BridgeSafety

Our Methodology

BridgeSafety grades every bridge in America using publicly available federal inspection data. We transform raw structural ratings into a single Condition Score so anyone can understand the state of the infrastructure they drive across every day. Here is exactly how we do it.

Data Source

Our sole data source is the FHWA National Bridge Inventory (NBI), maintained by the Federal Highway Administration. The NBI is the most comprehensive bridge database in the United States, covering every bridge on a public road longer than 20 feet. Each bridge is inspected at least once every 24 months by certified inspectors following the FHWA Recording and Coding Guide.

The NBI data is published annually as bulk ASCII files. Each record includes over 100 fields covering location, design, traffic, and condition ratings for the bridge's three primary structural components.

How We Calculate the Condition Score

Every bridge receives a Condition Score on a 0-100 scale, mapped to letter grades A through F. The score is a weighted composite of three NBI condition ratings plus an age adjustment:

  • Deck Condition — 33% weight. The condition of the bridge deck (the driving surface). NBI rates this 0-9, where 9 is excellent and 0 is failed. We normalize the 0-9 rating to a 0-100 scale.
  • Superstructure Condition — 33% weight. The condition of the primary structural members (beams, girders, trusses) that support the deck. Also rated 0-9 by inspectors and normalized to 0-100.
  • Substructure Condition — 34% weight. The condition of the foundations, piers, and abutments that transfer loads to the ground. Rated 0-9, normalized to 0-100.

An age penalty is applied to bridges over 50 years old. Older bridges face increased material fatigue and may use outdated design standards, so the age penalty provides additional differentiation among bridges with similar inspection ratings.

Letter grades: A (80-100), B (65-79), C (50-64), D (35-49), F (0-34).

Structurally Deficient Classification

A bridge is classified as structurally deficient when any of its three primary components (deck, superstructure, or substructure) receives an NBI condition rating of 4 or below on the 0-9 scale. This is the standard federal definition used by FHWA.

Structurally deficient does not mean a bridge is unsafe or likely to collapse. It means the bridge has one or more components in poor condition that require maintenance, rehabilitation, or replacement. State DOTs monitor these bridges with increased inspection frequency.

Data Collection Process

We download the annual NBI bulk data files from FHWA and parse each state's records. Bridge identifiers (structure numbers) are used to track individual bridges across years. We extract condition ratings, traffic counts (ADT), year built, location coordinates, and ownership information for each bridge.

State-level statistics are aggregated from individual bridge records: total bridges, deficient count, average age, and total daily traffic crossings.

Update Frequency

FHWA publishes updated NBI data annually, typically in the spring. We process and publish updates within two weeks of each release.

Known Limitations

  • NBI condition ratings are based on visual inspection and engineering judgment — they do not include load testing or material sampling.
  • Inspection frequency is typically every 24 months, so ratings may not reflect very recent deterioration or repairs.
  • Culverts and tunnels use different rating scales and are not included in our Condition Score.
  • The Condition Score is our own composite metric, not an official FHWA designation. All safety determinations are made by state DOTs and FHWA.
  • Privately-owned bridges and bridges shorter than 20 feet are not included in the NBI.

How to Cite This Data

If you use data from BridgeSafety, please cite:

BridgeSafety. "[Bridge Name] Condition Data." www.bridgesafety.org, 2026. Accessed [date].

Underlying data is sourced from the FHWA National Bridge Inventory and is in the public domain.