Bureau of Transportation Statistics

Special Report

U.S. Department of Transportation

Research and Innovative Technology Administration

September 2007

Highway Bridges in the United States—an Overview

by Jeffery Memmott, Ph.D.

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 traffi c 

(fi gures 1 and 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 traffi c (ADT) 

is carried on urban bridges—about 73 

percent of all traffi c crossing bridges 

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

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 traffi c. At the other extreme, rural 

local roads have about 35 percent of 

the bridges but carry less than 2 percent of the traffi c. 

About 27 percent of the bridges in the 

United States were built between 1957 

and 1971, refl ecting 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 defi cient and functionally obsolete 

bridges by age. 

Table 1: Number of Highway Bridges by Owner, 1996-2006

Owner 1996 1998 2000 2002 2004 2006

Federal 6,250 7,452 8,010 9,049 8,425 8,355

State 273,247 274,263 277,076 280,216 282,527 284,668

Local 299,125 298,280 298,791 299,275 300,385 301,912

Private/railroad 2,382 2,283 2,275 1,501 1,479 1,490

Unknown/unclassifi ed 484 441 415 400 397 375

Total 581,488 582,719 586,567 590,441 593,213 596,800

NOTE: Numbers include the 50 U.S. states, the District of Columbia, and Puerto Rico.

SOURCE: U.S. Department of Transportation, Federal Highway Administration, National Bridge Inventory, Highway Bridges by Owner, various years. http://www.fhwa.dot.gov/bridge/owner.htm as of Aug. 14, 2007.

SR-003
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Figure 1: Percent of Bridges by Highway Type

Rural interstate

Rural other arterial

Rural collector

Rural local

Urban interstate

Urban other arterial

Urban collector

Urban local

0

5

10

15

20

25

30

35

40

Percent

4.7

12.9

24.1

35.1

4.7

11.2

2.6

4.7

NOTE: Percentages include the 50 U.S. states, the District of 

Columbia, and Puerto Rico. 

SOURCE: U.S. Department of Transportation, Federal Highway 

Administration, 2006 Status of the Nation’s Highways, Bridges, 

and Transit: Conditions and Performance, Exhibit 2-16, p. 2-20, 

available at http://www.fhwa.dot.gov/policy/2006cpr/pdfs/chap2.

pdf

Figure 2: Percent of Average Daily Traffi c on Bridges 

 by Highway Type

Rural interstate

Rural other arterial

Rural collector

Rural local

Urban interstate

Urban other arterial

Urban collector

Urban local

0

5

10

15

20

25

30

35

40

Percent

10.6 10.5

4.6

1.6

34.7 33.3

2.3 2.3

NOTE: Percentages include the 50 U.S. states, the District of 

Columbia, and Puerto Rico. 

SOURCE: U.S. Department of Transportation, Federal Highway 

Administration, 2006 Status of the Nation’s Highways, Bridges, 

and Transit: Conditions and Performance, Exhibit 2-16, p. 2-20, 

available at http://www.fhwa.dot.gov/policy/2006cpr/pdfs/chap2.

pdf

Box A. Types of Highways

Highway bridges are located on all kinds of roads: the following terms are used to describe them, along 

with the functional classifi cations associated with each of the three major categories:

Arterials provide the highest level of mobility, at the highest speed, for long and uninterrupted travel. 

Arterials typically have higher design standards than other roads. They often include multiple lanes and 

have some degree of access control. The urban arterial system includes the functional classes interstate 

highways, other freeways and expressways, other principal arterials, and minor arterials. The rural network includes the functional classes interstate highways, other principal arterials, and minor arterials. In 

this report interstate highways are shown separately, and the other arterial functional classes are combined into the rural and urban other arterial categories.

Collectors provide a lower degree of mobility than arterials. They are designed for travel at lower 

speeds and for shorter distances. Generally, collectors are two-lane roads that collect travel from local 

roads and distribute it to the arterial system. The rural collector system includes the functional classes 

major collectors and minor collectors. In this report they have been combined into a single category. 

The urban collector system consists of a single functional class, urban collectors.

Local roads represent the largest element in the American public road network in terms of mileage. For 

rural and urban areas, all public road mileage below the collector system is considered local. Local roads 

provide basic access between residential and commercial properties, connecting with higher order highways. Both the urban and rural local road networks each consist of the lowest functional class, designated as the local functional class. 

SOURCE: adapted from the U.S. Department of Transportation, Federal Highway Administration and Federal Transit Administration, 2006 Status of the Nation’s Highways, Bridges, and Transit: Conditions and Performance report to Congress, p.2-7
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Figure 3: Distribution of Highway Bridges by Age and Condition in 2006

0-4

5-9

10-14

15-19

20-2425-2930-3435-3940-4445-4950-5455-5960-64

65-69

70-74

75-79

80-84

85-89

90-94

95-100

>100

0

10,000

20,000

30,000

40,000

50,000

60,000

Number of bridges

Age category in years

Non-deficient

Functionally obsolete

Structurally deficient

NOTE: Bridges that are both structurally defi cient and functionally obsolete are classifi ed as structurally defi cient. Numbers 

include the 50 U.S. states, the District of Columbia, and Puerto Rico. 

SOURCE: U.S. Department of Transportation, Federal Highway Administration, National Bridge Inventory, Bridge by Year Built, 

2006, available at http://www.fhwa.dot.gov/bridge/structyr.htm as of Aug. 8, 2007

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 

identifi ed as not defi cient, functionally obsolete, or structurally defi cient. Structurally defi cient takes precedence over 

functionally obsolete, so a bridge that is both structurally 

defi cient and functionally obsolete would be classifi ed as 

structurally defi cient.3

 (see box B for defi nitions)

Older bridges are more likely to be structurally defi cient 

and functionally obsolete than newer bridges. For example, the proportion of structurally defi cient 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. 

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

The proportion of structurally defi cient 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 defi cient, compared to almost 10 percent that are 

functionally obsolete. In contrast, in the 95 to 100 years old 

category, 53 percent are structurally defi cient, compared to 

21 percent that are functionally obsolete.

The percentage of structurally defi cient 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 defi cient and 13.4 percent as functionally obsolete. Figure 4 shows the trends in 

bridge defi ciencies from 1992 to 2006. 

The total number of bridges classifi ed as structurally defi -

cient declined from 119,000 bridges in 1992 to 74,000 in 

2006. The number of structurally defi cient bridges over the 

period 1992 to 2006 is shown in fi gure 5. 

The highest percentage of bridges classifi ed as structurally 

defi cient – 19 percent − are on local rural roads. Interstate 

highways have the lowest percent of structurally defi cient 

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 classifi ed as 
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Figure 4: Percentages of Highway Bridges with Defi ciencies, 1992-2006

1992

19931994199519961997

1998

1999

2000

2001

20022003

2004

2005

2006

0

5

10

15

20

25

Structurally deficient

Functionally obsolete

Percent

NOTE: Bridges that are both structurally defi cient and functionally obsolete are classifi ed as structurally defi cient. Numbers include 

the 50 U.S. states, the District of Columbia, and Puerto Rico. 

SOURCE: U.S. Department of Transportation, Federal Highway Administration, National Bridge Inventory, Defi cient Bridges by 

State and Highway System, various years, available at http://www.fhwa.dot.gov/bridge/britab.htm as of Aug. 14, 2007.

Box B. Structurally Defi cient and Obsolete Bridges as Defi ned by the FHWA

Bridges are considered structurally defi cient if signifi cant load-carrying elements are found to be in 

poor or worse condition due to deterioration and/or damage, or the adequacy of the waterway opening 

provided by the bridge is determined to be extremely insuffi cient to the point of causing intolerable traffi c 

interruptions. The fact that a bridge is “defi cient” does not immediately imply that it is likely to collapse 

or that it is unsafe. With hands-on inspection, unsafe conditions may be identifi ed and, if the bridge is 

determined to be unsafe, the structure must be closed. A “defi cient” bridge, when left open to traffi c, 

typically requires signifi cant maintenance and repair to remain in service and eventual rehabilitation or 

replacement to address defi ciencies. To remain in service, structurally defi cient bridges are often posted 

with weight limits to restrict the gross weight of vehicles using the bridges to less than the maximum 

weight typically allowed by statute.

Functional obsolescence is a function of the geometrics of the bridge in relation to the geometrics required 

by current design standards. While structural defi ciencies are generally the result of deterioration of the 

conditions of the bridge components, functional obsolescence results from changing traffi c demands on the 

structure. Facilities, including bridges, are designed to conform to the design standards in place at the time 

they are designed. Over time, improvements are made to the design requirements. As an example, a bridge 

designed in the 1930s would have shoulder widths in conformance with the design standards of the 1930s. 

However, the design standards have changed since the 1930s. Therefore, current design standards are based 

on different criteria and require wider bridge shoulders to meet current safety standards. The difference 

between the required, current-day shoulder width and the 1930s designed shoulder width represents a defi -

ciency. The magnitude of these types of defi ciencies determines whether the existing conditions cause the 

bridge to be classifi ed as functionally obsolete.

SOURCE: U.S. Department of Transportation, Federal Highway Administration, 2006 Status of the Nation’s Highway, Bridges, 

and Transit: Conditions and Performance, pp. 3-14, 3-15. http://www.fhwa.dot.gov/policy/2006cpr/pdfs/chap2.pdf
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functionally obsolete, compared to about 11 percent of 

rural bridges. The percentages of bridge defi ciencies vary 

by highway type as shown in fi gure 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

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

Figure 5: Number of Highway Bridges Categorized as Structurally Defi cient, 1992-2006

1992

1993

1994

19951996

1997

19981999

2000

2001

20022003

2004

20052006

0

20

40

60

80

100

120

140

Number (thousands)

119 112 107 104 102 98 93 88 87 84 81 80 78 76 74

NOTE: Bridges that are both structurally defi cient and functionally obsolete are classifi ed as structurally defi cient. Numbers include 

the 50 U.S. states, the District of Columbia, and Puerto Rico. 

SOURCE: U.S. Department of Transportation, Federal Highway Administration, National Bridge Inventory, Defi cient Bridges by 

State and Highway System, various years, available at http://www.fhwa.dot.gov/bridge/britab.htm as of Aug. 14, 2007.

Figure 6: Percentages of Highway Bridges with Defi ciencies by Highway Type in 2006

Rural interstate

Rural other arterial

Rural collector

Rural local

Urban interstate

Urban other arterial

Urban collector

Urban local

0

5

10

15

20

25

30

Structurally deficient Functionally obsolete

Percent

4.3

6.6

11.5

18.7

5.9

8.2

11.1 10.9 10.9 9.5 10.3 11.3

20.9

23.0

25.3

19.4

NOTE: Bridges that are both structurally defi cient and functionally obsolete are classifi ed as structurally defi cient. Percentages include the 50 U.S. states, the District of Columbia, and Puerto Rico. 

SOURCE: U.S. Department of Transportation, Federal Highway Administration, National Bridge Inventory, Count, Area, 

Length of Bridges by Highway System, 2006, available at http://www.fhwa.dot.gov/bridge/fc.htm as of Aug. 14, 2007.
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For related BTS data and publications

This report was prepared by Jeffery L. Memmott, Transportation Specialist, of the Bureau 

of Transportation Statistics (BTS). BTS is 

a component of USDOT’s Research and 

Innovative Technology Administration. The 

estimates in this report were developed from a 

variety of data sources and reviewed by staff 

in the Federal Highway Administration. 

The principal data sources are:

U.S. Department of Transportation, Federal 

Highway Administration, Status of the Nation’s 

Highways, Bridges, and Transit: Conditions 

and Performance, 2002, 2004, and 2006. 

http://www.fhwa.dot.gov/policy/2006cpr

U.S. Department of Transportation, Federal 

Highway Administration, National Bridge Inventory, Tables of Frequently Requested NBI 

Information. http://www.fhwa.dot.gov/bridge/

nbi.htm

For questions about this or other BTS reports, call 

1-800-853-1351, email answers@bts.gov, or visit 

www.bts.gov.

Data — 

• Commodity Flow Survey—survey reporting 

value, weight, and ton-miles by commodity, 

mode, origin, and destination.

• National Household Travel Survey – survey 

of daily and long-distance passenger travel in 

the United States, 2001.

Publications — 

• Transportation Statistics Annual Report 2006

• Government Transportation Financial Statistics 

Report 2003

• National Transportation Statistics

• State Transportation Statistics

About this Report

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 traffi c. 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 defi cient 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 defi cient 

and 13.4 percent functionally obsolete) of the bridges in 

the United States are currently classifi ed as either structurally defi cient or functionally obsolete. Bridges on rural 

local highways have the highest percentage of structurally 

defi cient bridges, with rural and urban interstates the lowest. The bridges with the highest traffi c volumes—urban 

interstates and urban other arterials—have low percentages of structurally defi cient 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. 

Table 2: Bridge Capital Outlays, Selected Years 

 (billions of current dollars)

Year

Bridge rehabilitation 

and replacement

Bridges for new highway 

construction Total

2004 10.5 1.6 12.0

2002 11.3 1.1 12.4

2000 7.6 1.2 8.8

1999 6.1 1.0 7.0

NOTE: Numbers may not add to totals due to rounding.

SOURCE: U.S. Department of Transportation, Federal Highway Administration, 

Status of the Nation’s Highways, Bridges, and Transit: Conditions and Performance, Years 1999, 2002, 2004, and 2006, Exhibit 6-13.