Question: U.S. Route 431 is a spur of another highway whose northern terminus is south of what city?

Context paragraphs:
Alabama State Route 26: State Route 26 (SR 26) is a 14.560 mi state highway in Russell County in the southeastern part of the U.S. state of Alabama.  The western terminus of the highway is at its intersection with SR 51, and the eastern terminus of the highway is at its intersection with U.S. Route 431 (US 431) at the unincorporated community of Seale, approximately 20 mi southwest of Phenix City.

Alabama State Route 165: State Route 165 (SR-165) is a state highway in the U.S. state of Alabama.  It runs through Holy Trinity, Alabama in the eastern part of the state.  Its northern terminus is at U.S. Route 431 (US 431) south of Phenix City, Alabama, and its southern terminus is at US 431 5 mi north of Eufaula, Alabama.

Tennessee State Route 161: State Route 161 (SR 161) is a north–south state highway located entirely in Robertson County in Middle Tennessee.  The route’s southern terminus sits 0.4 mi north of a junction between U.S. Route 431 {(US 431) and US 41.  Its northern terminus is at the Kentucky state line, where it becomes Kentucky Route 102 (KY 102).

U.S. Route 331: U.S. Route 331 is a spur of U.S. Route 31.  It currently runs for 151 miles (243 km) from Montgomery, Alabama at U.S. Route 80 and U.S. Route 82 to Santa Rosa Beach, Florida east of Fort Walton Beach at U.S. Route 98.  Unlike U.S. Route 131, U.S. Route 231, and U.S. Route 431, U.S. 331 never intersects with its "parent" route, U.S. Route 31; the two routes come within 4 mi of each other in Montgomery.

Alabama State Route 30: State Route 30 (SR 30) is a 19.012 mi state highway in Barbour County in the southeastern part of the U.S. state of Alabama.  The western terminus of the highway is at an intersection with SR 51 at Clayton.  The route continues until its intersection with U.S. Route 431 (US 431) south of Eufaula.

U.S. Route 31: U.S. Route 31 or U.S. Highway 31 (US 31) is a long north–south highway connecting southern Alabama to northern Michigan.  Its southern terminus is at an intersection with US 90/US 98 in Spanish Fort, Alabama.  Its northern terminus is at an interchange with Interstate 75 (I-75) south of Mackinaw City, Michigan.

U.S. Route 431: U.S. Route 431 is a spur of U.S. Route 31.  It currently runs for 556 miles (895 km) from Owensboro, Kentucky at U.S. Route 60 to Dothan, Alabama, at U.S. Route 231 and U.S. Route 84.

Alabama State Route 77: State Route 77 or SR 77 is a 124 mi north–south state highway in the eastern part of the U.S. state of Alabama.  The southern terminus of the highway is at an intersection with U.S. Route 431 (US 431) near LaFayette.  The northern terminus of the highway is at an intersection with US 431 just north of Attalla.  North of Talladega, the highway passes the entrance to Talladega Superspeedway, home of the NASCAR Sprint Cup, Infinity Series, and Camping World Truck Series races that are held annually.

Tennessee State Route 106: Tennessee State Route 106 (SR 106) is a north–south state highway in Middle Tennessee.  The highway runs concurrently with U.S. Route 431 (US 431) for much of its course as it is a secret, or hidden, designation since US 431 was extended into Tennessee and Kentucky from Alabama in 1953.  SR 106 traverses Marshall, Maury, Williamson, and Davidson counties in the state.  The only portion of SR 106 that is actually signed is on the Lewisburg Bypass in Marshall County from US 31A (SR 11) on the south side to US 431 and SR 50 on the east side.

Kentucky Route 107: Kentucky Route 107 (KY 107) is a 56.907 mi east–west state highway in west–central Kentucky.  The western (southern) terminus of the route is an "end of state maintenance" terminus near the Fort Campbell military reservation south of Donaldson Creek near La Fayette, and its northern (eastern) terminus is at an intersection with U.s. Route 431 (US 431) in Lewisburg.

Supporting paragraphs: U.S. Route 431 U.S. Route 31
Answer: Mackinaw City, Michigan