Alabama

Southern terminus of US 31 in Spanish Fort, AL

US 31 begins in Spanish Fort, Alabama at a junction with US 90 and 98. Formerly, the route originated in Mobile, co-routed with US 90 and 98. Whereas Interstate 65 leaves the Mobile Metropolitan Area via a route on the west side of the city, US 31 leaves the Mobile area via a route on the eastern side of the metro area, passing through Bay Minette. I-65 has effectively replaced US 31 as the preferred route for through traffic, making US 31 a local connecting route.

Near Atmore, the route passes extremely close to the Florida Panhandle's extreme northwestern corner, missing by approximately 1000 feet.

At Flomaton, US 31 begins a 15-mile virtual east–west concurrency with US 29 that continues to Brewton. North of Brewton, US 31 assumes a northeast–southwest trajectory, passing through rural areas and small towns in Escambia and Conecuh Counties. In Evergreen, US 31 north and US 84 share a brief concurrency heading north and east out of town.

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. The two routes do not junction each other again for another 22 miles. US 31 is routed along a bypass of Montgomery, the state capital. At Prattville, this route intersects U.S. Route 82.

North of Prattville, US 31 passes through rural areas of Autauga and Chilton Counties, primarily along two-lane roadways. Between Prattville and Alabaster, US 31 has three interchanges with I-65.

Between Saginaw and Warrior, US 31 is routed along multi-laned routes as it passes through the Birmingham-Hoover Metropolitan Area. It passes through suburbs such as Alabaster, Pelham, Hoover and Vestavia Hills. In Homewood, US 31 merges with U.S. Route 280 and is routed along the Elton B. Stephens Expressway, a 2.6-mile limited access highway that connects the south suburbs with downtown Birmingham. US 31 and 280 are co-routed until the expressway junctions I-20/ 59 just northeast of downtown. This interchange serves as the western terminus of US 280, and north of the interchange US 31 is routed along surface streets as it proceeds northwardly out of Birmingham.

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. At Warrior, the roadway shrinks to two lanes. Two miles north of Warrior, US 31 and I-65 begin a three-mile concurrency between the interstate route's Exits 284 and 287. The interstate route was constructed over US 31's roadway through this segment.

Between the split from I-65 and Garden City, US 31 is routed along a narrow two-lane route. North of Garden City, and passing through Cullman, Hartselle, Decatur and Athens, it is routed along multi-lane and often divided roadways.

At Athens, I-65 and US 31 begin a concurrency that continues to just north of the Tennessee state line. In both states, US 31 is largely unsigned.

In addition to junctions previously cited, US 31 junctions U.S. Route 80 in Montgomery, Interstate 459 in Hoover, U.S. Route 78 in Birmingham, U.S. Route 278 in Cullman, U.S. Route 72 Alternate in Decatur, and U.S. Route 72 in Athens.

Given US 31's former importance as a major connecting route in Alabama, in several cities it is known by the name of the city it leads to. For example, south of Montgomery it is still named Mobile Highway, and northwardly it is referred to as the Birmingham Highway. Similarly, south of Birmingham it is referred to as Montgomery Highway, and northwardly it is referred to as the Decatur Highway. Throughout north Alabama plaques and signage refer to the route as the Beeline Highway.

US 31 is co-routed with unsigned State Route 3 (SR 3) throughout the state until its junction with I-65 north of Athens.

Tennessee

US 31 northbound on 8th Avenue in Nashville

The first mile in Tennessee, US 31 runs concurrently with I-65, somewhat parallel to which it runs until it splits in downtown Nashville, to become US 31W and US 31E, a relatively uncommon occurrence in U.S. Highways, though not unlike that which occurs to US 49 and US 45 in Mississippi, U.S. Route 19 in North Carolina and Tennessee, and US 11 in Tennessee.

While upon crossing the state line from Alabama into Tennessee, US 31 (also State Route 7 [SR 7] at this point) runs to the east of I-65, it crosses over I-65, changing names from Main Street to Elkton Pike on the overpass. According to this name change, US 31 runs through Elkton shortly after making this crossing.

After passing through Elkton, US 31 wanders, away from I-65, towards the city of Pulaski. On its way to Pulaski, US 31 intersects with US 64 and does so once again in the center of the city. In Pulaski, US 31 serves as 1st Street in the downtown area. Just north of the downtown area of Pulaski, US 31 Alternate (US 31A) branches off US 31, serving first as East Grigsby Street, then as Lewisburg Highway, never to be visible from US 31 again until reaching downtown Nashville.

Continuing north, US 31 meanders, following Richland Creek for a portion of the way, named Columbia Highway; naturally, as follows the typical naming trend for former connecting routes, halfway to Columbia, the name changes to Pulaski Highway. US 31 runs by Lynnville in this process.

As US 31 enters Columbia, upon crossing James Campbell Boulevard (also SR 50), it is named Carmack Boulevard, at this point a decently wide road with two lanes on each side. While in Columbia, US 31 changes names two more times, once to South Garden Street (also temporarily sharing routes with US 412 and SR 99 in this area), and then to Nashville Highway while SR 7 departs from the street. It is a short stretch until US 31 hits an intersection with Bear Creek Pike. This is the northern terminus of US 43, and also the point at which US 412/SR 99 resumes its eastern trajectory, away from US 31. 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. At this location, there is an on-ramp onto Saturn Pkwy (SR 396), named as such because it was once a highway connecting I-65 to the assembly plant belonging to Saturn Corporation, which no longer acts independently from its parent company, General Motors. 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, a soon-to-be home of a golf course and luxury home development opened by Tiger Woods.

North of Thompson's Station, US 31 intersects I-840, serving as exit 28 for the southern loop around Nashville. Further up the highway, US 31 shares an interchange with Goose Creek Pike, a direct route to I-65. US 31 continues up through historic Franklin, as Columbia Avenue. In the middle of Franklin, US 31 comes to a five-point intersection with the routes of US 431 and SR 96, and a roundabout two blocks further north which continues the route of SR 96 to the east. US 31 then proceeds out of Franklin and to the northeast.

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. At this point, US 31 remains within this distance from I-65 until reaching its split in downtown Nashville, traveling through the sites of the Battle of Franklin and the Battle of Nashville in the process.

Entering Nashville, US 31 first shares an interchange with SR 155 (at this point Thompson Lane / Woodmont Boulevard), and, immediately afterwards, travels under an interchange between I-65 and I-440. After passing through Berry Hill, US 31 yet again travels under an interchange, this time between I-65 and I-40. Then, US 31 reaches another roundabout, where it intersects with US41, US 31A, and US 70S. Going through Nashville, US 31 retains several names: 8th Avenue South, Rosa L Parks Boulevard, James Robertson Pkwy, and 4th Avenue North, respectively as US 31 progresses northward. After making a semi-circle, crossing the Cumberland River, and passing under I-24, US 31 makes the split into US 31W and US 31E.

Kentucky

US 31E stays east of I-65, passing through Glasgow and Bardstown. 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. From here, it diverges to the west and passes through Fort Knox. North of Fort Knox, US 31W joins with US 60 in Kentucky and it roughly follows the Ohio River into Louisville. In southwestern Louisville, US 31W is called Dixie Highway. It is the major commercial street in this area.

In Downtown Louisville at Second Street, US 31E and US 31W rejoin to form US 31 and immediately crosses the Ohio River on the Clark Memorial Bridge only traveling 0.912 miles (1.468 km) before the route crosses into Indiana.^[2]

Indiana

US 31 travels concurrent with I-65 in Jeffersonville, after having crossed the Ohio River from Kentucky. It splits off north of Jeffersonville at Clarksville and proceeds to Indianapolis. At I-465 on the south side of Indianapolis, US 31 is now routed onto I-465 on the east side of the city. US 31 exits I-465 in extreme southern Hamilton County, and continues northward. 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.

Michigan

Northern terminus of US 31 in Mackinaw Township

US 31 follows the St. Joseph Valley Parkway northward into southern Michigan to an interchange with I-94 near Benton Harbor–St. Joseph, then runs concurrently north to I-196. US 31 then runs concurrently with I-196 along the shore of Lake Michigan north to the Holland area. US 31 departs from I-196 as a divided highway with many Michigan lefts north to Grand Haven, where it becomes a freeway again. This freeway continues northward through the Muskegon area until Ludington, where the freeway ends and US 31 turns east concurrently with US 10 to Scottville before turning northward again toward Manistee.^[3]

At Beulah, US 31 runs eastward into Grand Traverse County. South of Traverse City, US 31 turns northward along M-37, running concurrently into the city. Next to Grand Traverse Bay, US 31/M-37 turns easterly along M-72; the three highways run together along the bay before separating. M-37 turns northward up the Old Mission Peninsula, and M-72 turns easterly at Acme. US 31 continues northward along the eastern shore of Grand Traverse Bay, running eastward through Charlevoix and along Little Traverse Bay into Petoskey. There, US 31 intersects the northern end of US 131 before rounding the bay and heading northward and inland toward Mackinaw City. US 31 ends at a directional interchange with I-75 south of Mackinaw City.^[3]