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35 Intermodal Connectors. These highways provide access 3.1.2 System Size between major intermodal facilities and the other sub- systems making up the NHS. "Lane miles" is defined as road mileage, with parallel lanes counted separately and thus is greater than miles in terms of travel distance. In 2000, more than 50 percent of the Other Road Classifications U.S. total lane miles were rural local centerline roads. Prin- cipal arterials, including the interstate, composed only 7 per- These are part of the national road network, but only a cent of the national total lane miles, while major and minor small portion of the mileage falls within the NHS. collectors represented 15 and 9 percent of the national total lane miles, respectively. Table 3-1 shows lane miles and the Minor arterials. These are roads smaller than principal percentage of the U.S. total by road classification (i.e., func- arterials that connect small communities (e.g., popula- tional system) and area population size. tions 25,000). Bridges in the national road system may represent partic- Collectors. These roads provide a lower degree of mo- ular points of interest because they typically take more time bility than arterials. They are designed for travel at and money to replace than roadway. Of the 587,146 highway lower speeds and for shorter distances. For the most bridges in the United States in 2000, 77.6 percent were in part, collectors are two-lane roads that collect and dis- rural communities and 22.4 percent were in urban areas. tribute travel from the arterial system. The collector sys- Examining structures by numbers gives all bridges in the net- tem has two subsystems: major and minor collectors. work equal priority. Thus, a small local bridge is counted the Major collectors serve larger towns not accessed by same as a large urban bridge such as New York's George higher order roads and important industrial or agricul- Washington Bridge or San Francisco's Golden Gate Bridge. tural centers that generate significant traffic but are not Structure size can be accounted for by using bridge deck served by arterials. In urban areas, the collector system area. Table 3-2 describes, by functional classification and provides traffic circulation within residential neighbor- area population, the number of highway bridges and the per- hoods and commercial and industrial areas. Unlike arte- centage of the total number of bridges and total deck area. rials, collector roads may penetrate residential commu- nities, distributing traffic from the arterials to the ultimate destination for many motorists. Urban collec- 3.1.3 System Use tors also channel traffic from local streets onto the arte- rial system. Highway transportation in the United States plays a signif- Local roads. These roads represent the largest element in icant role in two major areas: providing personal mobility to the American public road network in terms of mileage. households and facilitating freight movement. The use of pri- For rural and urban areas, all public road mileage below vate automobiles on the U.S. highway network provides the collector system is considered local. Local roads pro- Americans with a high degree of personal mobility. Automo- vide basic access between residential and commercial bile transportation allows people to travel where they want, properties and connect with higher order highways. when they want, and with whom they want. The freedom TABLE 3-1 Highway Lane Miles and Percent of U.S. Total by Functional System and Area Population Size, 2000 Small Urban Functional Urban Rural (pop. 5,000 Total System (pop. > 50,000) (pop. <5,000) 49,999) Lane % U.S. Lane % U.S. % U.S. Lane % U.S. Lane miles miles Total miles Total Total miles Total Interstate 66,507 0.8% 7,626 0.1% 135,000 1.6% 209,133 3% Other Principal 37,113 0.4% 4,627 0.1% 253,192 3.1% 294,932 4% Arterial Minor Arterial 148,077 1.8% 37,702 0.5% 287,605 3.5% 473,384 6% Major Collector 180,434 2.2% 45,208 0.5% 872,647 10.6% 1,098,289 13% Minor Collector 143,620 1.7% 44,525 0.5% 544,976 6.6% 733,121 9% Local 961,484 11.6% 238,684 2.9% 4,230,598 51.3% 5,430,766 66% Total 1,537,235 18.6% 378,372 4.6% 6,324,018 76.6% 8,239,625 100% (Source: Highway Performance Monitoring System)