State Route 309 is an east–west highway in central Ohio. Its western terminus is at its interchange with U.S. Route 30 (US 30) near Delphos, and its eastern terminus is at its interchange with US 30 in Mansfield. Its current route takes it through the cities of Lima, Kenton, Marion, Galion, and Ontario; as well as the villages of Elida and Caledonia.

U.S. Route 30 (US 30) is a United States Numbered Highway that runs from Astoria, Oregon, to Atlantic City, New Jersey. In the state of Ohio, it is a major, 245-mile-long (394 km), east–west highway that runs through the northern section of the state. Overall, the highway runs through rural areas dominated by farm fields or woodlands; some segments are urban in character in the Mansfield and Canton areas.