US 45 is notable for incorporating, in its maiden alignment, the first paved road in the South, a 49-mile (79 km) segment in Lee County, Mississippi.

US 45 is concurrent with unsigned SR 17 between Mobile and Vinegar Bend, just north of Deer Park, in Washington County, Alabama.

The alternate roadway provides a more direct and entirely four-laned route between Meridian and Tupelo, bypassing Columbus to the west and, more closely, Starkville to the east.

Southern section From the Mississippi state line, US 45 extends north past the cities of Guys, Eastview, Selmer, Bethel Springs, Finger, Henderson, Pinson, and Jackson to Three Way, where the highway splits into US 45W and US 45E.

U.S. Route 45W US 45W is a 62.33-mile-long (100.31 km) state highway in West Tennessee, connecting Jackson with South Fulton via Humboldt, Trenton, Dyer, Rutherford, Kenton, and Union City.

U.S. Route 45E US 45E is a 61.23-mile-long (98.54 km) state highway in West Tennessee, connecting Jackson with South Fulton via Medina, Milan, Bradford, Greenfield, Sharon, and Martin.

U.S. 45 enters Kentucky at Fulton then northeast past Mayfield then heads directly north into Paducah as a four-lane highway.

It runs concurrent with Interstate 894 and U.S. Route 41 through the west side of metro Milwaukee to form a major artery through the metropolitan area.

A $112 million plan to straighten US 45 at McCullough Boulevard, Hilda Avenue and Front Street in a series of interchanges to solve congestion and safety issues in this area.