Exit number

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Image:I-4 east exits 111A-B.jpg Image:SR 429 north exit 33.jpg 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 Boulevard and Concourse and Linden Boulevard were given sequential numbers, one per intersection. 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).

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.

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Sequential numbers

The first exit numbers were sequential. This type of exit numbers 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, and current U.S. practice is to use 15A/15B. In France, 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 15AA and 15AB.

Occasionally sequential exits are renumbered 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 its exit numbers in Connecticut, was not renumbered, and the sequence now jumps from 30 down to 27 (the interchange on the state line had two exit 27s, and now has exits 30 and 27).

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 sequential numbers.

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Distance-based numbers

Image:Quebec exit number.jpg Image:Quebec exit gore sign.jpg As more highways were built, the limitations of sequential numbering became clear, and states began to experiment with distance-based (mile-based) exit numbers. 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. An example of a highway that does have an exit 0 is British Columbia provincial highway 1 on the Mainland.

In areas that use the metric system, distance based numbers are by kilometer rather than mile. A few highways, such as Delaware State Highway 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. 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 and when 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 to a kilometre-based system when Canada metricized.

More importantly, actual functionality is lost by using distance-based exit numbering: If one sees, for example, "Sarasota, next 4 exits", and then exit 34, it is possible to deduce that if one wishes to exit at the "last Sarasota exit", exit 37 is the number for which one must pay attention--given distance-based numbers, it is not possible to make any inferences at all; one must examine a map (and maps often do not list exit numbers).

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United States

Image:Mile tabs.jpg The use of exit numbers on the Interstate Highway System became mandatory in 1971. California was able to get a waiver because they had already built most of their freeways; the Cal-NExUS program began in 2002 to number California's exits.<ref>Cal-NExUS, Possible Questions and Answers</ref> Out of the 50 states and district in the United States with signed Interstate Highways, only nine still use sequential exit numbers, all in the northeast:

Alaska only has exit numbers on a surface road, the Johansen Expressway. It is unknown if they are mile-based or sequential.

Most other states began with sequential numbers and switched over later. Here is a list of these switches, in the order that they happened:

This list is incomplete; you can help by expanding it.

One highway (Interstate 19 in Arizona) has metric numbering, because it was constructed during the time when the U.S. was thought to be completely converting to metric.

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Canada

Of all the provinces in Canada, only two use sequential numbers: Nova Scotia and Newfoundland and Labrador. The northern territories and PEI use no exit numbers, as there are no freeways or expressways in those provinces/territories.

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Notes

  • Ontario has the oldest exit number system, which started posting exit numbers in the 1960s. Most short freeways do not have exit numbers, and until about 2000 (with Highways 11 and 69/400), incomplete freeways also did not have exit numbers.
  • Alberta has the newest exit number system. The province started posting exit numbers on Highway 2 in 2004 and plans to introduce them on Highway 1 in the near future (some maps from mapart showed exit numbers on Highway 2 and 1).
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Taiwan

Image:Taiwan Freeway Exit Sign.PNG Image:Taiwan Freeway New Exit Sign.PNG Taiwan uses distance-based exit numbers in kilometers. If two exits are located within the same kilometer mark, the English letters are appended to differentiate the exits. Taiwan did experiment with sequential exit numbers with National Freeway No. 1 for a couple of years, but abandoned the experiment in 2004.

Prior to 2006, Taiwan exit signs were generally near replicas of their US counterparts. However, the exit signs now not only indicate the distance number, but they also display the Chinese code-name for the interchange.

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New Zealand

New Zealand began introducing exit numbers in 2005. It uses a distance-based numbering system. The distance to the exit is measured from the origin of the highway. That is:

  • State Highway 1 (Northern motorway) measured from the origin of SH1 at Cape Reinga
  • State Highway 16 (Northwestern Motorway) measured from the origin at the Port
  • State Highway 20 (Southwestern Motorway) measured from the origin at SH1

There is not exit zero. If there is an exit within 1.499 km of the origin, Exit 1 is used. Exit 2 would be between 1.500 and 2.499 km of the origin. Subsequent 'exit zones' are at 1km intervals.

Letter suffixes are added at multi-exit interchanges, or where two or more exits exist within the same exit zone. For example, State Highway 1 (Southern Motorway) has an Exit 429A (Symonds St), Exit 429B (Wellesley St) and Exit 429C (Port).

Instead of replacing existing ramp and link signs, the exit numbers were added as supplementary information. Thus drivers can navigate either by exit number or name.

Exit numbers are only used for exits that may be used by all vehicle types. Bus- or emergency vehicle-only exits would not be numbered.

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Early exit numbers

This list is incomplete; you can help by expanding it.
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See also

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References

<references/>

  • Signs Numbering Exits Installed on Parkways, New York Times April 24, 1938 page 26
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External links

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