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.
The first exit numbers were of the sequential type. 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.
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 fudged 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 , 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.
Out of the 50 states and district in the United States with signed Interstate Highways, only eight 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:
Early exit numbers