County profile · official public datasets

Plymouth County, IA

432 public highway bridges tracked in the FHWA National Bridge Inventory: 52.1% rated Good, 26.4% Fair, 21.5% Poor.

432
Public bridges
52.1%
Good condition
21.5%
Poor condition

The verdict

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

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

60th
of 99 on Poor share
21.5%
Poor-condition share
432
public highway bridges
1974
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.

Plymouth County Bridge Snapshot

According to the Federal Highway Administration's 2024 National Bridge Inventory, Plymouth County ranks #60 of 99 Iowa counties with at least 5 public highway bridges on Poor-condition share (lower share ranks better; currently 21.5%). Plymouth County, Iowa contains 432 public highway bridges. Roughly 52.1% are rated “Good,” 26.4% “Fair,” and 21.5% “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

432

Public highway bridges in Plymouth County

Good condition

52.1%

225 of 432 rated 7-9

Fair condition

26.4%

114 of 432 rated 5-6

Poor condition

21.5%

93 of 432 rated 4 or below

In-state Poor-share rank

#60 of 99

Among Iowa counties with 5+ bridges, lower share ranks better

Condition distribution

Share of 432 NBI-tracked bridges by lowest applicable component rating.

Good
52.1% (225)
Fair
26.4% (114)
Poor
21.5% (93)

Plymouth County's Poor-condition bridges carry 3,697 vehicle crossings a day

That is 0.8% of the county's reported daily bridge traffic on 21.5% of its bridges: Poor-condition structures carry less traffic than their share of the inventory. Each Poor bridge averages 40 crossings a day against 938 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 432 of 432 bridges in this county that report an average daily traffic count. Source: FHWA National Bridge Inventory, 2024 release.

46.1% of Plymouth County's bridges have passed the 50-year mark

199 of the 432 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 52.1% 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 17% of the county's Poor-condition bridge traffic

FM over STEPHENS CREEK, built 1960, carries 628 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 93-bridge count understates how much of the daily burden sits on a single crossing.

Share is of the 3,697 daily crossings on Poor-rated bridges that report a traffic count. Source: FHWA National Bridge Inventory, 2024 release.

When Plymouth County's bridges were built

432 bridges with a recorded construction year, grouped by decade

bridges

What this shows Construction peaked in the 2000s, when 67 of the county's bridges still in the inventory were built, which is 15.5% of those with a recorded year. That cohort is now 15 to 24 years old in the 2024 inventory. Its average build year is 1974, 5 years older than the Iowa average of 1979.

Source FHWA National Bridge Inventory (year built) As of 2024 NBI release

How Plymouth County compares with Iowa's closest counties by inventory size

The 5 Iowa counties nearest Plymouth County in number of bridges, with their condition splits. Its 21.5% Poor share is the 4th-lowest of these 6. Its average build year of 1974 against a peer-group mean of 1982 is 8 years older.

Showing the 5 closest of 98 Iowa counties eligible for comparison.

Bridge inventory size and condition shares for Plymouth County and the 5 Iowa counties closest to it in inventory size
County Bridges Poor
Black Hawk County 489 6.5%
Woodbury County 455 21.1%
Sioux County 449 2.9%
Plymouth County (this page) 432 21.5%
Jasper County 382 29.1%
Benton County 374 23%

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 Plymouth 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 Plymouth County, both outside Iowa so the neighborhoods are not the same-state size table above.

Similar inventory size

Nearest county inventories outside IA (432 bridges here).

Similar average build year

Nearest mean year-built outside IA (1974 here).

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

All 432 Good 225 Fair 114 Poor 93

Good-condition bridges (225)

250TH ST

Good

Over: PERRY CREEK

091472201, Plymouth, IA

Built 2003 Load: 50.0 t ADT: 45

FM

Good

Over: DEER CREEK

091482204, Plymouth, IA

Built 2003 Load: 50.0 t ADT: 429

NOBLE AVE

Good

Over: PLYMOUTH CREEK

092441902, Plymouth, IA

Built 2000 Load: 50.0 t ADT: 80

FM

Good

Over: BULL RUN

092473201, Plymouth, IA

Built 1995 Load: 50.0 t ADT: 838

260TH ST

Good

Over: CREEK

091462901, Plymouth, IA

Built 2002 Load: 50.2 t ADT: 26

280TH ST

Good

Over: JOHNS CREEK

090440101, Plymouth, IA

Built 1992 Load: 52.7 t ADT: 5

300TH ST

Good

Over: PERRY CREEK

090471503, Plymouth, IA

Built 1991 Load: 52.7 t ADT: 71

308TH ST

Good

Over: W BR PERRY CREEK

090471603, Plymouth, IA

Built 1998 Load: 52.7 t ADT: 20

270TH ST

Good

Over: CLEAR CREEK

091443501, Plymouth, IA

Built 1991 Load: 52.7 t ADT: 26

120TH ST

Good

Over: WILLOW CREEK

093441501, Plymouth, IA

Built 1989 Load: 52.7 t ADT: 5

PIONEER AVE

Good

Over: WILLOW CREEK

093441502, Plymouth, IA

Built 1990 Load: 52.7 t ADT: 15

C70

Good

Over: MUD CREEK

090443101, Plymouth, IA

Built 1999 Load: 52.9 t ADT: 450

280TH ST

Good

Over: PERRY CREEK

090470201, Plymouth, IA

Built 2005 Load: 52.9 t ADT: 11

325TH ST

Good

Over: PERRY CREEK

090472705, Plymouth, IA

Built 2006 Load: 52.9 t ADT: 319

FM

Good

Over: LITTLE SIOUX

091432301, Plymouth, IA

Built 1991 Load: 52.9 t ADT: 139

220TH ST

Good

Over: PLYMOUTH CREEK

091450301, Plymouth, IA

Built 2004 Load: 52.9 t ADT: 15

FM

Good

Over: WHISKEY CREEK

092433501, Plymouth, IA

Built 1997 Load: 52.9 t ADT: 848

160TH ST

Good

Over: MINK CREEK

092460301, Plymouth, IA

Built 2003 Load: 52.9 t ADT: 26

FM

Good

Over: WESTFIELD CREEK

092481702, Plymouth, IA

Built 1988 Load: 52.9 t ADT: 180

140TH ST

Good

Over: DEEP CREEK

093432801, Plymouth, IA

Built 2001 Load: 52.9 t ADT: 30

150TH ST

Good

Over: DEEP CREEK

093433201, Plymouth, IA

Built 2004 Load: 52.9 t ADT: 60

140TH ST

Good

Over: WILLOW CREEK

093442901, Plymouth, IA

Built 2000 Load: 52.9 t ADT: 20

150TH ST

Good

Over: WILLOW CREEK

093443101, Plymouth, IA

Built 1992 Load: 52.9 t ADT: 71

C44 230TH ST

Good

Over: FLOYD RIVER

9146110310, Plymouth, IA

Built 2019 Load: 56.7 t ADT: 439

FM C-38

Good

Over: WESTFIELD CREEK

092493601, Plymouth, IA

Built 1987 Load: 57.5 t ADT: 589

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

Related

Browse all → | Rankings →
Data sourced from official public datasets. See our methodology for details. Retrieved and formatted by PlainBridges

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.