further up the highway, Saturn Pkwy has looped back around to begin its trajectory toward I-65, and there are more on and off ramps between SR 396 and US 31.

At this point, US 31 returns to its original two-lane, narrow road format, and has entered the rapidly-growing city of Spring Hill, where it serves as Main Street, then as Columbia Pike in the adjacent Thompson's Station.

North of Thompson's Station, US 31 intersects I-840, serving as exit 28 for the southern loop around Nashville.

As US 31 passes through Brentwood, the road widens, again to two lanes per side, and intersects with SR 254 at the point where it reaches less than a half of a mile from I-65 once again.

It enters Louisville from the southeast, where it is named Bardstown Road for most of its length in the city, while US 31W closely follows I-65 from the Tennessee state line, through Bowling Green, to Elizabethtown.

North of Fort Knox, US 31W joins with US 60 in Kentucky and it roughly follows the Ohio River into Louisville.

US 31 then passes through or near Carmel, Kokomo, Rochester, Plymouth and Lakeville, often following a historic Native American trail to what was once a French fur-trading outpost on Mackinac Island and later the British Fort Holmes.

As it passes Potato Creek State Park and approaches southern South Bend, US 31 travels concurrent with US 20 in Indiana and proceeds west then northward on the St. Joseph Valley Parkway, bypassing South Bend and proceeding into Michigan.

US 31 departs from I-196 as a divided highway with many Michigan lefts north to Grand Haven, where it becomes a freeway again.

There, US 31 intersects the northern end of US 131 before rounding the bay and heading northward and inland toward Mackinaw City.

South of Indianapolis, all segments of US 31 not traveling concurrently with I-65 have been reduced to roads largely of local use.

All of US 31 between Indianapolis and Ludington, Michigan, is divided highway—some of it is freeway, including a bypass route of South Bend, Indiana.

The segment between Indianapolis and South Bend is scheduled for upgrade or replacement with Interstate-standard freeway.

At its southern end, US 31 originally traveled further west than it currently does, passing through Spanish Fort, Alabama.