County profile · official public datasets

Marin County, CA

207 public highway bridges tracked in the FHWA National Bridge Inventory: 43.5% rated Good, 46.9% Fair, 9.7% Poor.

207
Public bridges: 43.5%
Good condition: 9.7%
Poor condition

The verdict

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

Marin 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.

44th
of 58 on Poor share: 9.7%
Poor-condition share: 207
public highway bridges: 1958
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.

Marin County Bridge Snapshot

According to the Federal Highway Administration's 2024 National Bridge Inventory, Marin County ranks #44 of 58 California counties with at least 5 public highway bridges on Poor-condition share (lower share ranks better; currently 9.7%). Marin County, California contains 207 public highway bridges. Roughly 43.5% are rated “Good,” 46.9% “Fair,” and 9.7% “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

207

Public highway bridges in Marin County

Good condition

43.5%

90 of 207 rated 7-9

Fair condition

46.9%

97 of 207 rated 5-6

Poor condition

9.7%

20 of 207 rated 4 or below

In-state Poor-share rank

#44 of 58

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

Condition distribution

Share of 207 NBI-tracked bridges by lowest applicable component rating.

Good

43.5% (90)

Fair

46.9% (97)

Poor

9.7% (20)

Marin County's Poor-condition bridges carry 181,015 vehicle crossings a day

That is 3.7% of the county's reported daily bridge traffic on 9.7% of its bridges: Poor-condition structures carry less traffic than their share of the inventory. Each Poor bridge averages 9,051 crossings a day against 27,674 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 207 of 207 bridges in this county that report an average daily traffic count. Source: FHWA National Bridge Inventory, 2024 release.

82.1% of Marin County's bridges have passed the 50-year mark

170 of the 207 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 43.5% 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 34% of the county's Poor-condition bridge traffic

U.S. HIGHWAY 101 over NOVATO CREEK, built 1974, carries 61,500 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 20-bridge count understates how much of the daily burden sits on a single crossing.

Share is of the 181,015 daily crossings on Poor-rated bridges that report a traffic count. Source: FHWA National Bridge Inventory, 2024 release.

pre-1900

1
1900s

4
1910s

2
1920s

23
1930s

29
1940s

9
1950s

37
1960s

40
1970s

27
1980s

12
1990s

7
2000s

7
2010s

7
2020s

2

What this shows Construction peaked in the 1960s, when 40 of the county's bridges still in the inventory were built, which is 19.3% of those with a recorded year. That cohort is now 55 to 64 years old in the 2024 inventory. Its average build year is 1958, 12 years older than the California average of 1970.

How Marin County compares with California's closest counties by inventory size

The 5 California counties nearest Marin County in number of bridges, with their condition splits. Its 9.7% Poor share is the 4th-lowest of these 6. Its average build year of 1958 against a peer-group mean of 1967 is 9 years older.

Showing the 5 closest of 57 California counties eligible for comparison.

Bridge inventory size and condition shares for Marin County and the 5 California counties closest to it in inventory size
County	Bridges	Good	Fair	Poor	Avg. built
Glenn County	242	36.8%	52.9%	10.3%	1954
Colusa County	211	40.8%	52.1%	7.1%	1968
Marin County (this page)	207	43.5%	46.9%	9.7%	1958
Trinity County	194	56.2%	39.7%	4.1%	1977
El Dorado County	192	46.9%	46.9%	6.3%	1975
Santa Cruz County	192	40.1%	42.2%	17.7%	1963

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

Similar inventory size

Nearest county inventories outside CA (207 bridges here).

Las Animas County, CO · 207 bridges
Shelby County, IA · 207 bridges
Washington County, IA · 207 bridges
Richland County, LA · 207 bridges

Similar average build year

Nearest mean year-built outside CA (1958 here).

Middlesex County, MA · avg 1958 · 818 bridges
Bucks County, PA · avg 1958 · 699 bridges
Cowley County, KS · avg 1958 · 337 bridges
Maui County, HI · avg 1958 · 185 bridges

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

All 207 Good 90 Fair 97 Poor 20

Good-condition bridges (90)

Sort:

Marin County Bridge Snapshot

Condition distribution

Marin County's Poor-condition bridges carry 181,015 vehicle crossings a day

82.1% of Marin County's bridges have passed the 50-year mark

One structure carries 34% of the county's Poor-condition bridge traffic

How Marin County compares with California's closest counties by inventory size

Nationwide counties with similar inventory size or average build year

Similar inventory size

Similar average build year

Good-condition bridges (90)

S101 ON & OFF RAMP

U.S. HIGHWAY 101

STATE ROUTE 1

STATE ROUTE 1

STATE ROUTE 1

101N OFFRAMP 2 ST

TOMALES PETALUMA R

IGNACIO BLVD

MEADWOOD DR

CALETA AVE

MUIR WOODS RD

SLAUGHTER HOUSE RD

SOUTH NOVATO BLVD

LAGUNITAS ROAD

NORTH HAMILTON PKW

CENTER ROAD

SOUTHERN HEIGHTS B

PT RYES PETLUMA RD

N&S101-E37 CONNCTR

2ND ST

U.S. HIGHWAY 101

LUCAS VALLEY ROAD

STATE ROUTE 1

INTERSTATE 580

DE LONG AVENUE

U.S. HIGHWAY 101

ATHERTON AVENUE

SIMMONS AVE

MRSHAL-PETALUMA RD

MEADOW WAY

S101-E37 & NOVATO-

LAS GALLINAS AVE

W37-S101&N101 OFF

DIABLO AVE

STATE ROUTE 37

THORSSON COURT

TAMALPAIS AVE

U.S. HIGHWAY 101

LAGUNITAS RD

U.S. HIGHWAY 101

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