Highway Bridges in the United States—an Overview

by Jeffery Memmott, Ph.D.

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Bridges are an integral part of the U.S. highway network, providing links across natural barriers, passage over railroads and highways, and freeway connections. The Federal Highway Administration (FHWA) maintains a database of our nation's highway bridges—the National Bridge Inventory (NBI)—with detailed information on all public road bridges greater than 20 feet. This special report gives a brief synopsis of that inventory, including bridge condition and the resources spent for maintenance and upgrades.

Bridge Characteristics

Most highway bridges are owned by state and local entities. Table 1 gives a breakdown of bridge ownership over the period 1996 to 2006. Slightly more than 50 percent of bridges are owned by local agencies, with state agencies owning about 48 percent. The remaining 2 percent are owned by federal agencies and private entities. Ownership is important because the owner of the bridge is responsible for the maintenance and operation of the structure, though in many cases agreements are made with other agencies to perform the actual maintenance and operation work.¹

Most bridges are in rural areas, but urban bridges carry the most traffic (figure 1 and figure 2). Almost 77 percent of all bridges are located on rural highways, with 59 percent of those bridges in the two lowest highway type categories, rural collector and rural local roads (see box A). In contrast, most of the average daily traffic (ADT) is carried on urban bridges—about 73 percent of all traffic crossing bridges in the United States. Urban interstate bridges, in particular, represent less than 5 percent of the total number of bridges but carry almost 35 percent of the traffic. At the other extreme, rural local roads have about 35 percent of the bridges but carry less than 2 percent of the traffic.

About 27 percent of the bridges in the United States were built between 1957 and 1971, reflecting increased bridge construction during the interstate construction era from the late 1950s through the early 1970s. However, a large number of bridges have been constructed in recent years; about 25 percent of the bridges are less than 20 years old. There are more than 9,900 bridges still in operation in the United States that are over 100 years old. Figure 3 shows the total number of bridges and the number of structurally deficient and functionally obsolete bridges by age.

Bridge Ratings

Bridges are inspected and rated across a number of criteria, including load-carrying capacity, clearances, waterway adequacy, and approach roadway alignment. The FHWA reports that routine inspections are typically conducted every 24 months, with some bridges warranting more frequent inspections. With FHWA approval, a State may increase the inspection interval up to, but not to exceed, 48 months.² These bridges will be in very good condition and conform to a very stringent list of requirements. As a result of inspections and evaluation of NBI data, bridges are identified as not deficient, functionally obsolete, or structurally deficient. Structurally deficient takes precedence over functionally obsolete, so a bridge that is both structurally deficient and functionally obsolete would be classified as structurally deficient.³ (see box B for definitions)

Older bridges are more likely to be structurally deficient and functionally obsolete than newer bridges. For example, the proportion of structurally deficient and functionally obsolete bridges is above 20 percent in the 35 to 39 years old category, over 40 percent in the 55 to 59 years old category, and over 50 percent in the 80 to 84 years old category.
The proportion of structurally deficient bridges is much lower than the proportion of functionally obsolete bridges for newer bridges, but rises much faster and is much higher for older bridges. For example, in the 15 to 19 years old category, about 3 percent of the bridges are structurally deficient, compared to almost 10 percent that are functionally obsolete. In contrast, in the 95 to 100 years old category, 53 percent are structurally deficient, compared to 21 percent that are functionally obsolete.

The percentage of structurally deficient bridges declined from 1992 to 2006, while the percentage of functionally obsolete bridges remained fairly constant over that time period. Of the bridges in the NBI database in 2006, 12.4 percent are listed as structurally deficient and 13.4 percent as functionally obsolete. Figure 4 shows the trends in bridge deficiencies from 1992 to 2006.

The total number of bridges classified as structurally deficient declined from 119,000 bridges in 1992 to 74,000 in 2006. The number of structurally deficient bridges over the period 1992 to 2006 is shown in figure 5.

The highest percentage of bridges classified as structurally deficient – 19 percent − are on local rural roads. Interstate highways have the lowest percent of structurally deficient bridges, with rural interstates at about 4 percent and urban interstates at about 6 percent. Functional obsolescence is much more prevalent in the urban functional categories. About 22 percent of all urban bridges are classified as functionally obsolete, compared to about 11 percent of rural bridges. The percentages of bridge deficiencies vary by highway type as shown in figure 6.

Spending on Bridges

Table 2, taken from the FHWA's Conditions and Performance Reports, gives total bridge capital outlays for selected years. In 2004 capital outlays for bridge rehabilitation and replacement amounted to $10.5 billion out of a total capital outlay of $12 billion (in 2004 dollars). The FHWA estimates future capital investment needs in biennial reports to Congress on highway, bridge, and transit conditions and performance. The most recent report, 2006 Status of the Nation's Highways, Bridges, and Transit: Conditions and Performance, draws primarily on 2004 data.⁴

Conclusion

Bridges play a critical role within the highway network and the overall transportation system in the United States. There are more than 590,000 highway bridges in the United States, and most are owned by state or local government entities. Most bridges are located on rural collector and rural local roads, but urban interstates and other urban arterial highways carry most of the traffic. The highest proportion of bridges were built during the peak interstate construction period from the late 1950s through the early 1970s, but there are many older bridges still in use. A large number of bridges have also been built in recent years. The number of structurally deficient bridges has been declining continuously since 1992. The number of functionally obsolete bridges has stayed relatively constant since 1992. Almost 26 percent (12.4 percent structurally deficient and 13.4 percent functionally obsolete) of the bridges in the United States are currently classified as either structurally deficient or functionally obsolete. Bridges on rural local highways have the highest percentage of structurally deficient bridges, with rural and urban interstates the lowest. The bridges with the highest traffic volumes—urban interstates and urban other arterials—have low percentages of structurally deficient bridges. Spending on bridge rehabilitation and replacement has generally been rising in recent years, accounting for $10.5 billion out of $12 billion in total bridge capital outlays in 2004.

¹ U.S. Department of Transportation, Federal Highway Administration, 2006 Status of the Nation's Highways, Bridges, and Transit: Conditions and Performance, p. 2-18. http://www.fhwa.dot.gov/policy/2006cpr/pdfs/chap2.pdf

² General guidelines are provided in FHWA Technical Advisory 5140.21.

³ U.S. Department of Transportation, Federal Highway Administration, 2006 Status of the Nation's Highways, Bridges, and Transit: Conditions and Performance, p. 3-12. http://www.fhwa.dot.gov/policy/2006cpr/pdfs/chap3.pdf

⁴ U.S. Department of Transportation, Federal Highway Administration, 2006 Status of the Nation's Highways, Bridges, and Transit: Conditions and Performance, http://www.fhwa.dot.gov/policy/2006cpr/pdfs.htm