County profile · official public datasets

St. Clair County, IL

486 public highway bridges tracked in the FHWA National Bridge Inventory: 50% rated Good, 40.9% Fair, 9.1% Poor.

486
Public bridges: 50%
Good condition: 9.1%
Poor condition

The verdict

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

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

68th
of 102 on Poor share: 9.1%
Poor-condition share: 486
public highway bridges: 1977
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.

St. Clair County Bridge Snapshot

According to the Federal Highway Administration's 2024 National Bridge Inventory, St. Clair County ranks #68 of 102 Illinois counties with at least 5 public highway bridges on Poor-condition share (lower share ranks better; currently 9.1%). St. Clair County, Illinois contains 486 public highway bridges. Roughly 50% are rated “Good,” 40.9% “Fair,” and 9.1% “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

486

Public highway bridges in St. Clair County

Good condition

50%

243 of 486 rated 7-9

Fair condition

40.9%

199 of 486 rated 5-6

Poor condition

9.1%

44 of 486 rated 4 or below

In-state Poor-share rank

#68 of 102

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

Condition distribution

Share of 486 NBI-tracked bridges by lowest applicable component rating.

Good

50% (243)

Fair

40.9% (199)

Poor

9.1% (44)

St. Clair County's Poor-condition bridges carry 535,350 vehicle crossings a day

That is 10% of the county's reported daily bridge traffic on 9.1% of its bridges: Poor-condition structures carry more traffic than their share of the inventory. Each Poor bridge averages 12,167 crossings a day against 9,231 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 484 of 486 bridges in this county that report an average daily traffic count. Source: FHWA National Bridge Inventory, 2024 release.

46.1% of St. Clair County's bridges have passed the 50-year mark

224 of the 486 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 50% 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 22% of the county's Poor-condition bridge traffic

I-55/64; US 40 PSC over TRRA, built 1966, carries 117,700 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 44-bridge count understates how much of the daily burden sits on a single crossing.

Share is of the 535,350 daily crossings on Poor-rated bridges that report a traffic count. Source: FHWA National Bridge Inventory, 2024 release.

pre-1900

1
1900s

6
1910s

1
1920s

12
1930s

18
1940s

17
1950s

34
1960s

84
1970s

74
1980s

79
1990s

63
2000s

42
2010s

53
2020s

2

What this shows Construction peaked in the 1960s, when 84 of the county's bridges still in the inventory were built, which is 17.3% of those with a recorded year. That cohort is now 55 to 64 years old in the 2024 inventory. Its average build year (1977) matches the Illinois average (1979).

How St. Clair County compares with Illinois's closest counties by inventory size

The 5 Illinois counties nearest St. Clair County in number of bridges, with their condition splits. Its 9.1% Poor share is the 5th-lowest of these 6. Its average build year of 1977 against a peer-group mean of 1979 is in line with the group.

Showing the 5 closest of 101 Illinois counties eligible for comparison.

Bridge inventory size and condition shares for St. Clair County and the 5 Illinois counties closest to it in inventory size
County	Bridges	Good	Fair	Poor	Avg. built
LaSalle County	528	39.4%	48.3%	12.3%	1978
Madison County	523	49.3%	42.3%	8.4%	1979
Vermilion County	494	27.7%	65%	7.3%	1979
St. Clair County (this page)	486	50%	40.9%	9.1%	1977
Winnebago County	467	31.3%	61.7%	7.1%	1978
Sangamon County	466	56.9%	36.9%	6.2%	1981

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 St. Clair 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 St. Clair County, both outside Illinois so the neighborhoods are not the same-state size table above.

Similar inventory size

Nearest county inventories outside IL (486 bridges here).

Frederick County, MD · 486 bridges
Cattaraugus County, NY · 484 bridges
Black Hawk County, IA · 489 bridges
Suffolk County, MA · 489 bridges

Similar average build year

Nearest mean year-built outside IL (1977 here).

San Diego County, CA · avg 1977 · 1,632 bridges
King County, WA · avg 1977 · 1,238 bridges
Riverside County, CA · avg 1977 · 1,117 bridges
Shelby County, TN · avg 1977 · 1,077 bridges

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

All 486 Good 243 Fair 199 Poor 44

Fair-condition bridges (199)

Sort:

St. Clair County Bridge Snapshot

Condition distribution

St. Clair County's Poor-condition bridges carry 535,350 vehicle crossings a day

46.1% of St. Clair County's bridges have passed the 50-year mark

One structure carries 22% of the county's Poor-condition bridge traffic

How St. Clair County compares with Illinois's closest counties by inventory size

Nationwide counties with similar inventory size or average build year

Similar inventory size

Similar average build year

Fair-condition bridges (199)

I-55/64; US 40 WB

ILL 15

IL 161

GRAY PLAZA

ILL 158

Davis St Ferry Rd

IL 3 SB Ramp F

FAU 9206-WATER ST

EMERALD MOUND TR 2

IL 203

ILL 177

SCOTT-TROY/ FAU 91

FAI 64 WB

FAI 64 EB

IL15 Ramp G to SB2

I-55/64; US 40 EB

SBI-4

OLD US 50

I-55/64; US 40 EB

I-55/70 & US 40 WB

I-64 EB

I-64 WB

FAI 64 EB

FAI 64 WB

ILL 158 SB

ILL 158 NB

US 50

FAI 64 WB

IL 3 NB Ramp E

81ST ST/CH35

FAI 255

ILL 177

BALDWIN RD/ FAS 84

FAI 64 EB

PARK ROAD TR 515A

SKAER RD/ TR 126

I-55/64; US 40 EB

S Main St/Old IL 3

STATE ST/ FAU 9182

FAI 255

FAI 64 WB

SUMMERFIELD CH 50

ILL 163

3RD ST TO EB I-55/

FAI 64 EB

I-55/64; US 40 WB

FAI 64 EB

ILL 163

I-255 RAMP E

FAI 255 SB

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