[6] From Millersville to the Kentucky state line, US 31W/SR 41 also marks the boundary between Robertson and Sumner Counties.

The highway enters the state of Kentucky and crosses I-65 for a third time, where it passes through Franklin and intersects KY 100.

In the city of Horse Cave, US 31W intersects KY 218; and then continues on to Munfordville before crossing I-65 for a fifth time at the interstate's exit 65 interchange, and paralleling it closely to the west and later, the east.

In Elizabethtown, US 31W intersects with the Wendell H. Ford Western Kentucky Parkway before crossing I-65 for a seventh and final time.

It also passes through Fort Knox near Radcliff, within sight of the United States Bullion Depository.

The 31W Treasure Hunt is an outdoor second-hand sale held annually for three days beginning the second Thursday in September.

The southern end can be reached via either KY 1136 on the south side, or following the Western Kentucky Parkway from the at-grade intersection with US 31W/KY 61to the interchange after the WK Parkway/I-65 junction.

Its component highways include the Wendell H. Ford Western Kentucky Parkway west from the US 31W/KY 61 junction to exit 135, and the final 3.512 miles (5.652 km) of the US 31W Bypass from the WK Parkway exit 135 interchange to US 31W on the north side of town.

It is 1.988 miles (3.199 km) long and does not intersect any other highways outside of the main alignment of US 31W (which runs concurrently with US 60 in that area).