U.S. Route 23 in Florida

U.S. Highway 23 (US 23) is a United States Numbered Highway that runs from Jacksonville, Florida, to Mackinaw City, Michigan. In the U.S. state of Florida, US 23 is concurrent with US 1 south of Alma, Georgia, except in Downtown Jacksonville. US 23 is also concurrent with US 301 between Homeland, Georgia, and Callahan. In the Jacksonville area, US 23 is the unsigned State Road 139 (SR 139), which also continues east from the south end of US 23 along SR 10A to SR 115 near the Mathews Bridge.

Route description
US 23 begins at US 1 (Main Street) at the northern end of Downtown Jacksonville, starting as a one-way pair, with the northbound lanes meeting with Florida State College at Jacksonville. It is also unsigned SR 139 from its southern terminus to its interchange with US 1 in northwestern Jacksonville (SR 139 continues east along SR 10A from the end of US 23 to SR 115). Additionally, it has a wrong-way concurrency with US 17 from the beginning at US 1 to exits 363B and 363C on Interstate 95 (I-95). West of I-95, US 23 ends the one-way pair, continuing as Kings Road through northwestern Jacksonville, as an off-grid road. A few miles to the northwest in the Grand Park section of Jacksonville, US 23 meets with US 1/SR 15 (Martin Luther King Jr. Parkway), running concurrently with the highway through the rest of its journey through Florida. The road continues northwest, intersecting with I-295, and eventually makes its way out of Jacksonville. At Callahan, US 1/US 23 meets US 301, beginning a three-way concurrency as the road continues northward through Hilliard and Boulogne toward the St. Marys River, leaving Florida and entering Georgia.

History
Beginning in 1950, US 23's southern terminus was extended into Florida approximately 319 mi south from Atlanta, Georgia, to Jacksonville. From 1956 until 1993, US 23 signs in Florida featured white numbering on a green shield. The "color-coding" of U.S. Highways by the Florida Department of Transportation was stopped when the state could no longer use federal funds to replace the signs with anything but the standard black-and-white version. Some green US 23 signs may still remain.