Editing Interstate Highway System (section)

===Auxiliary (three-digit) Interstates===
{{See also|List of auxiliary Interstate Highways}}
[[File:FHWA Auxiliary Route Numbering Diagram.svg|thumb|upright=2.5|Examples of the auxiliary Interstate Highway numbering system. An odd hundreds digit means the route connects at only one end to the rest of the interstate system, known as a "spur route" (see I-310 and I-510 in image). An even hundreds digit means the route connects at both ends, which could be a bypass route (which has two termini) (see I-210 and I-810 in image) or a radial route (known also as a beltway, beltline, or circumferential route) (see I-610 in image).]]

Auxiliary Interstate Highways are circumferential, radial, or spur highways that principally serve [[urban area]]s. These types of Interstate Highways are given three-digit route numbers, which consist of a single digit prefixed to the two-digit number of its parent Interstate Highway. Spur routes deviate from their parent and do not return; these are given an odd first digit. Circumferential and radial loop routes return to the parent, and are given an even first digit. Unlike primary Interstates, three-digit Interstates are signed as either east–west or north–south, depending on the general orientation of the route, without regard to the route number. For instance, [[Interstate 190 (Massachusetts)|I-190]] in Massachusetts is labeled north–south, while [[Interstate 195 (New Jersey)|I-195]] in New Jersey is labeled east–west. Some looped Interstate routes use [[inner–outer directions]] instead of compass directions, when the use of compass directions would create ambiguity. Due to the large number of these routes, auxiliary route numbers may be repeated in different states along the mainline.<ref name="fhwa_route_log">{{cite web |url = https://www.fhwa.dot.gov/planning/national_highway_system/interstate_highway_system/routefinder/index.cfm |author = Federal Highway Administration |publisher = Federal Highway Administration |date = March 22, 2007 |access-date = January 23, 2008 |title = FHWA Route Log and Finder List |archive-url = https://web.archive.org/web/20130605010643/http://www.fhwa.dot.gov/reports/routefinder/index.cfm |archive-date = June 5, 2013 |url-status = live }}</ref> Some auxiliary highways do not follow these guidelines, however.