Where does this data come from?
The bridge data used in this interactive map (and all national/state reports) is derived from the Federal Highway Administration’s (FHWA) National Bridge Inventory (NBI) which is composed of what states are required to report to the FHWA once a year. NBI data is released annually and provides a significant level of detail on the condition of over 700,000 bridges nationwide.
The bridges included in this report are defined as ‘highway bridges’ — about 600,000 total bridges of the 700,000 that states report on. According to the FHWA, ‘highway bridges’ are those that accommodate automobile traffic, are at least 20 ft. in length and have traffic passing on the bridge structure (not underneath it). These bridges may carry other modes, but they must carry automobiles to be part of this database. Bridges excluded from this analysis include those that do not accommodate automobiles (e.g.: pedestrian/cyclist-only bridges, railroad-only bridges), and bridges shorter than 20 feet, such as culverts.
Bridges are inspected every two years, unless they’re in “very good” condition (four years) or “structurally deficient” (every year, though many states inspect these bridges far more often.)
Finding bridges near you: About our interactive map
Our interactive mapping tool relies on geolocation data (latitude and longitude) provided by states in their regular reporting to the FHWA. Because every state is responsible for their own data collection and reporting, inaccurate coordinates abound in this dataset, and many bridges may not appear in the proper location. While geolocation data (coordinates on the map) may be wrong, the “facility” and “near” information in the right hand box when plotting a bridge is generally reliable. We’ve found some counties with all bad coordinates, or counties that plot all their bridges to one set of coordinates. (Iowa is consistently inaccurate, for example.)
Unfortunately, with 600,000 bridges in the federal database we’re working with, there’s little we can do to fix this, other than hope that this and other federal datasets are made open, accessible, transparent and standardized so taxpayers can easily analyze information like this to assess whether or not their elected leaders are doing a good job with their money. If we can’t easily track the performance of a bridge or where federal transportation dollars are going, how can we know if the state is doing a good job?
If you’re a media member or blogger looking for a specific bridge, we can often help track it down. Email us at info@t4america.org with more specific inquiries about the data.
Is this data completely current?
The data on the interactive map and any 2015 state reports was released in early 2015 by FHWA and reported by states to the FHWA in the previous year. It’s worth noting that inspection data in the NBI could have taken place in a range covering the previous 1-2 years, if not further back for some bridges in good condition. It’s perhaps most helpful to think of the NBI dataset as a summary of bridge conditions during a window of time, not an exact snapshot of bridge condition at one specific moment in time. States report their data to the feds at different points during the year, and bridges may be inspected again afterward or repairs made in the many months between their reporting date and the release of the FHWA dataset. Hence there might be some discrepancy between our data and what a state reports today. States will have the most up-to-date records of bridge condition, but this is the sole national collection of bridge inspection data.
About the definitions
Highway bridges have three major components, though there are others: 1) the deck, which is the top surface of the bridge that cars, trucks and people cross; 2) the superstructure, which supports the deck; and 2) the substructure, which uses the ground to support the superstructure. Each of these bridge features is given a rating between 0 and 9 when inspected, with 9 signifying the best condition.
Structurally deficient: Federal guidelines classify bridges as “structurally deficient” if one of these three key components is rated at 4 or less (poor or worse), meaning engineers have identified a major defect in its support structure or its deck. (There are exceptions and just a few other select criteria that can produce a SD classification, but this covers the vast majority of bridges rated “deficient.”) Deficient bridges require significant maintenance, rehabilitation or replacement. A state may have to restrict heavy vehicle traffic, conduct immediate repairs to allow unrestricted use or close the bridge to traffic until repairs can be completed. Federal law requires states to inspect all bridges 20 feet or longer at least every two years. Bridges in “very good” condition may go four years between inspections, while those rated “structurally deficient” must be inspected every year.
Not deficient: Bridges that are not structurally deficient are categorized as “not deficient” or “functionally obsolete.”
Functionally obsolete: Bridges rated “functionally obsolete” are those that have design elements that are no longer used today or up to current standards. This could refer to the length of an on-ramp, the width of lanes, the weight capacity, or any other element that has no bearing on its structural sufficiency. A bridge cannot be structurally deficient and functionally obsolete.
Fracture-critical: This is an additional data point in a bridge record, separate from deficiency or obsolescence. A fracture-critical bridge is one that does not contain redundant supporting elements, so failure of one of those components can lead to collapse. Scary as it sounds, fracture critical bridges are deemed perfectly safe as long as they remain structurally sound. There are 12,000 “fracture-critical” bridges in good repair. More concerning than those 12,000 are the nearly 8,000 U.S. bridges that are classified as structurally deficient and fracture critical.
