Status of the Nation's Highways, Bridges, and Transit:
2002 Conditions and Performance Report
|Chapter 3: System Conditions|
Part I: Description of Current System
Part II: Investment Performance Analyses
Part III: Bridges
Part IV: Special Topics
Part V: Supplemental Analyses of System Components
Three indicators are examined in this section: bridge condition ratings, the number of deficient bridges, and the percentage of deck area on deficient bridges. Each measure examines bridge conditions from a different perspective. Condition ratings provide a numerical evaluation of the condition of a bridge element. The number of deficient bridges is widely used by policymakers to describe bridge conditions nationwide, but it does not recognize the relative importance, from a mobility perspective, of an individual bridgeís contribution to the overall transportation system. The final indicatoróthe percentage of deck area on bridges classified as deficientóis increasingly used to document the state of bridge conditions; for example, the FY 2002 FHWA Performance Plan includes this measure as its new indicator. This chapter describes deck area on deficient bridges by owner and functional system. Information on National Highway System (NHS) bridges is described in Chapter 24.
Bridge Condition Ratings
The National Bridge Inventory (NBI) contains ratings on the conditions of three major bridge components: the deck, superstructure, and substructure. A bridge deck is the primary surface used for transportation. The deck is supported by the superstructure, which carries the load of the deck and the traffic. Within the superstructure are the girders, stringers, and other structural elements. The substructure is the foundation of the bridge and transfers the loads of the structure to the ground. The superstructure is supported by substructure elements, such as abutments and piers. Exhibit 3-21 describes bridge condition ratings in greater detail.
Condition ratings are used to describe the existing, inplace status of a component, not its as-built state. Engineers assign condition ratings by evaluating the severity of deterioration or disrepair and the extent to which it is widespread throughout the component being rated. A condition rating does not translate directly into an overall rating of a bridgeís condition, but it is a good indicator of the quality of specific elements.
Exhibit 3-22 illustrates the distribution of bridge condition ratings. Most bridge components are rated 7 or higher, indicating that they are in good, very good, or excellent condition. Another one-third of all bridge components are rated 5 or 6, indicating fair or satisfactory condition. The remainder of bridge components are rated 4 or lower, indicating a poor or worse condition.
Number of Deficient Bridges
The most commonly-cited indicator of bridge condition is the number of deficient bridges. There are two types of deficient bridges: structurally deficient and functionally obsolete. Bridges are considered structurally deficient if they are restricted to light vehicles, require immediate rehabilitation to remain open, or are closed. A deficient bridge may or may not be dangerous, but it does require significant maintenance, rehabilitation, or sometimes replacement. Bridges are considered functionally obsolete if they have deck geometry, load carrying capacity, clearance, or approach roadway alignment that no longer meets the criteria for the system of which the bridge is a part.
As shown by Exhibit 3-23, about 28.5 percent of the Nationís bridges were deficient in 2000. Of these deficient bridges, about 14.8 percent were structurally deficient and 13.8 percent were functionally obsolete.
The number of deficient bridges has steadily decreased over the past decade. In 1994, about 32.5 percent of the Nationís bridges were deficient, but that number had dropped by almost 4 percent by 2000. The long-term trend is consistent with expectations in the Federal Highway Administrationís 1998 Strategic Plan, which stated that less than 25 percent of the Nationís bridges should be deficient by 2008. Exhibit 3-24 describes the trend data in more detail.
A more specific way of looking at the number of deficient bridges is by owner. As Chapter 2 explained, ownership of bridges is largely divided among State and local governments (47.2 and 50.9 percent, respectively). The remaining bridges, totaling 1.4 percent, are split among the Federal Government, private companies, and entities for which ownership is unknown or not coded in the National Bridge Inventory.
Exhibit 3-25 examines bridge deficiencies by owner. This exhibit shows substantial differences by level of government and type of owner. The Federal Government, for example, has the smallest percentage of deficient bridges (24.8 percent), but also owns a relatively small number of bridges (8,221). States have almost the same percentage of deficient bridges (24.9 percent), but have a much larger number of bridges (277,106). About 31.8 percent of the 298,889 bridges owned by local governments are deficient, while 53.1 percent of the Nationís 2,299 private bridges are deficientóthe highest percentage of any owner type.
Most deficiencies on locally-owned bridges are structural, while most deficiencies on State and Federal bridges involve functional obsolescence. Exhibits 3-26 and 3-27 illustrate this phenomenon. About 69.5 percent of structurally deficient bridges were locally-owned, 29 percent were State-owned, and the remaining 1.5 percent were owned by the Federal Government, private companies, or other entities. Conversely, States owned about 54 percent of all functionally obsolete bridges. Local governments owned 43.4 percent of functionally obsolete bridges, and Federal, private, and other entities owned the remaining 2.6 percent.
Another way of looking at the number of deficient bridges is by rural and urban location. As Chapter 2 noted, 77.5 percent of bridges were in rural communities in 2000. About 27.6 percent of these rural bridges were deficient. At the same time, about 31.9 percent of the nationís urban bridges were deficient; therefore, urban bridges are more likely to be deficient than their rural counterparts.
Bridge condition in both urban and rural areas has steadily improved over the past decade. Exhibit 3-28 shows that the number of deficient rural bridges dropped from 31.8 percent in 1994 to 27.6 percent in 2000. More specifically, the number of structurally deficient rural bridges dropped from 20.2 percent in 1994 to 16.2 percent in 2000. The number of functionally obsolete rural bridges decreased less dramaticallyófrom 11.6 percent in 1994 to 11.4 percent in 2000.
Exhibit 3-28 also shows that the number of deficient urban bridges dropped from 35.3 percent in 1994 to 31.9 percent in 2000. The number of structurally deficient urban bridges decreased from 13 percent in 1994 to 9.9 percent in 2000, while the number of functionally obsolete bridges diminished only slightly, from 22.3 percent in 1994 to 22 percent in 2000. The significant drop in urban bridge deficiency, therefore, can largely be attributed to improvements in the structural integrity of bridges in metropolitan areas.
Exhibit 3-29 elaborates on a central conclusion of the previous section: that bridges are more likely to be deficient in urban areas. Bridges on urban Interstates, urban principal arterials, and urban minor arterials have a higher percentage of deficiencies than those on comparable rural functional systems. Local functional class bridges represent a break from this pattern. A larger percentage of rural local functional class bridges are deficient (34.7 percent) than urban local functional class bridges (31.6 percent).
The proportion of structurally deficient and functionally obsolete bridges varies by functional system. Generally, the percentage of bridges that are deficient is greater on lower functional systems. Interstate bridges, for example, have the lowest percentage of deficient bridges (16 percent in rural areas and 27 percent in urban areas). Urban minor arterials and urban collectors have the highest percentage of deficient bridges (37.3 percent for each system). The healthy condition of many higher-level bridges is striking, particularly since these account for a large share of VMT.
Exhibits 3-30 through 3-33 provide a historical perspective on the level of bridge deficiency by functional classification. Generally, bridge condition has improved on Interstates, other principal arterials, collectors, and local roads over the past decade. The greatest decline in deficiency occurred in the early to mid-1990s, particularly for Interstate bridges. Looking more specifically at the types of deficiency, structural deficiency consistently decreased on the systems profiled in Exhibits 3-30 through 3-33, while functional obsolescence either remained relatively constant or even increased slightly. On collectors, for instance, 16.1 percent of bridges were structurally deficient in 1994, but that number had dropped to 13.2 percent by 2000. At the same time, 11.9 percent of collector bridges were functionally obsolete in 1994, but that number had risen to 12.3 percent by 2000.
Deck Area on Deficient Bridges
A third indicator of bridge condition is deck area on deficient bridges. Engineers and policy analysts are increasingly using this measure to describe the condition of the Nationís bridges. The Federal Highway Administrationís FY 2002 Performance Plan, for example, includes this indicator for NHS and non-NHS bridges. This section examines the deck area on deficient bridges by owner and functional system.
As Exhibit 3-34 describes, the nationwide percentage of deck area on deficient bridges dropped from 30.9 percent in 1996 to 27.9 percent in 2000. Bridges with unknown or unclassified ownership had the largest percentage of deck area on deficient bridges (42.8 percent in 2000), followed by privately owned bridges (33.8 percent). Federally owned bridges had the smallest percentage of deck area on deficient bridges (25.8 percent in 2000).
Exhibit 3-35, describes this information by functional system. The percentage of deck area on bridges classified as deficient decreased on every functional system from 1996 to 2000. Urban Collector bridges had the largest percentage (39.6 percent). Using this indicator, the deck area on bridges classified as deficient was consistently larger for urban systems.
Exhibit 3-36 describes the percentage of deck area on deficient bridges in 2000, with data broken down by structural deficiency and functional obsolescence. On almost every functional system, the percentage of deck area on functionally obsolete bridges was far greater than the area for structurally deficient bridges. On urban Interstates, for example, 22.8 percent of the deck area on deficient bridges resulted from functionally obsolete bridges while 8.8 percent can be attributed to those bridges classified as structurally deficient.