South Dakota Highway 8 was one of only two single digit state highway numbers known to be used in South Dakota. It was a state route across north central and northwest South Dakota, generally following what is now South Dakota Highway 20. The first designation of this route, in 1926, was South Dakota Highway 18. By the late 1920s, U.S. Highway 18 was established across southern South Dakota. The existence of two highway 18's was corrected around 1935, when the northern highway was redesignated as South Dakota 8. This number remained in use until the late 1960s, when SD 20 was extended west across the Missouri River, absorbing the SD 8 alignment.
South Dakota Highway 9 was a designation that was used twice.
This road went from Minnesota west to Sioux Falls. When the U.S. highway system was implemented in 1926, th road was designated as part of U.S. Highway 16. After a short period of dual signage, the SD 9 designation was dropped.
South Dakota Highway 12 was a state route that ran across north central and northeast South Dakota. South Dakota 12 was one of the numbers assigned to the Glacier Trail. In 1926, U.S. Highway 12 was implemented, and ran 15 or 20 miles (32 km) to the south (US 12 was South Dakota 16 before that). Because the two Highway 12 routes were so close, the state highway was redesignated as South Dakota Highway 10 in 1927.
South Dakota Highway 16 was a state route running across much of northern South Dakota. South Dakota 16 was designated in the 1920s, as a number for the Yellowstone Trail across the state. When the U.S. highway system was implemented in 1926, this road was designated as part of U.S. Highway 12. After a short period of dual signage, the SD 16 designation was dropped.
South Dakota Highway 24 was a state route located in west central and northwest South Dakota. When initially established in the mid-1920s, the western terminus of South Dakota 24 was at Whitewood, west of Sturgis. It traveled east along what is now South Dakota Highway 34 to near Marcus, then northward to Faith. This northward segment was shifted east around 1931, to what is now South Dakota Highway 73; it became part of the latter's alignment in 1936.
Around 1940, the western terminus was pushed a bit further, to U.S. Highway 85. A branch of SD 24, called South Dakota Highway 24A, extended northwest from Whitewood to Belle Fourche. The mainline SD 24 was rerouted over this branch around 1948, to its new terminus at Belle Fourche. A further extension was made in the early 1950s west from Belle Fourche to the Wyoming border.
Between 1957 and 1960, the route was eliminated by a westward extension of SD 34.
When U.S. 281 was assigned in the early 1930s, it was routed along the SD 41 alignment between the North Dakota border and U.S. 14. SD 41 remained separate south from there, as U.S. 281 continued south via Huron and Mitchell. By 1933, this segment of U.S. 281 was moved onto SD 41 as well, southward to U.S. Highway 16 at Plankinton. Around 1936, the dual signage was dropped, and the northern terminus of SD 41 was located at Plankinton.
No changes took place in the 1940s, however, in the early 1950s, another alignment change in U.S. 281 took over the remainder of SD 41, and the number was discontinued.
South Dakota Highway 54 was a short state route in southeast Gregory County, South Dakota. South Dakota 54 was implemented around 1926 as part of the state highway network. Its western terminus was at South Dakota Highway 50 (present day U.S. Highway 18) at Bonesteel. It ran south and east through Fairfax to the Nebraska border. By 1935, it was pulled back to Fairfax, at the intersection with the new U.S. Highway 281. Around 1953, due to the pending flooding of its old alignment, U.S. 18 was rerouted south onto this route, and SD 54 was limited to a 1-mile (1.6 km) segment north of the border. This last segment was eliminated around 1960, when U.S. 281 was rerouted onto it.
Around 1932, U.S. Highway 183 was implemented along the entire alignment of SD 59. The dual signage was removed by 1935. The road became part of U.S. Highway 83 in the 1940s, when the alignments of U.S. 83 and U.S. 183 were swapped.