Guide · FHWA NBI
America's aging bridge infrastructure
The average US highway bridge is about 47 years old, and the Interstate-era cohort is now reaching the 50-year design life it was built for. Here is what the inventory's age profile actually shows.
- 47 yrs
- Average bridge age
- 45.2%
- Built 1974 or earlier
- 623,216
- Bridges nationwide
On this register · 2024 NBI
According to the Federal Highway Administration's 2024 National Bridge Inventory.
The Interstate-era bulge is why 6.8% of the 623,216-bridge file is Poor. Share still splits by place: Iowa 19.16% versus Nevada 1.14% among 49 inventories with 1,000+ bridges.
- 6.8%
- Poor nationwide
- 19.16%
- Iowa · highest share
- 1.14%
- Nevada · lowest share
- 6.03%
- Median large-inventory share
Year-built is stamped on 621,562 of 623,216 records; construction peaked in the 1960s (93,702 still in service).
US bridges by decade built
Count of bridges in the FHWA inventory, grouped by construction decade
- 1900s 5,009
1900s
5,009 bridges
- 1910s 5,008
1910s
5,008 bridges
- 1920s 15,826
1920s
15,826 bridges
- 1930s
1930s
37,507 bridges
- 1940s 23,088
1940s
23,088 bridges
- 1950s
1950s
58,190 bridges
- 1960s
1960s
93,702 bridges
- 1970s
1970s
79,015 bridges
- 1980s
1980s
76,815 bridges
- 1990s
1990s
80,962 bridges
- 2000s
2000s
72,517 bridges
- 2010s
2010s
57,289 bridges
- 2020s 16,634
2020s
16,634 bridges
What this shows Bridge building peaked in the 1960s with 93,702 bridges still in the inventory today. That Interstate-era surge means the single largest age cohort is now 64-plus years old, the structural reason the maintenance backlog is growing faster than new construction can offset.
The 50-Year Design Life
Bridges built during the postwar and Interstate Highway era, roughly the 1950s through the 1970s, were typically engineered for a service life of about 50 years. That assumption shaped the materials, the reinforcement detailing, and the maintenance schedules of an entire generation of structures. As of 2024, every bridge built in 1974 or earlier has reached or passed that mark: 280,669 bridges, or 45.2% of the 621,562 bridges with a recorded year built between 1900 and 2024.
Reaching the design life does not mean a bridge is unsafe or will fail. It means the elements that wear out first, the deck surface, the expansion joints, the bearings, and the protective coatings, are now past the point where they were expected to need major work. Inspections intensify, rehabilitation costs climb, and the probability of a "poor" condition rating rises sharply with each additional decade in service.
The Interstate-Era Cohort
The age profile above is not a smooth curve. It has a pronounced bulge: construction peaked in the 1960s, when 93,702 of today's surviving bridges were built to carry the new Interstate system and the suburban road network that grew around it. Because so many bridges were built in such a short window, they are now aging through the danger zone together rather than being spread evenly across the decades.
This concentration is what turns ordinary aging into a budget problem. A maintenance program sized for a steady trickle of bridges reaching 50 years instead faces a wave of them. The same dynamic shows up in the national condition numbers: 42,080 bridges (6.8%) are now rated poor, and another 306,278 (49.1%) sit in the "fair" band, structurally adequate today, but the next candidates for the poor list as their Interstate-era components wear out.
What the Age Data Can and Cannot Tell You
What it captures: the year-built field in the National Bridge Inventory is one of the most consistently reported items in the dataset, which makes the national age profile reliable. Combined with the 0–9 condition ratings for deck, superstructure, and substructure, it lets you see how age and condition move together across states and decades.
What it misses: age alone does not determine condition. A well-maintained 1950s bridge can rate better than a neglected 1990s one, and many older bridges have been rehabilitated or partially rebuilt - the inventory records a separate year-reconstructed field for exactly this reason. Traffic load, climate, de-icing salt exposure, and inspection quality all shape how fast a given bridge ages. Use the age profile to understand the scale of the backlog, not to judge an individual structure.
How to dig deeper: the condition and rehabilitation history of any specific bridge is on its record page, and you can compare states on the condition rankings. To understand the rating scale behind these figures, see the guide on reading condition ratings.
Frequently Asked Questions
How old is the average US bridge?
The average US highway bridge with a recorded construction year is about 47 years old, based on the year-built field in the FHWA National Bridge Inventory. Roughly 45.2% of the 621,562 bridges with year built between 1900 and 2024 were built in 1974 or earlier, which puts them at or beyond the 50-year design life assumed for mid-century bridge construction.
Why does bridge age matter?
Most bridges built during the Interstate era were engineered for an expected service life of about 50 years. As that cohort passes 50 years in service, components such as decks, bearings, and expansion joints reach the end of their design life, raising inspection frequency, rehabilitation cost, and the share of bridges rated in poor condition.
How many US bridges are in poor condition?
Of the 623,216 bridges in the FHWA inventory, 42,080 (6.8%) are rated in poor condition, meaning a key component (deck, superstructure, or substructure) is rated 4 or lower on the 0–9 NBI scale. Poor condition does not mean unsafe; it means the bridge needs significant rehabilitation or replacement.
Where does this data come from?
All figures come from the Federal Highway Administration (FHWA) National Bridge Inventory, the official record of the nation's highway bridges, updated annually. PlainBridges standardizes the public NBI files for searching and analysis. See our methodology page for the full processing pipeline.
What to do with this
Age tells you which bridges are due for a closer look, not which ones are actually in trouble.
- The 1960s were the peak construction decade at 93,702 bridges, that cohort is now 64 years into service life. Reading condition ratings
- 45.2% of dated bridges are past the 50-year design life, but age alone doesn't determine condition, check a specific bridge's own record. Search bridges
- Compare how age and condition line up state by state rather than assuming the national pattern applies locally. State condition rankings
Age-profile figures are the current FHWA 2024 NBI file's year-built field; a bridge's year-reconstructed field, not shown here, can mean an older bridge has already been substantially rebuilt.
Sources: Federal Highway Administration (FHWA) National Bridge Inventory, year-built and condition fields. See our methodology.