SR 666 begins at a
traffic signal with
SR 60 and
SR 146, on the northeast side of Zanesville. The route heads east as a two-lane highway, before curving northeast. While passing through Zanesville the route passes woodland, with some
commercial businesses. The highway makes a sharp curve due west, before another sharp curve northeast. After the second curve the route leaves Zanesville, running parallel to the Muskingum River and an
Ohio Central Railroad track. The road makes a sharp curve northwest and passes through a one-lane 10'11"-clearance underpass under the Ohio Central Railroad track. After passing under the railroad track the route curves north-northeast and the railroad track curves due east. The roadway passes through the
Zanesville State Forest Nursery, followed by a curve northwest, passing through the state forest nursery again. After leaving the state forest nursery, the highway passes through farmland heading due north. The route curves northeast passing through the state forest nursery for the last time and the highway has an intersection with County Road 40 (CR 40), in rural
Jefferson Township. Following this intersection, SR 666 turns due west, before curving towards the north. This section of roadway passes mostly through farmland, with wooded areas when the route is close to the river. SR 666 ends at an intersection with
SR 208 near Dresden, on the east bank of the Muskingum River. SR 666 is not part of the
National Highway System, a system of routes important to the nation's economy, mobility and defense. The highway is maintained by the
Ohio Department of Transportation (ODOT). ODOT's 2010
annual average daily traffic (AADT) calculations showed that the lowest traffic levels were present on the section that is at the intersection with
Washington Township Road 112 (Bateman Road), where only 510 vehicles used the highway daily; the peak traffic volume was 5,340 vehicles AADT along a section of SR 666 near its southern terminus. ==History==