County profile · official public datasets

Merrimack County, NH

326 public highway bridges tracked in the FHWA National Bridge Inventory: 45.1% rated Good, 46.9% Fair, 8% Poor.

326
Public bridges: 45.1%
Good condition: 8%
Poor condition

The verdict

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

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

6th
of 10 on Poor share: 8%
Poor-condition share: 326
public highway bridges: 1966
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.

Merrimack County Bridge Snapshot

According to the Federal Highway Administration's 2024 National Bridge Inventory, Merrimack County ranks #6 of 10 New Hampshire counties with at least 5 public highway bridges on Poor-condition share (lower share ranks better; currently 8%). Merrimack County, New Hampshire contains 326 public highway bridges. Roughly 45.1% are rated “Good,” 46.9% “Fair,” and 8% “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

326

Public highway bridges in Merrimack County

Good condition

45.1%

147 of 326 rated 7-9

Fair condition

46.9%

153 of 326 rated 5-6

Poor condition

26 of 326 rated 4 or below

In-state Poor-share rank

#6 of 10

Among New Hampshire counties with 5+ bridges, lower share ranks better

Condition distribution

Share of 326 NBI-tracked bridges by lowest applicable component rating.

Good

45.1% (147)

Fair

46.9% (153)

Poor

8% (26)

Merrimack County's Poor-condition bridges carry 269,174 vehicle crossings a day

That is 9.1% of the county's reported daily bridge traffic on 8% of its bridges: Poor-condition structures carry more traffic than their share of the inventory. Each Poor bridge averages 10,353 crossings a day against 7,847 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 326 bridges in this county that report an average daily traffic count. Source: FHWA National Bridge Inventory, 2024 release.

65.6% of Merrimack County's bridges have passed the 50-year mark

214 of the 326 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 45.1% 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 17.4% of the county's Poor-condition bridge traffic

I-89 over SOUTH STREET, built 1959, carries 46,769 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 26-bridge count understates how much of the daily burden sits on a single crossing.

Share is of the 269,174 daily crossings on Poor-rated bridges that report a traffic count. Source: FHWA National Bridge Inventory, 2024 release.

pre-1900

5
1900s

2
1910s

3
1920s

9
1930s

32
1940s

15
1950s

71
1960s

70
1970s

29
1980s

29
1990s

12
2000s

23
2010s

20
2020s

6

What this shows Construction peaked in the 1950s, when 71 of the county's bridges still in the inventory were built, which is 21.8% of those with a recorded year. That cohort is now 65 to 74 years old in the 2024 inventory. Its average build year (1966) matches the New Hampshire average (1968).

How Merrimack County compares with New Hampshire's closest counties by inventory size

The 5 New Hampshire counties nearest Merrimack County in number of bridges, with their condition splits. Its 8% Poor share is the 4th-lowest of these 6. Its average build year of 1966 against a peer-group mean of 1968 is in line with the group.

Showing the 5 closest of 9 New Hampshire counties eligible for comparison.

Bridge inventory size and condition shares for Merrimack County and the 5 New Hampshire counties closest to it in inventory size
County	Bridges	Good	Fair	Poor	Avg. built
Hillsborough County	385	51.4%	39.7%	8.8%	1972
Merrimack County (this page)	326	45.1%	46.9%	8%	1966
Rockingham County	308	66.6%	26.9%	6.5%	1977
Carroll County	193	54.9%	40.9%	4.1%	1966
Cheshire County	192	41.7%	46.9%	11.5%	1960
Coos County	188	49.5%	44.1%	6.4%	1964

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 Merrimack 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 Merrimack County, both outside New Hampshire so the neighborhoods are not the same-state size table above.

Similar inventory size

Nearest county inventories outside NH (326 bridges here).

Miami County, KS · 326 bridges
Montgomery County, KS · 326 bridges
Washington County, OR · 326 bridges
St. Clair County, MI · 327 bridges

Similar average build year

Nearest mean year-built outside NH (1966 here).

Butler County, KS · avg 1966 · 591 bridges
Queens County, NY · avg 1966 · 514 bridges
Lehigh County, PA · avg 1966 · 381 bridges
Osage County, OK · avg 1966 · 366 bridges

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

All 326 Good 147 Fair 153 Poor 26

Good-condition bridges (147)

Sort:

Merrimack County Bridge Snapshot

Condition distribution

Merrimack County's Poor-condition bridges carry 269,174 vehicle crossings a day

65.6% of Merrimack County's bridges have passed the 50-year mark

One structure carries 17.4% of the county's Poor-condition bridge traffic

How Merrimack County compares with New Hampshire'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 (147)

ISLAND ROAD

CONCORD ROAD

GOOSE HOLE ROAD

TWIN BRIDGE ROAD

NH107

BIRCHDALE ROAD

FAIRGROUNDS ROAD

BUNKER HILL ROAD

CENTER HILL ROAD

WEST MEADOW ROAD

NH 9(LOUDON RD)

I-93 FEE TPK

BIRCHDALE ROAD

NH106

NH132

I-89 NB

I-93 US 4 SB

I-93 US 4 NB

W. SALISBURY ROAD

WARNER ROAD

I-89 NB

I-89 SB

CHICHESTER ROAD

WEBSTER MILLS ROAD

MANSION ROAD

US 4 US202 NH 9

NH 9

ACCESS ROAD

WEST ROAD

ECHO VALLEY FARM R

WOODLAND LANE

US 4

CROSS MILL ROAD

US 3

MAIN STREET

I-89 NB

BREEZY HILL ROAD

I-89(RAMP D)

I-89 NB

NORTH VILLAGE ROAD

WESTERN AVE

SUTTON ROAD

BROOKSIDE DRIVE

US202 NH 9

MAIN STREET

NH 11

I-89 SB

I-89 NB

VILLAGE ROAD

US 3 NH 11 NH127

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