County profile · official public datasets

Sully County, SD

31 public highway bridges tracked in the FHWA National Bridge Inventory: 45.2% rated Good, 35.5% Fair, 19.4% Poor.

Compiled by PlainBridges

31
Public bridges: 45.2%
Good condition: 19.4%
Poor condition

The verdict

According to the Federal Highway Administration's 2024 National Bridge Inventory.

Sully County posts a higher-than-median Poor-condition share among rated counties in the state across rated county bridge inventory.

44th
of 66 on Poor share: 19.4%
Poor-condition share: 31
public highway bridges: 1979
avg year built

Poor is an FHWA condition category from the lowest applicable component rating (4 or below), not a live crossing-safety or closure decision. Rankings cover counties with at least five public highway bridges.

Sully County Bridge Snapshot

According to the Federal Highway Administration's 2024 National Bridge Inventory, Sully County ranks #44 of 66 South Dakota counties with at least 5 public highway bridges on Poor-condition share (lower share ranks better; currently 19.4%). Sully County, South Dakota contains 31 public highway bridges. Roughly 45.2% are rated “Good,” 35.5% “Fair,” and 19.4% “Poor.” Poor means the lowest applicable deck, superstructure, substructure, or culvert rating is 4 or below on the 0–9 NBI scale. It is a condition category, not a live safety or closure decision.

Responsible agencies inspect covered bridges on risk-based intervals under current NBIS rules and submit inventory data to FHWA annually. The list below shows every NBI-tracked bridge in the county. Use the “Sort by” controls to change the ordering, and click any row for its reported inspection date, traffic count, and condition fields.

Source: FHWA National Bridge Inventory. See methodology for full definitions.

Total bridges

31

Public highway bridges in Sully County

Good condition

45.2%

14 of 31 rated 7-9

Fair condition

35.5%

11 of 31 rated 5-6

Poor condition

19.4%

6 of 31 rated 4 or below

In-state Poor-share rank

#44 of 66

Among South Dakota counties with 5+ bridges, lower share ranks better

Condition distribution

Share of 31 NBI-tracked bridges by lowest applicable component rating.

Good

45.2% (14)

Fair

35.5% (11)

Poor

19.4% (6)

Sully County's Poor-condition bridges carry 214 vehicle crossings a day

That is 3.9% of the county's reported daily bridge traffic on 19.4% of its bridges: Poor-condition structures carry less traffic than their share of the inventory. Each Poor bridge averages 36 crossings a day against 283 for a Good-condition one. A Poor rating is a condition category, not a closure or a live safety judgement; it means the lowest applicable component rating is 4 or below.

Computed over the 31 of 31 bridges in this county that report an average daily traffic count. Source: FHWA National Bridge Inventory, 2024 release.

51.6% of Sully County's bridges have passed the 50-year mark

16 of the 31 bridges with a recorded construction year were built in 1974 or earlier. Age alone is not a defect: a well-maintained older bridge can rate Good, and 45.2% of this county's inventory does. Even so, this older cohort is the population from which future Poor ratings are drawn, which is why the age profile below matters alongside the condition split above.

Cutoff derives from the 2024 release year minus a 50-year design life. Source: FHWA National Bridge Inventory, 2024 release.

One structure carries 35% of the county's Poor-condition bridge traffic

188 STREET over CK, built 1964, carries 75 vehicles a day, the most of any Poor-rated structure here. The county's Poor-condition exposure is concentrated rather than evenly spread, so the 6-bridge count understates how much of the daily burden sits on a single crossing.

Share is of the 214 daily crossings on Poor-rated bridges that report a traffic count. Source: FHWA National Bridge Inventory, 2024 release.

1910s

2
1930s

3
1950s

1
1960s

8
1970s

2
1980s

1
1990s

4
2000s

4
2010s

3
2020s

3

What this shows Construction peaked in the 1960s, when 8 of the county's bridges still in the inventory were built, which is 25.8% of those with a recorded year. That cohort is now 55 to 64 years old in the 2024 inventory. Its average build year (1979) matches the South Dakota average (1975).

How Sully County compares with South Dakota's closest counties by inventory size

The 5 South Dakota counties nearest Sully County in number of bridges, with their condition splits. Its 19.4% Poor share is the 5th-lowest of these 6. Its average build year of 1979 against a peer-group mean of 1976 is in line with the group.

Showing the 5 closest of 65 South Dakota counties eligible for comparison.

Bridge inventory size and condition shares for Sully County and the 5 South Dakota counties closest to it in inventory size
County	Bridges	Good	Fair	Poor	Avg. built
Day County	34	35.3%	29.4%	35.3%	1963
Oglala Lakota County	34	55.9%	38.2%	5.9%	1992
Walworth County	33	36.4%	60.6%	3%	1972
Sully County (this page)	31	45.2%	35.5%	19.4%	1979
Ziebach County	30	26.7%	63.3%	10%	1969
Dewey County	28	57.1%	28.6%	14.3%	1986

Each share is of that county's own total tracked bridges; peers are the counties with 5 or more bridges whose inventory size is closest to Sully County's. Source: FHWA National Bridge Inventory, 2024 release.

Nationwide counties with similar inventory size or average build year

Two NBI-derived peer sets for Sully County, both outside South Dakota so the neighborhoods are not the same-state size table above.

Similar inventory size

Nearest county inventories outside SD (31 bridges here).

Hinsdale County, CO · 31 bridges
Kiowa County, CO · 31 bridges
Ogemaw County, MI · 31 bridges
Grant County, MN · 31 bridges

Similar average build year

Nearest mean year-built outside SD (1979 here).

Pima County, AZ · avg 1979 · 1,150 bridges
Jefferson County, KY · avg 1979 · 662 bridges
Spartanburg County, SC · avg 1979 · 638 bridges
McPherson County, KS · avg 1979 · 586 bridges

Source: FHWA National Bridge Inventory, 2024 release. Peers floor at 10 bridges so stub inventories cannot own the match.

All 31 Good 14 Fair 11 Poor 6

All bridges (31)

Sort:

300 AVENUE

Over: CK

3W & 3.8N AGAR, Sully, SD

Built 1981 Load: 19.1 t ADT: 48

308 AVENUE

Over: OKOBOJO CK

4E & 0.4S AGAR, Sully, SD

Built 1974 Load: 19.1 t ADT: 25

172ND ST

Over: OKOBOJO CK

4N & 2.9W AGAR, Sully, SD

Built 1912 Load: 23.7 t ADT: 25

182 STREET

Over: CK

4W & 3N ONIDA, Sully, SD

Built 1960 Load: 24.5 t ADT: 49

306 AVENUE

Over: OKOBOJO CK

2E & 0.8S AGAR, Sully, SD

Built 1968 Load: 24.5 t ADT: 25

193 STREET

Over: CK

7S & 1.2E Onida, Sully, SD

Built 1968 Load: 24.5 t ADT: 25

309 AVENUE

Over: OKOBOJO CK

5E & 0.3N AGAR, Sully, SD

Built 1967 Load: 24.5 t ADT: 20

FAS 6155

Over: OKOBOJO CK

6.6E & 2.2N AGAR, Sully, SD

Built 1936 Load: 24.5 t ADT: 47

291 AVENUE

Over: OKOBOJO CK

3.5E & 0.6N OKOBOJO, Sully, SD

Built 1936 Load: 25.7 t ADT: 250

185 STREET

Over: MEDICINE KNOLL CK

14.2E ONIDA, Sully, SD

Built 1930 Load: 28.1 t ADT: 150

SD1804

Over: OKOBOJO CK

5.2 N HUGHES CO LINE, Sully, SD

Built 1972 Load: 47.4 t ADT: 634

296 AVENUE

Over: OKOBOJO CK

15.5W & 4.5S ONIDA, Sully, SD

Built 2010 Load: 23.6 t ADT: 48

196 ST

Over: COW CK

6.9S & 1.6E OKOBOJO, Sully, SD

Built 2006 Load: 24.5 t ADT: 180

295TH AVE

Over: CK

8W & 0.1S of Onida, Sully, SD

Built 2014 Load: 24.5 t ADT: 26

185 ST

Over: CK

6.2W ONIDA, Sully, SD

Built 2017 Load: 24.5 t ADT: 760

305 AVENUE

Over: CK

1E & 9.5S ONIDA, Sully, SD

Built 2009 Load: 24.5 t ADT: 35

317 AVENUE

Over: MEDICINE KNOLL CK

12.5 & 7.2S ONIDA, Sully, SD

Built 1995 Load: 24.5 t ADT: 25

185 STREET

Over: MEDICINE CK

13.6E ONIDA, Sully, SD

Built 2021 Load: 33.6 t ADT: 150

185 ST

Over: MEDICINE KNOLL CK

14.8E ONIDA, Sully, SD

Built 2005 Load: 38.6 t ADT: 25

172 STREET

Over: CK

4N & 4.3W OF AGAR, Sully, SD

Built 2023 Load: 41.3 t ADT: 48

305 AVENUE

Over: OKOBOJO CK

1.9S & 0.8E AGAR, Sully, SD

Built 1990 Load: 41.7 t ADT: 80

176 STREET

Over: OKOBOJO CK

4E AGAR, Sully, SD

Built 2023 Load: 43.1 t ADT: 230

US083

Over: OKOBOJO CK

3.4 S OF AGAR, Sully, SD

Built 1998 Load: 59.1 t ADT: 1,452

SD1804

Over: CK

6.0 N JCT CO RD TO ONIDA, Sully, SD

Built 1995 Load: 62.2 t ADT: 262

SD1804

Over: COW CK

0.4 N HUGHES CO LINE, Sully, SD

Built 2007 Load: 95.1 t ADT: 634

195 STREET

Over: MEDICINE KNOLL CK

10S & 8.2E ONIDA, Sully, SD

Built 1913 Load: 0.0 t ADT: 10

302 AVENUE

Over: OKOBOJO CK

3.7N & 2.5W ONIDA, Sully, SD

Built 1962 Load: 10.4 t ADT: 20

188 STREET

Over: CK

20.3W & 3S ONIDA, Sully, SD

Built 1964 Load: 24.5 t ADT: 75

301 AVENUE

Over: Okobojo Creek

3W & 3.4N ONIDA, Sully, SD

Built 1961 Load: 24.5 t ADT: 20

315 AVENUE

Over: MEDICINE CK

11E & 7.4S ONIDA, Sully, SD

Built 1956 Load: 24.5 t ADT: 64

320 AVENUE

Over: CK

16E & 10.2S ONIDA, Sully, SD

Built 1961 Load: 24.5 t ADT: 25

Read our methodology - how this data is sourced, computed, and verified.

Related

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Data sourced from official public datasets. See our methodology for details. Retrieved and formatted by PlainBridges

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Compiled by PlainBridges, Independent data publisher

Every figure on PlainBridges is rendered directly from federal source data, no number is typed in by an editor. This page draws directly on federal source data, no figure is typed in by an editor. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error. NBI condition categories reflect the latest reported inventory record, not a live safety, closure, or travel decision.