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An exit number is a number assigned to a road junction, usually an exit from a freeway. It is usually marked on the same sign as the destinations of the exit, as well as a sign in the gore. Exit numbers typically reset at political borders such as state lines.〔()〕 In some areas, such as New Jersey (and its New Jersey Turnpike), the numbers are a part of popular culture; a clichéd greeting is "what exit?". Some non-freeways use exit numbers. Typically these are rural roads built to expressway standards, and either only the actual exits are numbered, or the at-grade intersections are also numbered. An extreme case of this is in New York City, where the Grand Concourse and Linden Boulevard were given sequential numbers, one per intersection (both boulevards no longer have exit numbers as of 2011). A milder version of this has been recently used on the West Side Highway, also in New York, where only the major intersections are numbered (possibly to match the planned exits on the cancelled Westway freeway). Another case is the Nanaimo Parkway in Nanaimo, British Columbia carrying Highway 19, where all exits are numbered though all except one are at-grade intersections. Some other intersections on Highway 19 outside Nanaimo are also given numbers. As a means of educating motorists, some state highway maps include a brief explanation of the exit numbering system on an inset. Iowa DOT maps from the 1980s and 1990s included a picture or drawing of a milepost and briefly described how Iowa had included milepost references near interchanges on the map. ==Sequential numbers== The first exit numbers were sequential. This type of exit numbering usually begins with exit 1 at the beginning of the road; each subsequent exit is given the next number. The first implementations gave each ramp its own number, even when two ramps existed for two directions of a road; later implementations used directional suffixes, as in 15N/15S or 15E/15W; current U.S. and Canadian practice is to use 15A/15B. In France and Japan, decimals are used, as in 15.1 and 15.2. Toll roads, especially those using tickets, lend themselves nicely to sequential numbering, as each toll plaza gets its own number. Problems arise when exits are added. For instance, an exit between 15 and 16 would typically be 15A. On the New York State Thruway, an exit was added between 21 and 21A, leading to the sequence 21 - 21B - 21A - 22. In Florida, some new exits got the suffix C, so that if it had or acquired separate exits for the two directions, they would be 15CA and 15CB rather than 15AB. There are also occurrences of this happening on the New Jersey Turnpike; the original interchanges opened in 1951, with newer exits as recently as 1982. On the Baltimore Beltway, there is an exit 12B-C (MD 372), as well as 12A (US 1). There is also an exit 8A (I-895) and an exit 8 (MD 168). Some sequential exits are renumbered (remaining sequential) due to added exits. For instance, the Hutchinson River Parkway in New York was renumbered so that its northernmost exit, 27, became 30. However, the Merritt Parkway, which continued the "Hutch"′s exit numbers in Connecticut, was not renumbered. This means the Route 120A interchange is numbered 27 in Connecticut ''and'' 30 in New York. The Atlantic City-Brigantine Connector in Atlantic City, New Jersey uses letters (without numbers) for its exits; it has many exits in a short distance, and the South Jersey Transportation Authority may have wanted to avoid numbers, as the Atlantic City Expressway's lowest numbers (mile-based) are in Atlantic City. ==Distance-based numbers== As more highways were built, the limitations of sequential numbering became clear, and states began to experiment with distance-based (mile-based) exit numbers. The first mile-based system known was implemented on the Garden State Parkway in New Jersey in the late 1950s. Michigan also implemented mile-based junction numbers on Interstate 94 in the 1960s. In this system, the number of miles from the beginning of the highway to the exit is used for the exit number. If two exits would end up with the same number, the numbers are sometimes modified slightly; this is often impossible and exits are given sequential or directional suffixes, just as with sequential numbers. An exit can be numbered by where the exit in the direction of increased mileage leaves the freeway, or by where the road that the exit serves crosses the freeway (which is occasionally ambiguous). From this number, the integer exit number can be determined by rounding up, rounding down, or rounding to the nearest integer. Many jurisdictions prefer to avoid an exit 0. To this end, the numbers are either rounded up to get the exit number, or any exit that would get the number 0 is instead numbered 1. Examples of highways with an exit 0 are British Columbia Highway 1 on the mainland, Interstate 70 in Wheeling, West Virginia along the West Virginia-Ohio border, Interstate 90 on the Montana side of the Idaho-Montana border, and Interstate 65 in two locations: at its terminus with Interstate 10 in Mobile, Alabama, and just north of the Ohio River in Jeffersonville, Indiana. In areas that use the metric system, distance-based numbers are by kilometer rather than mile. A few highways, such as Delaware Route 1 and Interstate 19, have been renumbered from miles to kilometers, even in areas that typically use miles. Distance-based numbers have several advantages. They match the mileposts along the road; it is thus easy to calculate how far one has to go. Additionally, most new exits don't need letter suffixes, as in a sequential system. On the other hand, there are some disadvantages to changing from a sequential system. Suffixes are required when the same mile of highway contains multiple exits. Businesses and motorists have to adapt to the changes, and it costs money to replace the signs (as well as for temporary "old exit" tabs to ease the transition). Additionally, some argue that it is pointless to change to mile-based numbers, as the numbers would have to be replaced again if the U.S. switches to the metric system. Ontario's Highway 401 changed to a mile-based system from a sequential system of exit numbers, then had to switch again to a kilometre-based system when Canada metricized in 1977. 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「exit number」の詳細全文を読む スポンサード リンク
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