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Method

How the bridge condition figures are made

How the pages of the Bridges hub read what their publishers put out, and what that record can and cannot say. The method every topic shares — where a figure comes from, snapshots, freshness and corrections — is on the site’s methodology page.

What the inventory holds

The Federal Highway Administration publishes the National Bridge Inventory as one file a year: one row for each highway bridge on the public roads, with its state, county and structure number, its owner’s type and its condition ratings. The pages read the file’s counts from the publisher’s own page, and FHWA is the cite beside every figure.

A poor rating is a condition rating

Each bridge is rated on four items: its deck, its superstructure, its substructure and its culvert. The file’s band takes the lowest of the applicable ratings. A lowest rating of 0 to 4 is rated poor, 5 or 6 is fair, and 7 to 9 is good. The band is FHWA’s download rule, and it is a condition rating. It is not a finding that a bridge is unsafe, and the file does not say that a poor bridge will fail or close.

Every share has its count beside it

A share is a percent of the bridges it is counted from: a state’s own bridges, an owner type’s own bridges, or every bridge in the year for an owner type’s share of the national total. Each share is printed with its count and its denominator, so a jurisdiction with few bridges reads as few bridges. Nothing is cut for being small.

How the state ranking is made

The 50 states and the District of Columbia are ranked by the share of their bridges rated poor, highest first, and the ranking says how many places it holds. A tie gives every tied place the same rank, so every place at the top of the ranking is ranked 1. Puerto Rico and the U.S. Virgin Islands are printed beneath it and are not ranked with the states.

Owner types

The file gives each bridge an owner type code and no owner’s name. The pages print each code in the wording of FHWA’s 1995 Recording and Coding Guide, whose Item 22 uses the list of Item 21. The file also carries codes the 1995 guide does not define; each of those is printed as its code, marked “not defined in the 1995 guide”, and no wording is supplied for it. A bridge with no owner code is one row with no type, and it is not ranked.

Names of places

A state or territory is named as the Census Bureau’s state estimates spell it, from the same file the demographics hub reads. A place with no name in that file is not printed under a made-up one.

Where the pages are listed

The hub is listed on the front door once its contact sheet has been read. Its pages are in the search index and the sitemap, and each is marked with the hub’s own title and figures.

What the figures cannot show

The file records each bridge’s condition in one year. It does not record whether a bridge is safe, what it costs to repair, or who is responsible for a failure, and no page says that it does. One year is not a trend, so no trend is drawn. No page says that the age, the traffic or the condition of a bridge causes any other.

What this data cannot show

  • A poor rating is a condition rating from FHWA’s download rule. It does not say that a bridge is unsafe, will fail or has a closure.
  • The file records each bridge once a year, and the site holds one year of it, so no page draws a trend.
  • Every share is a percent of the bridges it is counted from, and each count is printed beside it. The file names no owner, only an owner type.

Sources

Data retrieved