Interstate Highway System - Signage - Exit Numbering

Exit Numbering

The majority of Interstates have exit numbers. All traffic signs and lane markings on the Interstates are supposed to be designed in compliance with the Manual on Uniform Traffic Control Devices (MUTCD). There are, however, many local and regional variations in signage.

For many years California was the only state that did not use an exit numbering system. It was granted an exemption in the 1950s due to having an already largely completed and signed highway system; placing exit number signage across the state was deemed too expensive. California began to incorporate exit numbers on its freeways in 2002—Interstate, U.S., and state routes alike. Caltrans commonly installs exit number signage only when a freeway or interchange is built, reconstructed, retrofitted, or repaired to control costs, and are usually just tacked onto the top-right corner of an already existing sign. Newer signs along the freeways follow this practice as well. Most exits along California's Interstates now have exit number signage, particularly in rural areas.

Exit numbers correspond to Interstate mileage markers in most states. On I‑19 in Arizona, however, length is measured in kilometers instead of miles because, at the time of construction, a push for the United States to change to a metric system of measurement had gained enough traction that it was mistakenly assumed that all highway measurements would eventually be changed to metric; proximity to metric-using Mexico may also have been a factor, as I‑19 indirectly connects I‑10 to the Mexican Federal Highway system via surface streets in Nogales. Mileage count increases from west to east on most even-numbered Interstates; on odd-numbered Interstates mileage count increases from south to north. Some tollways, including the New York State Thruway and Jane Addams Memorial Tollway, use sequential exit-numbering schemes. Exits on the New York State Thruway count up from Yonkers traveling north, and then west from Albany. On the Jane Addams Memorial Tollway mileage markers count up from O'Hare International Airport traveling west, which is the starting point of the tollway.

As of November 2010 the Illinois State Tollway Authority has redone the mileage markers to be uniform with the rest of the state on I‑90 (Jane Addams Memorial/Northwest Tollway) and the I‑94 section of the Tri‑State Tollway, which previously had matched the I‑294 section starting in the south at I‑80/I‑94/IL Route 394. The tollway is also currently in the process of adding exit number tabs to the exits.

Many northeastern states label exit numbers sequentially, regardless of how many miles have passed between exits. States in which Interstate exits are still numbered sequentially are Connecticut, Delaware, Massachusetts, New Hampshire, New York, Rhode Island, and Vermont; as such, five of the main Interstate highways that remain completely within these states (87, 88, 89, 91, and 93) have interchanges numbered sequentially along their entire routes. Maine, Pennsylvania, Virginia, Georgia, and Florida followed this system for a number of years, but since converted to mileage-based exit numbers. Georgia renumbered in 2000, while Maine did so in 2004. The Pennsylvania Turnpike uses both mile marker numbers and sequential numbers. Mile marker numbers are used for signage, while sequential numbers are used for numbering interchanges internally. The New Jersey Turnpike, including the portions which are signed as I‑95 and I‑78, also has sequential numbering, but other Interstates within New Jersey use mile markers.

I‑87 in New York State is numbered in three sections. The first section is the portion of I‑87 that makes up the Major Deegan Expressway, with interchanges numbered sequentially from 1 to 14. The second section of I‑87 is a part of the New York State Thruway that starts in Yonkers (exit 1) and continues north to Albany (exit 24); at Albany, the Thruway turns west and becomes I‑90 for exits 25 to 61. From Albany north to the Canadian border, the exits on I‑87 are numbered sequentially from 1 to 44 along the New York State Northway. This often leads to confusion as there is more than one exit on I‑87 with the same number. For example, exit 4 on Thruway section of I‑87 connects with the Cross County Parkway in Yonkers, but exit 4 on the Northway is the exit for the Albany airport. These two exits share a number but are located 150 miles (240 km) apart.