Counting Poor Bridges and Weighting Them by Deck Area Give Two Different Answers

A bridge classified as Poor sounds like a bridge about to close, and the number of them in a state reads like a scoreboard. The federal system does not keep score that way. The National Bridge Inventory carries a condition class for each public highway bridge, but the measure written into federal regulation does not add the bridges up. It adds up their decks. In the 2026 inventory, 41,148 of 624,939 bridges were classified Poor — 6.58 percent of the structures, and 4.77 percent of the deck area.

That 1.81 percentage point gap is not a rounding artifact. It is the design of the measure. On the network the regulation actually governs, the same comparison runs the other way: the Poor class holds 2.81 percent of the count and 3.96 percent of the deck area. What follows is the chain that turns an inspector's coding sheet into a percentage, and the arithmetic that decides which way the gap opens.

The Chain

Seven steps separate a person standing under a girder from a percentage in a federal table. Two happen in the field. The rest is definition and arithmetic, and the definition has already been rewritten once.

  1. A bridge is inspected on an interval. Under 23 CFR 650.311, “Each bridge must be inspected at regular intervals not to exceed the interval established using one of the risk-based methods outlined in paragraph (a)(1) or (2) of this section.” The first of those methods, Method 1, is described as follows: “Inspection intervals are determined by a simplified assessment of risk to classify each bridge into one of three categories with an inspection interval as described below.” The second, Method 2, is described as follows: “Inspection intervals are determined by a more rigorous assessment of risk to classify each bridge, or a group of bridges, into one of four categories, with inspection intervals not to exceed 12, 24, 48, or 72 months.”
  2. Components are rated separately. The regulation names the deck, the superstructure and the substructure as NBI Items 58, 59 and 60, and rates a culvert once instead, under NBI Item 62. The page that defines the 2026 tables uses newer codes for the same four components: “Bridge Condition is determined by the lowest rating of National Bridge Inventory (NBI) condition ratings for Item BC01 (Deck), Item BC02 (Superstructure), Item BC03 (Substructure), or Item BC04 (Culvert).”
  3. The lowest of those ratings sets the class. The rule opens by fixing its own scope: “The condition of bridges carrying the NHS, which includes on- and off-ramps connected to the NHS, shall be classified as Good, Fair, or Poor following the criteria specified in this paragraph.” Under 23 CFR 490.409, “When the lowest rating of the 3 NBI items for a bridge (Items 58—Deck, 59—Superstructure, 60—Substructure) is 7, 8, or 9, the bridge will be classified as Good.” The Poor test is written the same way: “When the lowest rating of the 3 NBI items for a bridge is 4, 3, 2, 1, or 0, the bridge will be classified as Poor.” Fair occupies 5 or 6.
  4. A separate rule sets the minimum condition level. Under 23 CFR 490.411, “Beginning with calendar year 2018 and thereafter, a bridge will be classified as Structurally Deficient when one of its NBI Items, 58—Deck, 59—Superstructure, 60—Substructure, or 62—Culverts, is 4 or less.” Before that calendar year the same classification also attached when Item 67, Structural Evaluation, or Item 71, Waterway Adequacy, was 2 or less.
  5. Each structure is given a size. Deck area is the product of a length item and a width item. The bridge stops being one entry here and becomes a quantity of square feet.
  6. The square feet are pooled, within one network and one state at a time. The regulation directs that “For all bridges carrying the NHS, which includes on- and off-ramps connected to the NHS and bridges carrying the NHS that cross a State border, FHWA shall calculate a ratio of the total deck area of all bridges classified as Structurally Deficient to the total deck area of all applicable bridges for each State.” The next sentence fixes the precision: “The percentage of deck area of bridges classified as Structurally Deficient shall be computed by FHWA to the one tenth of a percent as follows:” Bridges that do not carry the National Highway System are outside this calculation.
  7. The ratio is compared with a floor. “State DOTs will maintain bridges so that the percentage of the deck area of bridges classified as Structurally Deficient does not exceed 10.0 percent. This minimum condition level is applicable to bridges carrying the NHS, which includes on- and off-ramps connected to the NHS within a State, and bridges carrying the NHS that cross a State border.”
From one inspection to one percentage Five of the seven steps are definition and arithmetic. Size enters only at step five. 1 Inspection Interval set by one of two risk-based methods 2 Component ratings Items 58, 59 and 60, or Item 62 for culverts 3 Lowest rating The lowest of those ratings sets the class 4 Condition class Good 7 to 9, Fair 5 or 6, Poor 4 or less 5 Deck area Length multiplied by width 6 Ratio, per state Poor NHS deck area over all NHS deck area, in each State 7 Comparison Measured against 10.0 percent What comes out A structure enters as its deck, not as a unit Steps and thresholds as written in 23 CFR 650.311, 23 CFR 490.409 and 23 CFR 490.411. Item numbers are National Bridge Inventory items.

What the Class Is Built From

Two features of that chain do most of the work. The first is the lowest-rating rule. A bridge whose deck and superstructure are coded 7 and whose substructure is coded 4 is Poor, and the classification carries no trace of the two components rated Good. The class is a floor, not an average.

The second is that the inventory runs two vocabularies at once. The performance measures in 23 CFR 490.409 speak of Good, Fair and Poor; the minimum condition level in 23 CFR 490.411 speaks of Structurally Deficient. Since calendar year 2018 the two tests turn on the same four items at the same cutoff, so a bridge that is Poor by the measure is Structurally Deficient by the floor. Before 2018 they could part company, because the older wording also swept in bridges rated 2 or less on Item 67 or Item 71. A chart of the older years is therefore not a chart of the same quantity.

Both vocabularies then hand the result to the same weighting. Neither one counts bridges.

Where the Two Counts Separate

The 2026 tables under Bridge Condition by Highway System give counts and deck areas side by side. Across all public highway bridges, the Poor class holds 41,148 of 624,939 structures and 217,000,522 of 4,548,152,281 square feet of deck. That is 6.58 percent by count against 4.77 percent by area, carried here to two decimals where the regulation's own measure is computed to one tenth.

Restrict the same file to bridges carrying the National Highway System and the relationship inverts. There the Poor class holds 4,144 of 147,243 structures and 102,912,580 of 2,598,042,855 square feet, which is 2.81 percent by count against 3.96 percent by area. The deck-area measure reads 1.15 points worse than a headcount on the network where the 10.0 percent floor applies, and 1.81 points better than a headcount across the inventory as a whole.

Taking the printed total rows of both tables, 624,939 structures less 147,243 leaves 477,696, and 4,548,152,281 square feet less 2,598,042,855 leaves 1,950,109,426 — the rest of the public stock. The Poor rows subtract the same way, to 37,004 structures and 114,087,942 square feet. That is 7.75 percent by count against 5.85 percent by area. These off-network figures are arithmetic on two published rows, not rows FHWA prints, which is worth stating plainly before they are used.

The averages behind those percentages are the reason. Divide deck area by count and the National Highway System yields 17,645 square feet per structure against 4,082 off it, a factor of 4.3. On the network, the average Good bridge covers 15,854 square feet, the average Fair bridge 18,649, and the average Poor bridge 24,834. Off the network the order reverses: 4,212 for Good, 4,126 for Fair and 3,083 for Poor.

Average deck area per bridge, by condition class On the National Highway System the Poor class holds the largest structures. Off it, the smallest. Square feet. 0 10,000 20,000 30,000 15,854 18,649 24,834 4,212 4,126 3,083 group average 17,645 group average 4,082 Good Fair Poor Good Fair Poor Carrying the National Highway System All other public bridges Source: Federal Highway Administration, National Bridge Inventory, Bridge Condition by Highway System, 2026. Right-hand values are the all-bridges total rows minus the National Highway System total rows.

The Multiplier That Decides the Direction

The relationship between the two answers is fixed. The share by deck area equals the share by count multiplied by the average size of the Poor structures divided by the average size of all structures in the same group. On the National Highway System that multiplier is 24,834 over 17,645, or about 1.41, which lifts 2.81 percent to 3.96 percent. Off the network it is 3,083 over 4,082, about 0.755, which pulls 7.75 percent down to 5.85 percent.

Everything therefore rests on the relative size of the structures in the Poor class. The average Poor bridge on the National Highway System covers about 8.1 times the deck area of the average Poor bridge off it, 24,834 square feet against 3,083 square feet. Two structures identical in a headline count are separated by roughly an order of magnitude in the quantity that does the weighting.

The inventory does not explain that difference, and neither does the regulation. Any account of why the National Highway System carries larger structures in the Poor class would need evidence outside these two tables.

The same class, two different weights Rectangles are drawn in proportion to average deck area. A headcount treats them as equals. 24,834 sq ft Average Poor bridge carrying the NHS 3,083 sq ft Average Poor bridge off the NHS Counted as structures 1 1 Counted as deck area 8.1 1 Averages computed from Federal Highway Administration, National Bridge Inventory, Bridge Condition by Highway System, 2026.

What the 2026 Snapshot Cannot Fix

The percentages above are firm arithmetic on a file that its own documentation qualifies in four ways, and the qualifications sit inside the numbers.

The first is age. A condition class is written at an inspection, and the intervals quoted above run out to 72 months under Method 2. A single year's file is therefore a mosaic of ratings recorded across a window several years wide.

The second is status. FHWA states that “Each data set is submitted in the spring and may be corrected or updated throughout the year.” It adds that “The data for public highway bridges is considered final and is archived by June 15th of each calendar year and published to this website shortly thereafter.” The 2026 page carries the date 8/4/2026, which falls after that archive date, so these figures are the archived set rather than a mid-season one. Which side of June 15 a given year's table sits on is not visible in the figures themselves.

The third is the width input itself. FHWA states that “Bridge Deck Area is determined by multiplying Length (SNBI Item BG02) by Width (SNBI Item BG05). During 2026 and 2027 as the data is being transitioned from the Coding Guide to the SNBI, if SNBI Item BG05 (Bridge Width) is greater than or equal to 10 feet, this value is used for Width in the calculation. If SNBI Item BG05 is less than 10 feet or null, as in cases under the Coding Guide where the roadway is on a fill over the structure and headwalls do not affect the flow of traffic, SNBI Item BG09 (Approach Roadway Width) is used for Width in the calculation. Beginning in 2028, only SNBI Item BG05 will be used for Width in the calculation, regardless of its value.” The quantity that carries the entire weighting is drawn from one of two width items during this transition, and the agency has published the year it ends.

The fourth is coverage. In the 2026 tables Guam's row carries dashes rather than figures in both tables, and the U.S. Virgin Islands row carries dashes in the National Highway System table. The page prints no note explaining the absence.

Where This Doesn't Apply

The 10.0 percent minimum condition level is a state-level test applied to NHS bridges. The national 3.96 percent computed above is an aggregate and says nothing about any individual state. Nothing here identifies which states sit above it.

The condition class is also not a verdict on whether a bridge may be crossed. Load restriction is tracked separately, which is why FHWA publishes Bridge Condition by Posting Status as its own table. A Poor rating and a posted bridge are different records.

Deck area is a measure of surface, not of use. Neither method weights a structure by the traffic it carries.

The reversal described here belongs to this pair of populations in this year's file. It follows from the average sizes in the 2026 tables, and a different year has to be recomputed.

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