Methodology & Data Sources

Data Source

All bridge data on PlainBridge comes from the Federal Highway Administration (FHWA) National Bridge Inventory (NBI) — the federal database mandated by Congress since 1968 following the Silver Bridge collapse. Every public highway bridge over 20 feet in the U.S. is required to be inspected and reported to the NBI.

Source URL: fhwa.dot.gov/bridge/nbi.cfm

Coverage

PlainBridge covers 575,000+ public highway bridges across all 50 states plus territories, sourced from the most recent annual NBI dataset.

How We Process the Data

  1. Download the annual NBI dataset from FHWA
  2. Parse all 116 NBI data items including condition ratings, structural details, traffic counts, and geographic data
  3. Apply FHWA's official condition classification (Good/Fair/Poor) based on component ratings
  4. Calculate or confirm structurally deficient (SD) status using FHWA's federal definition
  5. Organize into searchable pages by bridge, county, and state

Condition Ratings Explained

FHWA inspectors rate four bridge components on a 0–9 scale:

  • Deck — The road surface and supporting structure
  • Superstructure — Beams, trusses, and spans that carry the load
  • Substructure — Piers, abutments, and foundations
  • Culvert — For pipe/box structures carrying drainage

A bridge is classified as Good if all rated components score 7 or above, Fair if all score between 5 and 6 inclusive, and Poor if any component scores 4 or below. These thresholds follow FHWA's official MAP-21 classification system established under the Moving Ahead for Progress in the 21st Century Act. Approximately 7.5 percent of U.S. bridges are classified as structurally deficient, requiring prioritized maintenance or replacement.

Load rating vs. sufficiency rating

Each bridge page shows the NBI inventory load rating (item 66) — the load level, in metric tons, a structure is rated to carry for an indefinite period. A lower number means the bridge is more load-restricted. This is a real, reported NBI field and is distinct from the FHWA sufficiency rating.

The sufficiency rating is a separate composite 0–100 score calculated from structural adequacy (55%), functional obsolescence (30%), and essentiality to public use (15%); below 50 may qualify a bridge for federal replacement funding and below 80 for rehabilitation. We do not currently surface the composite sufficiency rating per bridge — our structurally-deficient classification is derived directly from the published component condition ratings (deck, superstructure, substructure rated 4 or below), which is the criterion that flags every deficient bridge in this dataset.

Data Currency

FHWA releases updated NBI data annually, typically in spring covering the prior calendar year. States must submit inspection data every two years per bridge (many inspect annually). We update our database when new annual NBI data becomes available.

Limitations

  • Condition data reflects the most recent NBI inspection, which may lag current conditions by up to a year
  • A "Poor" or "Structurally Deficient" rating does not mean a bridge is closed or imminently unsafe — it means it needs attention or monitoring
  • NBI does not cover pedestrian-only bridges, rail bridges, or bridges under 20 feet
  • For current closure, load restriction, or emergency information, contact your state DOT

How the Source Agency Collects Data

The National Bridge Inventory is maintained by the Federal Highway Administration under the authority of the National Bridge Inspection Standards (NBIS), established by the Federal-Aid Highway Act of 1968. Every state Department of Transportation is responsible for inspecting all public highway bridges within its jurisdiction at least once every 24 months, using FHWA-certified bridge inspectors. Inspectors evaluate 116 standardized data items per bridge, including structural condition ratings, load capacity, geometric dimensions, and traffic volumes — all defined in the FHWA Recording and Coding Guide for the Structure Inventory.

States submit their inspection data annually to FHWA, which compiles it into the national NBI dataset. The FHWA publishes this dataset each spring, covering inspections from the prior calendar year. The U.S. Bureau of Transportation Statistics (BTS) publishes annual bridge condition summaries derived from the same NBI feed, and the U.S. Department of Transportation uses NBI condition data to allocate Highway Bridge Program funding under MAP-21 and the Infrastructure Investment and Jobs Act.

Data Accuracy Commitment

PlainBridge presents FHWA data without modification. Condition ratings, inventory load ratings, and structural classifications are displayed exactly as reported in the NBI. We do not compute our own safety ratings or editorialize about bridge conditions. If you find any data that appears incorrect, please contact us and we will verify against the NBI source data.

Contact

Questions about our methodology or found a data error? Reach us at hello@plainbridges.com or through our contact page.