County profile · official public datasets

St. Clair County, MI

327 public highway bridges tracked in the FHWA National Bridge Inventory: 41.3% rated Good, 50.5% Fair, 8.3% Poor.

327
Public bridges: 41.3%
Good condition: 8.3%
Poor condition

The verdict

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

St. Clair County pairs a better-than-median Poor-condition share among rated counties in the state with a newer average build year than the state inventory.

Top 39%
lowest Poor share in state: 8.3%
Poor-condition share: 327
public highway bridges: 1985
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 #32 of 83 Michigan counties with at least 5 public highway bridges on Poor-condition share (lower share ranks better; currently 8.3%). St. Clair County, Michigan contains 327 public highway bridges. Roughly 41.3% are rated “Good,” 50.5% “Fair,” and 8.3% “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

327

Public highway bridges in St. Clair County

Good condition

41.3%

135 of 327 rated 7-9

Fair condition

50.5%

165 of 327 rated 5-6

Poor condition

8.3%

27 of 327 rated 4 or below

In-state Poor-share rank

#32 of 83

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

Condition distribution

Share of 327 NBI-tracked bridges by lowest applicable component rating.

Good

41.3% (135)

Fair

50.5% (165)

Poor

8.3% (27)

St. Clair County's Poor-condition bridges carry 38,796 vehicle crossings a day

That is 3% of the county's reported daily bridge traffic on 8.3% of its bridges: Poor-condition structures carry less traffic than their share of the inventory. Each Poor bridge averages 1,437 crossings a day against 4,382 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 326 of 327 bridges in this county that report an average daily traffic count. Source: FHWA National Bridge Inventory, 2024 release.

26.9% of St. Clair County's bridges have passed the 50-year mark

88 of the 327 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 41.3% 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 55.7% of the county's Poor-condition bridge traffic

I-94 WB over M-25 CONN, built 1964, carries 21,619 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 27-bridge count understates how much of the daily burden sits on a single crossing.

Share is of the 38,796 daily crossings on Poor-rated bridges that report a traffic count. Source: FHWA National Bridge Inventory, 2024 release.

1910s

2
1920s

9
1930s

17
1940s

14
1950s

8
1960s

31
1970s

25
1980s

77
1990s

34
2000s

28
2010s

69
2020s

13

What this shows Construction peaked in the 1980s, when 77 of the county's bridges still in the inventory were built, which is 23.5% of those with a recorded year. That cohort is now 35 to 44 years old in the 2024 inventory. Its average build year is 1985, 9 years newer than the Michigan average of 1976.

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

The 5 Michigan counties nearest St. Clair County in number of bridges, with their condition splits. Its 8.3% Poor share is the 2nd-lowest of these 6. Its average build year of 1985 against a peer-group mean of 1975 is 10 years newer.

Showing the 5 closest of 82 Michigan counties eligible for comparison.

Bridge inventory size and condition shares for St. Clair County and the 5 Michigan counties closest to it in inventory size
County	Bridges	Good	Fair	Poor	Avg. built
Genesee County	371	22.9%	61.7%	15.4%	1968
St. Clair County (this page)	327	41.3%	50.5%	8.3%	1985
Saginaw County	315	28.9%	52.7%	18.4%	1979
Monroe County	297	30%	54.5%	15.5%	1974
Berrien County	272	27.6%	66.5%	5.9%	1979
Washtenaw County	262	23.3%	56.5%	20.2%	1975

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 Michigan so the neighborhoods are not the same-state size table above.

Similar inventory size

Nearest county inventories outside MI (327 bridges here).

Henry County, IL · 328 bridges
Nassau County, NY · 328 bridges
Miami County, KS · 326 bridges
Montgomery County, KS · 326 bridges

Similar average build year

Nearest mean year-built outside MI (1985 here).

Hillsborough County, FL · avg 1985 · 840 bridges
Lancaster County, NE · avg 1985 · 661 bridges
Nueces County, TX · avg 1985 · 528 bridges
Grayson County, TX · avg 1985 · 513 bridges

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

All 327 Good 135 Fair 165 Poor 27

All bridges (327)

Sort:

St. Clair County Bridge Snapshot

Condition distribution

St. Clair County's Poor-condition bridges carry 38,796 vehicle crossings a day

26.9% of St. Clair County's bridges have passed the 50-year mark

One structure carries 55.7% of the county's Poor-condition bridge traffic

How St. Clair County compares with Michigan'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 (327)

I-94 EB

I-69 EB

M-29

PALMS ROAD

I-69 WB

GOODELLS ROAD

M-19 (KINNEY ROAD)

MIDDLE CHANNEL DRV

STATEROAD

REEVES RD

MARTIN ROAD

ARENDT ROAD

BRAIDWOOD ROAD

M-25

I-94 WB

BABCOCK ROAD

I-69 EB

I-94 EB

RATTLE RUN ROAD

KRONNER ROAD

I-69 EB

I-94 & I-69 WB

CHANNELSYDE DRIVE

CAPAC ROAD

M-25

BRENNAN ROAD

I-69 WB

I-69 WB

STAPLETON RD

MAYER ROAD

M-29

LAPEER RD(OLDM-21)

BROWN ROAD

FULTON ROAD

GRISWOLD ROAD

NORMAN ROAD

MARTIN ROAD

I-69 WB

DUCE ROAD

AUDUBON DRIVE

BROADBRIDGE ROAD

M-29

LAPEER RD (M-21)

CRIBBINS ROAD

ANCHOR BAY DRIVE

MARINE CITY HWY

BORDMAN ROAD

NORMAN ROAD

BURT ROAD

MASON ROAD

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