State Route 166 (SR 166) starts at the Alabama line west of Carrollton in Carroll County. It goes around Carrollton on a by-pass route constructed in the mid-1900s. From there, SR 166 enters Douglas County where it remains mostly rural. After a short north–south concurrency with SR 92, it enters Fulton County by crossing the Chattahoochee River. Once in Fulton County, SR 166 intersects Fulton Industrial Boulevard/Cascade-Palmetto Highway (State Route 70/State Route 154). SR 154 joins SR 166 at this intersection. Campbellton Road (SR 166 / SR 154) crosses Camp Creek Parkway (State Route 6), entering the Sandtown community on its way to the Ben Hill community.

SR 166 and SR 154 leave the Ben Hill community as Arthur B. Langford Jr. Parkway, which is a limited access freeway that was originally named the Lakewood Freeway and was planned as Interstate 420. It continues into East Point, passing the Fort McPherson Army base. SR 154 leaves Langford Parkway at Main Street (U.S. Route 29 (US 29) / SR 14 / SR 139). After crossing the Downtown Connector, S.R. 166 ends into Lakewood Avenue in southeast Atlanta (Lakewood Heights).

SR 166 was originally supposed to extend (as I-420) from I-20 near Douglasville in the west to the DeKalb County seat of Decatur in the east. The eastern portion of this was fought bitterly by residents in court, and was eventually cancelled, along with a number of other interstate highways in Atlanta. The old I-420 was planned to meet the cancelled portion of Georgia 400 near Moreland Avenue, and then end at an interchange with I-20 in Decatur. 166 is multiplexed with SR 154 from SR 70 (Fulton Industrial Boulevard/Cascade-Palmetto Highway) in the west to the US 29 interchange at Lee Street/Main Street near Fort McPherson in the east.

In 1995, the Georgia Senate moved to rename the then Lakewood Freeway to the Arthur Langford, Jr. Memorial Parkway in honor of the former Atlanta city councilman and Georgia state senator. Since 2003, GDOT has widened 166 between Greenbriar Parkway and Perkerson Road.

The entire road is in Fulton County.

Interstate 420 (I-420) was a highway planned to split off I-20 at Douglasville, Georgia on the west side of metro Atlanta, then continue eastward to the south of downtown Atlanta, merging at a highway interchange with proposed Interstate 475 and traveling northeastward with it back to I-20. The only portion of it actually built was state route 166, known as Lakewood Freeway and now Langford Parkway. The portion west of I-285 on the west side, and east of I-75 were never built, due to public opposition from communities in its path.[citation needed] The I-675 it was to connect to was however built and currently runs between Interstate 75 near Stockbridge in Henry County to Interstate 285 near Panthersville in DeKalb County. I-475 was later used for the bypass route around Macon, Georgia.