Interstate 90 in South Dakota

I-90 enters South Dakota in Lawrence County as a four-lane divided highway. It enters concurrently with US Highway 14 (US 14) and passes through the town of Spearfish, where it shares another concurrency with US 85 from exit 10 to exit 17. From there it passes several miles north and east of the tourist town of Deadwood before entering Meade County, going just to the west of Sturgis. Another concurrency is with South Dakota Highway 34 (SD 34) from exit 23 at Whitewood to exit 30, the west exit of Sturgis, where there starts a concurrency with SD 79. The freeway generally passes along the north and east edges of the Black Hills.

The route then enters Pennington County, where it passes through the northern edge of Rapid City, gateway to the Black Hills and the nearest passing to Mount Rushmore National Memorial, as well as other scenic attractions including Custer State Park and the Needles Highway. Rapid City is also where the interstate has its westernmost auxiliary route, I-190, which links the Interstate to the western edge of the downtown area. SD 79's concurrency ends at exit 61 on the east side of the city. From Rapid City, the route heads east, passing between Box Elder to the south and Ellsworth Air Force Base to the north before heading toward Wall on to Jackson County. Exits 110 and 131 for Badlands National Park to the south (SD 240) are within 15 miles (24 km) either way of the county line. The concurrency for US 14 ends at exit 112 just east of Wall; here, US 14 runs due east and will connect to the city of Philip and the capital city of Pierre.

The Lake Francis Case portion of the Missouri River viewed from a rest stop just south of Chamberlain; the pre-Interstate Chamberlain Bridge is in the center

From there, the route passes mostly east–west through Jackson and Jones counties. There are short concurrencies with SD 73 (from the west entering Kadoka) and SD 63 in Jackson County. I-90 crosses the Central–Mountain timezone boundary on the west edge of Jones County between milemarkers 174 and 175. Then, at Murdo, a concurrency begins with US 83 at exit 192 and continues to exit 212. It is in Lyman County, at exit 212 in Vivian, that the highway provides its closest link to the capital of Pierre, along US 83, some 35 miles (56 km) north. The road crosses the Missouri River at Lake Francis Case, between the towns of Oacoma and Chamberlain, also crossing into Brule County. I-90 Business (I-90 Bus.) runs from exit 260 at Oacoma to downtown Chamberlain across the Missouri River on the nearby Chamberlain Bridge and returns to I-90 at exit 265. Beginning there is a short concurrency with SD 50 heading to Pukwana, and also in Brule County is another short concurrency with SD 45 starting at Kimball.

I-90 then continues through Aurora County (connecting to White Lake and Plankinton), Davison County (where Mt. Vernon, Mitchell, and the Corn Palace are located), Hanson County (Alexandria), and McCook County (Salem). Then comes the state's most populous county, Minnehaha. After passing the communities of Humboldt and Hartford, the Interstate intersects the only other major Interstate in the state, I-29, at Sioux Falls, the largest city. Just east of I-29 is where I-90 meets the northern terminus of I-229. East of Brandon and northeast of Valley Springs, it then ends its trip through South Dakota of 413 miles (665 km) before entering Minnesota and Rock County, just west of the towns of Beaver Creek and Manley.

Since April 2015, the speed limit is 80 mph (130 km/h) between the Rapid City and the Minnesota state line, except for the segment between I-229 and Marion Road in Sioux Falls, where the speed limit is 65 mph (105 km/h). The speed limit remains 75 mph (121 km/h) between Rapid City and the Wyoming state line.

I-90 was constructed in the 1970s. The four-lane Interstate replaced old US 16 as the main road connecting Rapid City and Sioux Falls. Much of Old US 16 remains intact; however, I-90 has absorbed sections and likewise has reshaped the economies of many southern South Dakota towns.[4] Driver-oriented businesses and hotels opened near new exits on I-90, along with roadside attractions to lure in tourists.[5] The final section of I-90 in the state opened in November 1976 between the Wyoming state line and Spearfish.[6]

On May 24, 2012, the South Dakota Transportation Commission declared the entire length of I-90 as part of the National Purple Heart Trail, naming it the Purple Heart Memorial Highway, named after the military decoration of the same name.[7]

• Media related to Interstate 90 in South Dakota at Wikimedia Commons