State Route 15 is a north–south and east–west route in northwestern Ohio. Its southern (eastern) terminus is at its interchange with U.S. Route 23/State Route 103 (US 23/SR 103) near Carey, and its northern (western) terminus is at the Michigan state line north of Pioneer, where the route continues in Michigan as M-99. The route is signed east–west from Carey to Bryan, and it is signed north–south from there to the Michigan state line.

U.S. Route 23 (US 23) is a United States Numbered Highway that runs from Jacksonville, Florida, to Mackinaw City, Michigan. In the state of Ohio, it is a major north–south state highway that runs from the Kentucky border at Portsmouth to the Michigan border at Sylvania.