SR 166 Conn. begins at an intersection with US 27/SR 1 on the northern edge of Carrollton, where the roadway continues as Columbia Drive. The highway leaves the city limits and travels on a nearly due east direction until it intersects SR 113. It travels in a southeasterly direction and crosses over the Little Tallapoosa River, where it re-enters the city limits of Carrollton. After a gradual curve to the south-southeast, it meets its eastern terminus, an intersection with the SR 166 mainline.