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Status of the Nation's Highways, Bridges, and Transit:
2004 Conditions and Performance
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Chapter 16 (Continued)

Physical Conditions

Chapter 3 describes the physical conditions of highways throughout the United States. There are numerous ways to examine physical conditions. This section focuses on Interstate pavement condition, lane width, alignment adequacy, bridge deficiencies, and bridge age.

Pavement Condition

Exhibit 16-5 shows the percentage of total Interstate miles with "Acceptable" or better ride quality by function class for select years from 1995 to 2002. Exhibit 16-6 shows the percentage of Interstate pavement meeting a standard of "Good" ride quality. (Data for other functional systems can be found in Exhibit 3-14.) Since 1995, the number of Interstate miles rated as having "Good" ride quality has increased for all three population subsets of Interstate highways.

Exhibit 16-5 Percent of Interstate Miles with Acceptable Ride Quality, 1995-2002
Location of Interstates19951997199920002002
Rural Areas94.5%95.9%97.6%97.8%97.8%
Small Urban Areas94.4%95.8%95.4%95.7%95.3%
Urbanized Areas90.0%90.0%92.2%93.0%91.7%
Source: Highway Performance Monitoring System.

Exhibit 16-6 Percent of Interstate Miles with Good Ride Quality, 1995-2002
Location of Interstates19951997199920002002
Rural Areas51.8%56.9%65.4%68.5%71.9%
Small Urban Areas49.8%51.4%58.2%61.6%64.9%
Urbanized Areas41.4%39.3%45.0%48.2%48.7%
Source: Highway Performance Monitoring System.

In 2002, rural area Interstates had the greatest percentage of miles with "Acceptable" or better ride quality. About 98 percent of rural area Interstates met this standard. As a subset of the miles with "Acceptable" ride quality, 71.9 percent of rural Interstate miles met standards required for classification as "Good" ride quality.

For small urban Interstate miles, 95.3 percent met the criteria for "Acceptable" ride quality. As a subset of the miles with "Acceptable" ride quality, 64.9 percent met the standards to be classified as "Good" ride quality in the year 2002.

Q. How has the percent of Interstate travel occurring on pavements with "Acceptable "and "Good" ride quality changed since 1995?
A.

As discussed in Chapter 3, another way to evaluate ride quality is to consider the vehicle miles traveled on routes with "Acceptable" or "Good" ride quality, rather than simply looking at the miles of pavement themselves (see Exhibit 3-15). On this basis, the percentage of rural Interstate travel on pavements with "Acceptable" ride quality rose from 94.5 percent in 1995 to 97.3 percent in 2002, while the percentage of travel on pavements with "Good" ride quality rose from 53.3 percent to 72.2 percent.

Conditions also improved for urbanized Interstates, as the percentage of travel on pavements with "Acceptable" ride quality rose from 88.8 percent to 89.3 percent, while the percentage of travel on pavements with "Good" ride quality rose from 39.1 percent to 43.8 percent.

For small Urban Interstates, performance was mixed, as the percentage of travel on pavements with "Acceptable" ride quality declined from 94.9 percent to 94.6 percent, while the percentage of travel on pavements with "Good" ride quality rose from 51.4 percent to 65.1 percent.

In 2002, 91.7 percent of urbanized Interstate miles met the criteria for "Acceptable" ride quality. As a subset of this group meeting "Acceptable" ride quality, 48.7 percent of the urbanized Interstate miles met the standards to be classified as having "Good" ride quality.

Lane Width, Alignment, and Access Control

As described in Chapter 3, roadway alignment affects the level of service and safety of the highway system. Inadequate alignment may result in speed reductions as well as impaired sight distance. In particular, trucks are affected by inadequate roadway alignment with regard to speed.

There are two types of alignment: horizontal (curvature) and vertical (gradient). Alignment adequacy is evaluated on a scale from Code 1 (best) to Code 4 (worst). Exhibit 16-7 summarizes alignment for rural Interstates (alignment is normally not an issue in urban areas). More than 93.3 percent of rural Interstate miles are classified as Code 1 for vertical and 95.7 percent are classified as Code 1 for horizontal alignment.

Exhibit 16-7 Rural Interstate Vertical/Horizontal Alignment Status for 2002 (Percent of Miles)
    Vertical Horizontal
Code 1: All curves and grades meet appropriate design standards. 93.3% 95.7%
Code 2: Some curves or grades are below design standards for new construction, but curves can be negotiated safely at prevailing speed limits. Truck speed is not substantially affected. 5.9% 1.1%
Code 3: Infrequent curves or grades occur that impair sight distance or severely affect truck speeds. May have reduced speed limits. 0.3% 0.8%
Code 4: Frequent grades occur that impair sight distance or severely affect truck speeds. Generally, curves are unsafe or uncomfortable at prevailing speed limit, or the speed limit is severely restricted due to the design speed limits of the curves. 0.5% 2.4%
Source: Highway Performance Monitoring System.

Lane width can have an impact on highway safety and operational performance. Currently, higher functional systems such as Interstates are expected to have 12-foot lanes. As shown in Exhibit 16-8, approximately 99.8 percent of rural Interstate miles and 98.5 percent of urban Interstate miles have minimum 12-foot lanes widths (see also Exhibits 3-19 and 3 20 in Chapter 3).

Exhibit 16-8, interstate lane width. Bar chart plotting values for five lane width categories. On small urban or urbanized interstates, values for lane width of less than nine feet, nine feet, and 10 feet are 0.02% each. The value for lane width of 11 feet is 1.48%, and the value for above 12 feet is 98.46%. On rural interstates, the value for lane width of less than nine feet is 0.02%, 0% for nine feet, and 0.01% for 10 feet, and 0.18% for 11 feet. The value for above 12 feet is 99.78%. Source: Highway Performance Monitoring System.

The vast majority of the Interstate mileage consists of divided highways with a minimum of four lanes and with full access control. The Interstate Systems for Alaska and Puerto Rico are not required to meet this standard. For Alaska and Puerto Rico, the requirement is that construction is adequate for current and probable future traffic demands and the needs of the locality. In Alaska, 1,034 miles of rural Interstate are not required to have a minimum of four lanes and full access control. For urban Interstates, 104 miles do not meet the specified criteria for access control; 53 of these miles are in Puerto Rico and the remaining miles are in Alaska.

Bridge Conditions

Exhibit 3-33 in Chapter 3 identifies bridge deficiencies by functional system, while Exhibit 3-35 shows the percentage of rural and urban bridge deficiencies for the Interstate System in particular. Approximately 15.8 percent of all rural Interstate bridges were deficient in 2002, including 1,104 that were structurally deficient (about 4.0 percent of the total number) and 3,210 that were functionally obsolete (11.8 percent of the total number). Among rural functional systems, only other principal arterials had a lower percentage of bridge deficiencies.

About 26.3 percent of all urban Interstate bridges were deficient in 2002. This included 1,715 structurally deficient bridges (6.1 percent of total urban Interstate bridges), and 5,617 functionally obsolete bridges (20.1 percent of the total). Among urban functional systems, the Interstate System had the lowest percentage of deficient bridges.

The number of deficient bridges has steadily declined in recent years. In 1994, for example, 18.5 percent of rural Interstate bridges were deficient. That number has declined to 15.8 percent. The number of deficient urban Interstate bridges also declined, from 30.6 percent in 1994 to 26.3 percent.

The Federal Highway Administration also looks at bridge deficiencies by the percent of deficient deck area. Approximately 17.9 percent of the rural Interstate bridge deck area was deficient in 1996. This has decreased to 14.6 percent in 2002. This is the lowest percent deficient deck area for all rural functional classes.

Q. How old are most Interstate bridges?
A.

The aging of Interstate bridges is a significant concern for the Federal Highway Administration and its State and local partners.

Exhibit 16-9 describes the age of rural Interstate bridges. About 47.9 percent of rural Interstate bridges were built during the early years of the Interstate System, from 1961 to 1970. More than 68.2 percent of all rural Interstate bridges in 2002 were at least 30 years old.

Exhibit 16-10 describes the age of urban Interstate bridges. About 41.2 percent of urban Interstate bridges were built between 1961 and 1970. Over 61.5 percent of all urban Interstate bridges in 2002 were at least 30 years old.

Exhibit 16-9, age composition of rural interstate bridges, 2002. Pie chart in seven segments. Bridges constructed in 2001 and after account for 0.3%; the value for 1991 to 2000 is 2.9%; 1981 to 1990 is 6.8%; 1971 to 1980 is 21.7%, 1961 to 1970 is 47.9%, 1951 to 1960 is 17.2% and 1950 and before is 3.2%. Source:  National Bridge Inventory.

Exhibit 16-10, age composition of urban interstate bridges, 2002. Pie chart in seven segments. Bridges constructed in 2001 and after account for 0.7%; the value for 1991 to 2000 is 7.7%; 1981 to 1990 is 11.4%; 1971 to 1980 is 18.7%, 1961 to 1970 is 41.2%, 1951 to 1960 is 18.4% and 1950 and before is 2%. Source:  National Bridge Inventory.

The percent of deficient deck area on urban Interstate bridges was 34.2 percent in 1996. By 2002, this had decreased to 31.0 percent.

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