Editing Interstate Highway System (section)

===Mile markers and exit numbers===
On one- or two-digit Interstates, the mile marker numbering almost always begins at the southern or western state line. If an Interstate originates within a state, the numbering begins from the location where the road begins in the south or west.  As with all guidelines for Interstate routes, however, numerous exceptions exist.

Three-digit Interstates with an even first number that form a complete circumferential (circle) bypass around a city feature mile markers that are numbered in a clockwise direction, beginning just west of an Interstate that bisects the circumferential route near a south polar location. In other words, mile marker&nbsp;1 on [[I-465]], a {{convert|53|mi|km|adj=on}} route around Indianapolis, is just west of its junction with [[I-65]] on the south side of Indianapolis (on the south leg of I-465), and mile marker&nbsp;53 is just east of this same junction. An exception is [[Interstate 495 (Capital Beltway)|I-495]] in the [[Washington metropolitan area]], with mileposts increasing counterclockwise because part of that road is also part of [[I-95]].

Most Interstate Highways use distance-based [[exit numbers in the United States|exit numbers]] so that the exit number is the same as the nearest mile marker.  If multiple exits occur within the same mile, letter suffixes may be appended to the numbers in alphabetical order starting with A.<ref name="INDOT-Understanding Interstate Route Numbering, Mile Markers & Exit Numbering" >{{cite web |url = http://www.in.gov/indot/2488.htm |access-date = November 26, 2011 |title = Understanding Interstate Route Numbering, Mile Markers & Exit Numbering |author = Indiana Department of Transportation |date = n.d. |publisher = Indiana Department of Transportation |archive-url = https://web.archive.org/web/20130515104711/http://www.in.gov/indot/2488.htm |archive-date = May 15, 2013 |url-status = live }}</ref>  A small number of Interstate Highways (mostly in the Northeastern United States) use sequential-based exit numbering schemes (where each exit is numbered in order starting with 1, without regard for the mile markers on the road).  One Interstate Highway, [[Interstate 19|I-19]] in Arizona, is signed with kilometer-based exit numbers. In the state of New York, most Interstate Highways use sequential exit numbering, with some exceptions.<ref name="Is New York State planning to change its Interstate exit numbering system from a sequential system to a distance-based milepost system?" >{{cite web |url = https://www.dot.ny.gov/about-nysdot/faq/nys-interstate-exit--system-sequential-or-milepost-system |access-date = January 1, 2003 |title = Is New York State planning to change its Interstate exit numbering system from a sequential system to a distance-based milepost system? |author = New York State Department of Transportation |date = n.d. |publisher = New York State Department of Transportation |archive-date = March 22, 2019 |archive-url = https://web.archive.org/web/20190322034420/https://www.dot.ny.gov/about-nysdot/faq/nys-interstate-exit--system-sequential-or-milepost-system |url-status = live }}</ref>