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|maint=state or local governments; numbers and routings assigned by AASHTO |interstate= |us=U.S. Highway nn (US nn) U.S. Route nn (US nn) |state= |notes= |links=US }} The system of United States Numbered Highways (often called U.S. Routes or U.S. Highways) is an integrated network of roads and highways numbered within a nationwide grid in the contiguous United States. As the designation and numbering of these highways were coordinated among the states, they are sometimes called Federal Highways, but the roadways have always been maintained by state or local governments since their initial designation in 1926. The route numbers and locations are coordinated by the American Association of State Highway and Transportation Officials (AASHTO).〔 The only federal involvement in AASHTO is a nonvoting seat for the United States Department of Transportation. Generally, north-to-south highways are odd-numbered, with lowest numbers in the east, the area of the founding thirteen states of the United States, and highest in the west. Similarly, east-to-west highways are typically even-numbered, with the lowest numbers in the north, where roads were first improved most intensively, and highest in the south. Major north–south routes have numbers ending in "1" while major east–west routes have numbers ending in "0". Three-digit numbered highways are spur routes of parent highways but are not necessarily connected to their parents. Some divided routes exist to provide two alignments for one route, even though many splits have been eliminated. Special routes, usually posted with a banner, can provide various routes, such as an alternate, bypass or business route, for a U.S. Highway. Before the U.S. Routes were designated, auto trails designated by auto trail associations were the main means of marking roads through the United States. In 1925, the Joint Board on Interstate Highways, recommended by the American Association of State Highway Officials (AASHO), worked to form a national numbering system to rationalize the roads. After several meetings, a final report was approved by the U.S. Department of Agriculture in November 1925. They received complaints from across the country about the assignment of routes, so the Board made several modifications; the U.S. Highway System was approved in November 1926. As a result of compromises made to get the U.S. Highway System approved, many routes were divided, with alignments to serve different towns. In subsequent years, AASHTO called for such splits in U.S. Routes to be eliminated. Expansion of the system continued until 1956, when the Interstate Highway System was formed. After construction was completed, many U.S. Routes were replaced by Interstate Highways for through traffic. Despite the Interstate system, U.S. Highways still form many important regional connections, and new routes are still being added. ==System details== In general, U.S. Routes do not have a minimum design standard, unlike the later Interstate Highways, and are not usually built to freeway standards. Some stretches of U.S. Routes do meet those standards. Many are designated using the main streets of the cities and towns through which they run. New additions to the system, however, must "substantially meet the current AASHTO design standards". As of 1989, the United States Numbered Highways system has a total length of .〔 Except for toll bridges and tunnels, very few U.S. Routes are toll roads. AASHTO policy says that a toll road may only be included as a special route, and that "a toll-free routing between the same termini shall continue to be retained and marked as a part of the U.S. Numbered System."〔 U.S. Route 3 (US 3) meets this obligation; in New Hampshire, it does not follow tolled portions of the Everett Turnpike. But US Routes in the system do use parts of four toll roads: * US 51 uses part of the Jane Addams Memorial Tollway in Illinois; the old road is Illinois Route 251. * US 278 uses the tolled Cross Island Parkway in South Carolina; the old road is US 278 Business. * US 412 uses the Cimarron Turnpike in Oklahoma; the old road is US 64. * US 412 also uses the Cherokee Turnpike in Oklahoma; the old road is Alternate US 412. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「United States Numbered Highways」の詳細全文を読む スポンサード リンク
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