National Bridge Inventory: Research Data Guide

The National Bridge Inventory (NBI) is the authoritative federal database of public highway bridges compiled annually by the Federal Highway Administration (FHWA). Mandated under the National Bridge Inspection Standards (23 CFR Part 650 Subpart C and 23 U.S.C. 144), it aggregates structural, geometric, and condition appraisal records for infrastructure research and deterioration modeling across the United States.

What it is

The National Bridge Inventory is a centralized repository of standardized engineering appraisals and administrative records for highway bridges carrying vehicular traffic. Initiated by the U.S. Congress in the Federal-Aid Highway Act of 1968 after the 1967 Silver Bridge collapse, the program established uniform safety inspection criteria nationwide with the initial publication of the National Bridge Inspection Standards (NBIS) in 1971.

For civil and structural engineering researchers, the NBI provides a continuous longitudinal dataset spanning five decades. Historically governed by the FHWA Recording and Coding Guide for the Structure Inventory and Appraisal of the Nation’s Bridges, each bridge record captures over 100 standardized attributes, including design types, span geometry, material specifications, functional classifications, and component-level condition appraisals. When designing automated data workflows, investigators should consult guidance on choosing an appropriate metadata schema for research datasets to map legacy flat files into normalized database architectures.

Who runs it and what it covers

The inventory is managed by the Office of Bridges and Structures within the Federal Highway Administration (FHWA) under the U.S. Department of Transportation (USDOT). Field inspections and inventory updates are conducted by state departments of transportation, federal land-management agencies (such as the National Park Service and U.S. Forest Service), and Tribal governments, which submit annual data to FHWA pursuant to 23 CFR 650.315.

The inventory covers vehicular bridges on public roads carrying highway traffic with an opening span exceeding 20 feet (6.1 meters). In official annual dataset releases, FHWA recorded 623,218 public highway bridges in the 2024 archive and 624,193 bridges in the 2025 archive. Agencies submit inspection data each spring; FHWA validates the submissions and publishes the finalized public dataset on its website by June 15th of each calendar year through federal portals and open data repositories for public datasets.

For structural health research, condition ratings follow a standardized 0–9 numerical scale. Effective with the 2018 data archive, FHWA formally eliminated the legacy term “Structurally Deficient” (SD) from the National Bridge Inspection Program to align with MAP-21 (Public Law 112-141) performance measures. Bridges are evaluated under three performance categories:

• Good: The lowest condition rating among primary structural components is 7, 8, or 9 on the 0–9 scale.
• Fair: The lowest condition rating among primary structural components is 5 or 6.
• Poor: The lowest condition rating among primary structural components is 4 or less. A bridge is classified in Poor condition if Item 58 (Deck), Item 59 (Superstructure), Item 60 (Substructure), or Item 62 (Culvert) is coded 4, 3, 2, 1, or 0.

How to search or use it

Extracting and analyzing NBI data requires understanding legacy file layouts and maintaining sound data stewardship principles across multi-year data joins. Researchers should follow these seven steps:

1. Access the data repository: Visit the FHWA Bridge Inspection portal to access the Download NBI ASCII Files directory, or use the LTBP InfoBridge portal (infobridge.fhwa.dot.gov) for interactive mapping and multi-attribute filtering.
2. Select geographic coverage and annual archive: Choose between individual state data files or nationwide consolidated archives. Historical datasets are available from 1992 through 2025.
3. Choose file layout format: NBI datasets are distributed as fixed-width ASCII (445-character record width) or comma-delimited text files (CSV format with single-quote ' qualifiers). Delimited files import directly into statistical packages, while fixed-width layouts require exact column slicing.
4. Define the composite primary key: To join records longitudinally across multiple years, use Item 1 (State Code, character positions 1–3) combined with Item 8 (Structure Number, character positions 4–18). Because structure numbering schemes are defined independently by each state, Item 8 alone is not unique nationwide.
5. Extract primary condition appraisal fields: Isolate Item 58 (Deck Condition), Item 59 (Superstructure Condition), Item 60 (Substructure Condition), and Item 62 (Culvert Condition). On the 0–9 scale, code 9 indicates excellent condition, code 7 indicates minor problems, and codes 4 or lower indicate advanced section loss or structural deterioration.
6. Incorporate traffic and operational variables: Extract Item 29 (Average Daily Traffic, ADT), Item 30 (Year of ADT), Item 109 (Average Daily Truck Traffic), Item 27 (Year Built), and Item 106 (Year Reconstructed) to parameterize degradation rates and loading history.
7. Apply the SNBI crosswalk: Because 2025 was the final reporting year using the legacy Coding Guide format, consult the FHWA Data Crosswalk and Detailed Coding Map to translate historical items into the modern Specifications for the National Bridge Inventory (SNBI) structure.

Strengths and limits

Methodological planning requires weighing the breadth of the NBI against its structural constraints.

Strengths:

• Universal national scope: Encompasses over 624,000 structures across all 50 states, federal lands, and territories.
• Multi-decade longitudinal depth: Continuous annual archives since the 1990s support long-term asset deterioration modeling and survival analysis.
• Open access: Fully in the public domain with no paywalls, usage restrictions, or proprietary licenses.
• Multimodal linkages: Includes geographic coordinates, highway functional classes, and National Highway System flags for spatial risk analysis.

Limits:

• Visual inspection subjectivity: Ratings on the 0–9 scale are primarily based on visual observation, creating inter-inspector variance.
• Inspection interval lag: Routine inspections occur on 24-month cycles, meaning condition ratings reflect the most recent field inspection rather than real-time performance.
• Structure renumbering: States occasionally renumber bridges during substantial reconstruction, requiring spatial coordinate matching to reconcile gaps.
• Legacy schema complexity: The 445-character fixed-width layout lacks embedded headers, requiring external specification documentation for ingestion.

Alternatives

Infrastructure researchers can leverage several complementary federal datasets and platforms depending on analytical scope.

Platform / Dataset	Data Format	Structural Coverage	Granularity	Primary Research Use
NBI Legacy ASCII	445-character fixed-width or delimited CSV	All public highway bridges >20 ft (~624,000 structures)	Component-level (0–9 condition scale for deck, super, sub)	Historical deterioration modeling, national baseline statistics, and network risk screening.
Specifications for the NBI (SNBI)	Relational / JSON schema-based data elements	All public highway bridges >20 ft	Expanded component and sub-element distress metrics with defect codes	Modern regulatory compliance, risk-based inspection interval analyses, and detailed asset management.
LTBP InfoBridge	Interactive web portal & tabular exports	National bridge population with LTBP test corridors	Integrated NBI, element-level, and environmental sensor data	Multi-criteria spatial querying, exploratory visual data analysis, and condition trend benchmarking.
National Bridge Element (NBE) Data	CSV / relational database files	National Highway System (NHS) bridges	Detailed material elements (e.g., reinforced concrete deck, steel girders, bearings)	Element-level condition state modeling (States 1–4) and lifecycle maintenance optimization.

Frequently asked questions

What is the difference between the legacy NBI Coding Guide and the SNBI?

The legacy Recording and Coding Guide governed NBI reporting from 1995 through the 2025 reporting year, using a flat 445-character fixed-width record layout. The Specifications for the National Bridge Inventory (SNBI), established under the 2022 NBIS Final Rule, introduces a relational data model with expanded defect categories, component condition ratings, and risk-based inspection interval tracking.

When does FHWA release new National Bridge Inventory datasets each year?

State agencies and federal partners submit inspection data each spring. The FHWA reviews, validates, and consolidates the records, officially releasing the finalized annual dataset for public highway bridges by June 15th of each calendar year on the FHWA website.

What does a “Poor” condition rating mean in the National Bridge Inventory?

Under federal performance measures adopted in 2018, a bridge is classified in “Poor” condition if the lowest rating among Item 58 (Deck), Item 59 (Superstructure), Item 60 (Substructure), or Item 62 (Culvert) is 4 or less on the 0–9 scale. A Poor rating signifies advanced section loss or structural deterioration requiring maintenance or rehabilitation, but does not indicate that the bridge is unsafe for traffic.

What is the composite primary key for tracking a bridge over time in NBI?

Researchers must track structures longitudinally using a composite key consisting of Item 1 (State Code, character positions 1–3) combined with Item 8 (Structure Number, character positions 4–18). Because structure numbering schemes are defined independently by each state DOT, using Item 8 alone produces collision errors across states.

How do researchers cite the National Bridge Inventory in academic papers?

Researchers citing NBI data should follow formal dataset citation standards, explicitly identifying the Federal Highway Administration as author, specifying the annual dataset release year (e.g., 2024 or 2025), citing the official download URL (https://www.fhwa.dot.gov/bridge/nbi.cfm), and noting the access date and state submittal version.

Sources

• Federal Highway Administration. “National Bridge Inventory (NBI).” U.S. Department of Transportation. URL: https://www.fhwa.dot.gov/bridge/nbi.cfm (read 2026-11-07).
• Federal Highway Administration. “Download NBI ASCII Files.” U.S. Department of Transportation. URL: https://www.fhwa.dot.gov/bridge/nbi/ascii.cfm (read 2026-11-07).
• Federal Highway Administration. “National Bridge Inspection Standards (NBIS).” 23 CFR Part 650 Subpart C. URL: https://www.fhwa.dot.gov/bridge/nbis.cfm (read 2026-11-07).
• Federal Highway Administration. “NBI Record Format (445-Character Layout).” URL: https://www.fhwa.dot.gov/bridge/nbi/format.cfm (read 2026-11-07).
• Federal Highway Administration. “2024 and 2025 NBI ASCII Files and State Totals.” URL: https://www.fhwa.dot.gov/bridge/nbi/ascii2024.cfm and https://www.fhwa.dot.gov/bridge/nbi/ascii2025.cfm (read 2026-11-07).
• Federal Highway Administration. “LTBP InfoBridge Web Portal.” Long-Term Bridge Performance Program. URL: https://infobridge.fhwa.dot.gov/ (read 2026-11-07).