Interstate 90 in South Dakota
In the U.S. state of South Dakota, Interstate 90 (I-90) traverses east–west through the southern half of the state. Route description. Interstate 90 enters South Dakota in Lawrence County as a four-lane divided highway. It enters concurrently with U.S. Route 14, and passes through the town of Spearfish, where it shares another concurrency with U.S. Route 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 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, Interstate 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 towards Wall on to Jackson County. Exits 110 and 131 for Badlands National Park to the south (SD 240) are within 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. 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, Interstate 29, at Sioux Falls, the largest city. Just east of Interstate 29 is where Interstate 90 meets the northern terminus of Interstate 229. East of Brandon and northeast of Valley Springs, it then ends  its trip through South Dakota of before entering Minnesota and Rock County, just west of the towns of Beaver Creek and Manley. Since April 2015 the speed limit is , except it is between I-229 and Marion Road in Sioux Falls; and through Rapid City. The speed limit remains between Rapid City and the Wyoming state line. Codified law. The South Dakota section of I-90 is defined at South Dakota Codified Laws § 31-4-184. History. I-90 was constructed in the 1970s. The four-lane interstate replaced old highway 16 as the main road connecting Rapid City and Sioux Falls. Much  of Old 16 remains intact, however I-90 has absorbed sections and likewise has reshaped the economies of many southern South Dakota towns. The final section of I-90 in the state opened in November 1976 between the Wyoming state line and Spearfish. 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. Removal of exit 66. In the city of Box Elder, exit 66, which crossed South Dakota Highway 435, was closed on October 1, 2003, after a 10-month delay . The exit was closed and removed as a result of the department of transportation's efforts to move traffic away from a potential crash site at the end of the Ellsworth AFB runway. The partial cloverleaf interchange of exit 66 was also demolished shortly thereafter. Exit 67, which was constructed not too long before the closure of exit 66, is now used as the main entrance and exit feeder to the base, and it is less likely to be impacted by a potential crash. Local citizens and officials feared the lack of entrances which access the Ellsworth Air Force Base  would cause traffic issues, and moving businesses from near the old exit would cause some to close and affect tax revenue. Land near exit 66 began to be purchased in 2004, including the former McDonald's property.