South Dakota/Highway Classification
With the permission of user elijahmathews, I (SD Mapman) have heavily based this page off of the ND highway classification page.
General Guidelines
These are based off of what the current state of classification is in South Dakota plus the new trunk road guidance.
- highway=motorway - This should only apply to I-90, I-29, I-229, and I-190. South Dakota has no non-interstate freeways, with only 6 "motorway islands" throughout the state (US 16/16A, Cambell St/Elk Vale Rd (Rapid City), Elk Vale Rd/SD 44, SD 50 Vermillion West, SD 50 Vermillion East, and US 14 Brookings West).
- highway=trunk - This should apply to high-speed divided highways in addition to a set of long-haul two-lane highways connecting major cities (list and discussion below).
- expressway=yes - Rurally, this should apply to the Heartland Expressway (SD 79 from Nebraska to Rapid City), the Eastern Dakota & Pierre expressway system (US 83 from I-90 to Pierre, SD 37 from I-90 Exit 330 to Huron, US 12 from Aberdeen to I-29, US 281 from SD 20 to Aberdeen, and SD 50 from Yankton to I-29), US 85 between Spearfish and Belle Fourche, and US 16 between Keystone and Rapid City. In Sioux Falls, this should apply to Russell St between I-29 and Minnesota Ave and the future SD 100/Veterans Parkway. In Rapid City, this should apply to the Catron Blvd/Elk Vale Rd beltway along the east side of town.
- highway=primary - This should be used for any remaining US highways and signed Interstate Business routes along with the most important state highways.
- highway=secondary - This should be used for the remaining state highways along with unusually important county roads.
- If a county road and state highway are marked as secondary in the same geographic area, check to see if they are in fact of similar classification level and if not, mark the state highway as higher than the county road.
- highway=tertiary - This should be used for most county roads.
- highway=unclassified - This should be used for the remaining unpaved gravel roads.
Population Centers
The following is a list of the twenty largest cities in South Dakota, along with their populations as of the 2020 United States census and the major highways that serve them.
| City | Population | Highways |
|---|---|---|
| Sioux Falls | 192,517 |        
|
| Rapid City | 74,703 |       
|
| Aberdeen | 28,495 |  
|
| Brookings | 23,377 |
|
| Watertown | 22,655 |   
|
| Mitchell | 15,660 |  
|
| Yankton | 15,411 |   
|
| Huron | 14,263 |
|
| Pierre | 14,091 |   
|
| Spearfish | 12,193 |  
|
| Box Elder | 11,746 |
|
| Vermillion | 11,695 | 
|
| Brandon | 11,048 |
|
| Sturgis | 7,020 |
|
| Harrisburg | 6,732 |
|
| Madison | 6,191 |
|
| Belle Fourche | 5,617 |
|
| Tea | 5,598 |
|
| Dell Rapids | 3,996 |
|
| Milbank | 3,554 | 
|
Note: Dell Rapids and Milbank can probably be removed from consideration, but I included them to fill out an even table.
Additionally, since the population of South Dakota is heavily weighted toward East River, the 10 largest cities in West River are presented below.
| City | Population | Highways |
|---|---|---|
| Rapid City | 74,703 |
|
| Spearfish | 12,193 |
|
| Box Elder | 11,746 |
|
| Sturgis | 7,020 |
|
| Belle Fourche | 5,617 |
|
| Hot Springs | 3,395 |   
|
| Lead | 2,982 |
|
| Summerset | 2,972 |
|
| Fort Pierre | 2,115 | 
|
| Custer | 1,919 |  
|
Inventory and proposals
In all tables below, a city name in bold as a terminus denotes that the highway classification reaches its terminus at the state border.
Motorways
Proposal
South Dakota's motorways are primarily limited to I-29 and I-90, which are limited-access freeways for their entire lengths across the state.
| Highway | S/W Terminus | N/E Terminus | Length (mi) |
|---|---|---|---|
|
Dakota Dunes | Rosholt | 253 |
|
Spearfish | Valley Springs | 413 |
There are additional limited-access freeways in Rapid City and Sioux Falls, I-190 and I-229. There are no non-Interstate full freeways in SD, with non-interstate grade separation limited to isolated interchanges.
| Highway | S/W Terminus | N/E Terminus | Length (mi) |
|---|---|---|---|
 |
Rapid City | Rapid City | 1 |
|
Tea | Sioux Falls | 10 |
Trunk highways
Proposal
The previous interpretation of "trunk highway" in South Dakota was effectively high-speed divided/four-lane highway. The proposed trunk highway network below is largely an expansion of the previous network of trunk highways. Any necessary cross-state collaboration will be discussed when the issue arises.
Note: SD mileposts are based off the WY/MT border (for E-W routes) and the NE border (for N-S routes). This can lead to routes having a "zero" milepost of 354 (e.g.) depending on where they start. Terminus in this case refers to the terminus of the trunk segment of the route, not necessarily the route itself.
| Highway | S/W Terminus City | N/E Terminus City | S/W Terminus Milepost | N/E Terminus Milepost | Length (mi) | Justification/Comments |
|---|---|---|---|---|---|---|
| US 12 |
Lemmon | Big Stone City | 80.50 | 399.74 | 319.24 | US 12 is the major E-W collector route for Northern SD, linking Lemmon, Mobridge, and Aberdeen with the Bakken to the west and the Twin Cities to the east. |
| SD 34 |
Sturgis | Hayes | 33.89 | 170.45 | 136.56 | SD 34 in combination with US 14 East River is the major E-W collector route for Central SD, linking Sturgis, Pierre, Huron, and Brookings. This route continues into MN towards Mankato. |
| US 14 |
Hayes | Brookings | 190.03 | 418.40 | 228.37 | US 14 in combination with SD 34 West River is the major E-W collector route for Central SD, linking Sturgis, Pierre, Huron, and Brookings. This route continues into MN towards Mankato. |
| US 14 Bypass |
Brookings | Brookings | 418.11 | 423.24 | 5.13 | Bypass for through traffic around Brookings. |
| US 14 |
Brookings | Elkton | 423.19 | 439.75 | 16.56 | US 14 in combination with SD 34 West River is the major E-W collector route for Central SD, linking Sturgis, Pierre, Huron, and Brookings. This route continues into MN towards Mankato. |
| SD 50 |
Yankton | Burbank | 383.83 | 417.02 | 33.19 | SD 50 between Yankton and I-29 forms part of the Eastern Dakota Expressway system, linking one of the larger cities in the state and one of the state universities with I-29. |
| US 18 |
Edgemont | Hot Springs | 0.00 | 62.27 | 62.27 | US 18 serves as the link from Wyoming and Colorado to the Heartland Expressway. This is the most convenient route for travelers from Rapid City and points east to reach Cheyenne, Casper, and Denver. Partially concurrent with US 385 and SD 79. Due to South Dakota's milepost priority system, this also includes the Heartland Expressway system from Hot Springs to Oelrichs. |
| US 385 |
Oelrichs | Oelrichs | 0.00 | 13.08 | 13.08 | This is part of the Heartland Expressway, which was intended to be a four-lane divided highway linking Denver and Rapid City. |
| SD 79 |
Hot Springs | Rapid City | 27.00 | 74.54 | 47.54 | This is part of the Heartland Expressway, which was intended to be a four-lane divided highway linking Denver and Rapid City. |
| US 16 Truck |
Rapid City | Rapid City | 67.63 | 73.11 | 5.48 | Heartland Expressway portion of the bypass for through traffic around Rapid City. |
| US 85 |
Spearfish | Ludlow | 44.69 | 154.88 | 110.19 | This is part of the Theodore Roosevelt Expressway, which connects the North Dakota oilfields with places south. |
| US 212 |
Belle Fourche | Belle Fourche | 0.00 | 13.46 | 13.46 | This is technically the fastest way from the Black Hills to Billings, MT. |
| US 83 |
Mission | Herreid | 0.00 | 240.73 | 240.73 | US 83 is the major N-S collector route for central SD, linking Pierre with Bismarck and North Platte. |
| SD 37 |
Mitchell | Huron | 73.08 | 127.74 | 54.66 | SD 37 to Huron is part of the Eastern Dakota Expressway System. |
| US 281 |
Wolsey | Frederick | 124.25 | 229.27 | 105.02 | US 28 is the major N-S collector route for east-central SD, linking Huron with Aberdeen and Jamestown, ND. |
| US 81 |
Yankton | Salem | 0.00 | 57.97 | 57.97 | This is an extension of the US 81 through route originating in Salina, KS. |
Additionally, due to the development of the Black Hills, the major tourist areas are served by "spur" highways to/from I-90. For instance, to access Custer, SD and the Southern Hills tourist areas (Mt. Rushmore, Crazy Horse, etc.) the vast majority of traffic takes US 16 to/from Rapid City, rather than coming from, say, Newcastle WY. Three rural trunk spurs are proposed to account for this phenomenon.
| Highway | S/W Terminus City | N/E Terminus City | S/W Terminus Milepost | N/E Terminus Milepost | Length (mi) | Justification/Comments |
|---|---|---|---|---|---|---|
| US 16 |
Custer | Hill City | 26.96 | 40.51 | 13.55 | US 16 is the collector route for the Southern Hills tourism area. |
| US 16 Truck |
Hill City | Hill City | 40.51 | 41.08 | 0.57 | Hill City bypass. |
| US 16 |
Hill City | Rapid City | 41.06 | 69.00 | 27.94 | US 16 is the collector route for the Southern Hills tourism area. |
| US 16 Truck |
Rapid City | Rapid City | 64.19 | 67.63 | 3.44 | Non-Heartland Expressway portion of the bypass for through traffic around Rapid City. |
| US 85 |
Deadwood | Spearfish | 29.18 | 36.96 | 7.78 | This is the link for eastbound travelers to reach the gaming center of Deadwood and the winter sports area of Lead. |
| US 14A |
Lead | Sturgis | 36.93 | 53.03 | 16.10 | This is the link for westbound travelers to reach the gaming center of Deadwood and the winter sports area of Lead. |
Additionally, Russell Street and Veterans Parkway in Sioux Falls will be included in the system, with Russell Street acting like a spur freeway and Veterans Parkway acting as a bypass (when finished).
Other highways
Proposal
The primary and secondary classification system in South Dakota is adequate as-is. HFCS can be used as a resource, but following that tends to over-classify highways, and should be utilized with a mapper's best judgement. Generally, identical-looking roads will be classified higher in higher population areas (the Sioux Falls metro area has a higher density of primary roads than Brookings, for example). See the general guidelines for further information.