Bridge Report Card Methodology
BridgeLookup report cards turn public Federal Highway Administration National Bridge Inventory records into a county-level Bridge Condition Exposure Score. The score is meant to help reporters, researchers, civic groups, and residents find places where bridge maintenance questions may deserve a closer look.
Important limitation
The report card is not an official government grade and it is not a bridge safety certification. A poor-condition bridge is not automatically unsafe. Unsafe bridges are closed, posted, or restricted by the responsible transportation agency.
Data Source
BridgeLookup uses the Federal Highway Administration's National Bridge Inventory (NBI), an annual bridge dataset assembled from state transportation agency submissions. The local BridgeLookup database currently reflects the site dataset last updated on 2026-04-03.
The NBI includes bridge location, ownership, condition ratings, average daily traffic, truck share, structure dimensions, year built, inspection fields, and scour-critical foundation ratings. Report cards only use fields that are available consistently enough for county-level comparison.
Which Counties Are Scored?
County report-card rankings include counties with at least 10 tracked bridges. This avoids giving tiny inventories unstable grades based on one or two structures. Individual county report-card pages may exist for smaller counties, but national ranking tables focus on counties with enough records to compare more fairly.
Score Formula
Each county starts at 100 points. BridgeLookup subtracts weighted penalties for five factors:
| Factor | Weight | Why it matters | Penalty rule |
|---|---|---|---|
| Poor-condition bridge share | 40 | Shows how much of the local bridge inventory has significant component deterioration. | min(40, poor % × 2.0) |
| Poor-condition traffic exposure | 25 | Prioritizes poor-condition bridges that carry more daily travel and would disrupt more people if repaired, restricted, or replaced. | min(25, poor-ADT % × 0.75) |
| Scour-critical bridge share | 20 | Captures foundation vulnerability during water and flood events. | min(20, scour-critical % × 2.0) |
| Average age above national baseline | 10 | Older inventories often require more inspection and maintenance, though age alone is not a condition rating. | min(10, years above national average × 0.4) |
| Traffic-data completeness | 5 | Counties with no measurable traffic records get a small confidence penalty because exposure is harder to compare. | 0–5 points |
Final score = 100 minus all penalties, rounded to the nearest whole number and capped between 0 and 100.
Letter Grades
| Score | Grade | Interpretation |
|---|---|---|
| 90–100 | A | Lower relative bridge-condition exposure. |
| 80–89 | B | Some exposure, but comparatively limited. |
| 70–79 | C | Moderate exposure worth monitoring. |
| 60–69 | D | Elevated exposure across one or more factors. |
| 0–59 | F | High relative exposure and a strong candidate for local follow-up. |
Definitions
| Poor condition | Overall NBI condition of poor, usually because the lowest major bridge component rating is 4 or below. |
| Poor-condition traffic exposure | Average daily traffic on poor-condition bridges. Nationally this dataset shows 163,724,147 daily crossings on poor-condition bridges out of 4,986,527,728 measured daily crossings. |
| Scour-critical | NBI scour-critical ratings 0 through 3. BridgeLookup counts 19,774 such bridges nationally. |
| Average bridge age | Calculated from year built for bridges with a valid year-built field. |
How to Use These Pages
Use report cards as a starting point for questions, not as final proof of a problem. Strong local follow-up questions include: Which agency owns the highest-exposure bridges? Are repairs programmed? Are weight restrictions posted? Has recent inspection data changed since the annual NBI submission?
Browse county report cards, view the weakest county scores, or download the county report-card CSV.