The final 17 miles (27 km) are part of a heavily traveled[citation needed] corridor providing the most direct route between Detroit, Toledo, Findlay, Marion and Columbus.

[citation needed] In 1926 the designation was reapplied to a route from Carey to the Indiana state line.

It replaced the rest of SR 9 in 1931, rerouted along its current route to the Michigan state line.

[citation needed] In 1966, SR 15 was rerouted on an expressway from I-75 in Findlay to US 23 in Carey, its new southern (eastern) terminus.

The highway runs a few miles south of its previous routing, which was recertified as SR 568 the same year.