Auxiliary Interstates are divided into three types:
spur,
loop, and
bypass routes. The first digit of the three digits usually signifies whether a route is a bypass, spur, or beltway. The last two digits are derived from the main Interstate Highway. For instance,
I-115 contains an odd number in the first digit (1), which indicates that this freeway is a spur. The last two digits signify the highway's origin. In this case, the "15" in I-115 shows that it is a supplement to
I-15. Exceptions to the standard numbering guidelines exist for a number of reasons. In some cases, original routes were changed, extended, or abandoned, leaving discrepancies in the system. In other cases, it may not be possible to use the proper number because the limited set of available numbers has been exhausted, causing a "non-standard" number to be used.
Spur route A
spur route's number usually has an odd number for its first digit. It is usually one of the following: • It may serve another section of a city or metropolitan area not served by the main freeway (most often the
central business district), terminating at a regular city street/avenue or at a substandard freeway, such as
I-185 in
Columbus, Georgia. • It may represent the first portion of a contemplated extended freeway, one that downgrades to below Interstate standards with plans to upgrade it later on. An example is
I-369 in Texas, which currently does not connect with parent route I-69. • It may connect two unrelated Interstate highways—as
I-390 in
New York and
I-355 in
Illinois do. • States differ on their interpretation of the numbering convention in this case. In the I-390 example above, the route has both ends at Interstates, but not at the same Interstate on both ends, and is assigned an odd first digit. Another example is
I-275 in
Knoxville, Tennessee; it is a connector between
I-40 and
I-75 (a similar case of having both ends at Interstates but not at the same Interstate) and is assigned an even first digit. • This may even vary within the same state. For example, in
Michigan, both
I-196 and
I-696 intersect
I-96 at one end and
I-94 at the other end. Examples include: •
I-110 in
California links
I-10 with the
Port of Los Angeles. This freeway was built before Interstate Highways as the
Harbor Freeway. Later, it was adopted into the Interstate Highway System. •
I-180 in
North-Central Pennsylvania connects
Williamsport and
Lycoming County with
I-80 in
Milton, which does not actually enter Lycoming County. I-180 also serves as a connection to those traveling to
New York and
I-86 via
U.S. Route 15 and the Future
I-99 Corridor at its western terminus. •
I-190 in New York connects the cities of
Niagara Falls and
Buffalo with
I-90. Sometimes, a three-digit Interstate Highway branches off from another three-digit Interstate Highway. These spurs do not connect directly with their parent highways, but are associated with them via the three-digit highways they do intersect with. Examples include: •
I-380 in
Northern California is located at the
San Francisco Bay Area. This highway begins at
I-280, and it connects with US 101 and
San Francisco International Airport. •
I-190 in
Massachusetts branches off from
I-290 near
Worcester. •
I-795 in
Baltimore County, Maryland branches off from
I-695.
Bypass A
bypass route may traverse around a city, or may run through it with the mainline bypassing. In a typical 3-digit Interstate Highway, bypasses usually have both its two termini junctioned with another Interstate highway. Bypass routes are preceded by an even number in the first digit. Examples include: •
I-220 in
Louisiana serves as a bypass of downtown
Shreveport. •
I-440 forms a loop around the south side of
Nashville. •
I-890 travels through downtown
Schenectady, which
I-90 does not go through. In the case of an auxiliary Interstate highway which has both ends at Interstates but not the same Interstate, some states treat these as bypasses while others treat these as spurs—see
Spur route above.
Beltway A
beltway (also known as a
loop route) completely surrounds a large metropolitan city, and it is often connected with multiple junctions to other routes. Unlike other auxiliary Interstate Highways (and by extension, all
primary Interstate Highways), beltways do not have termini; however, they have a place where the highway mileage resets to zero. Beltways are also preceded by an even number in the first digit. Some examples of beltways include: •
I-275 in
Ohio,
Kentucky and
Indiana encircles the city of
Cincinnati. •
I-465 in
Indiana encircles the city of
Indianapolis. •
I-495 in
Maryland and
Virginia encircles the city of
Washington, D.C., and is referred to as "The Capital Beltway". ==Auxiliary Interstates==