County profile · official public datasets

Ziebach County, SD

30 public highway bridges tracked in the FHWA National Bridge Inventory: 26.7% rated Good, 63.3% Fair, 10% Poor.

30
Public bridges: 26.7%
Good condition: 10%
Poor condition

The verdict

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

Ziebach County pairs a better-than-median Poor-condition share among rated counties in the state with an older average build year than the state inventory.

Top 32%
lowest Poor share in state: 10%
Poor-condition share: 30
public highway bridges: 1969
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.

Ziebach County Bridge Snapshot

According to the Federal Highway Administration's 2024 National Bridge Inventory, Ziebach County ranks #21 of 66 South Dakota counties with at least 5 public highway bridges on Poor-condition share (lower share ranks better; currently 10%). Ziebach County, South Dakota contains 30 public highway bridges. Roughly 26.7% are rated “Good,” 63.3% “Fair,” and 10% “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

Public highway bridges in Ziebach County

Good condition

26.7%

8 of 30 rated 7-9

Fair condition

63.3%

19 of 30 rated 5-6

Poor condition

10%

3 of 30 rated 4 or below

In-state Poor-share rank

#21 of 66

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

Condition distribution

Share of 30 NBI-tracked bridges by lowest applicable component rating.

Good

26.7% (8)

Fair

63.3% (19)

Poor

10% (3)

Ziebach County's Poor-condition bridges carry 682 vehicle crossings a day

That is 6.2% of the county's reported daily bridge traffic on 10% of its bridges: Poor-condition structures carry less traffic than their share of the inventory. Each Poor bridge averages 227 crossings a day against 389 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 30 of 30 bridges in this county that report an average daily traffic count. Source: FHWA National Bridge Inventory, 2024 release.

60% of Ziebach County's bridges have passed the 50-year mark

18 of the 30 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 26.7% 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 96.2% of the county's Poor-condition bridge traffic

IRR SD063 over DUPREE CK, built 1961, carries 656 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 3-bridge count understates how much of the daily burden sits on a single crossing.

Share is of the 682 daily crossings on Poor-rated bridges that report a traffic count. Source: FHWA National Bridge Inventory, 2024 release.

1930s

3
1940s

1
1950s

4
1960s

9
1970s

3
1980s

5
1990s

4
2000s

1

What this shows Construction peaked in the 1960s, when 9 of the county's bridges still in the inventory were built, which is 30% of those with a recorded year. That cohort is now 55 to 64 years old in the 2024 inventory. Its average build year is 1969, 6 years older than the South Dakota average of 1975.

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

The 5 South Dakota counties nearest Ziebach County in number of bridges, with their condition splits. Its 10% Poor share is the 3rd-lowest of these 6. Its average build year of 1969 against a peer-group mean of 1976 is 7 years older.

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

Bridge inventory size and condition shares for Ziebach 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
Walworth County	33	36.4%	60.6%	3%	1972
Sully County	31	45.2%	35.5%	19.4%	1979
Ziebach County (this page)	30	26.7%	63.3%	10%	1969
Dewey County	28	57.1%	28.6%	14.3%	1986
Mellette County	28	32.1%	60.7%	7.1%	1980

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 Ziebach 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 Ziebach County, both outside South Dakota so the neighborhoods are not the same-state size table above.

Similar inventory size

Nearest county inventories outside SD (30 bridges here).

Nome County, AK · 30 bridges
Bleckley County, GA · 30 bridges
Lake County, MI · 30 bridges
Box Butte County, NE · 30 bridges

Similar average build year

Nearest mean year-built outside SD (1969 here).

Cook County, IL · avg 1969 · 1,721 bridges
Fresno County, CA · avg 1969 · 916 bridges
Cuyahoga County, OH · avg 1969 · 871 bridges
Ventura County, CA · avg 1969 · 501 bridges

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

All 30 Good 8 Fair 19 Poor 3

All bridges (30)

Sort:

Ziebach County Bridge Snapshot

Condition distribution

Ziebach County's Poor-condition bridges carry 682 vehicle crossings a day

60% of Ziebach County's bridges have passed the 50-year mark

One structure carries 96.2% of the county's Poor-condition bridge traffic

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

Nationwide counties with similar inventory size or average build year

Similar inventory size

Similar average build year

All bridges (30)

IRR COUNTY ROAD

173 ST (MEADE CO)

IRR SD065

IRON LIGHTENING RD

IRR BIA RTE 9

IRR BIA RTE 6

LEEDOM PIKE RD

THUNDER BUTTE ROAD

IRR US212

THUNDER BUTTE ROAD

IRR BIA RTE 6

IRR BIA RTE 12

IRR BIA RTE 12

IRR BIA RTE 6

IRR US212

IRR US212

IRR SD063

IRR SD063

IRR SD065

FOUR CORNER ROAD

IRR SD020

IRR BIA RTE 13

IRR BIA RTE 11

IRR BIA RTE 6

IRR SD065

IRR SD065

IRR SD063

IRR COUNTY ROAD

FOUR CORNERS ROAD

IRR SD063

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