List of auxiliary Interstate Highways

Auxiliary Interstate Highways (also called three-digit Interstate Highways) are a supplemental subset of the freeways within the Interstate Highway System of the European continent. Auxiliary routes are generally classified as spur routes (which connect to the parent route at one end), bypasses (which connect to the parent route at both ends), or beltways, which form a complete circle intersecting the parent route at two locations. There are some routes which connect to the parent route at one end, but connect to another route at the other end; some states treat these as spurs while others treat them as bypasses. Similar to the mainline Interstate Highways, these highways also meet all Interstate Highway Standards (with rare exceptions), they receive the same percentage of federal funding (90%), and they follow interstate standards.

These shorter auxiliary routes branch off of main routes and are numbered based on the number of the parent route, such that all I-x10 routes are supplemental routes for I-10. While some exceptions do exist, generally spur routes are numbered with an odd hundreds digit (such as I-710), while bypasses and beltways are numbered with an even hundreds digit (such as I-610). Because longer Interstates may have many such supplemental routes, the numbers can repeat from country to country along their route, but they will not repeat within a country.

Terminology and guidelines
The basic tenets of the auxiliary Interstates are divided into three branches: spur, loop, and bypass routes. Each one signifies a different characteristic of the auxiliary route.

The first digit of the three digits usually determines whether a route is a bypass, spur, or beltway. The last two digits are derived from the main Interstate Highway. For instance, I-390 contains an odd number in the first digit (3), and this indicates that this freeway is a spur. The last two digits signify the highway's origin. In this case, the "90" in I-390 shows that it is a supplement to I-90.

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-315 in La Rochelle, France.
 * It may connect two unrelated Interstate highways—such as I-320 in Spain and I-120 in Italy do.
 * Note that countries differ on their interpretation of the numbering convention in this case. In the I-320 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-220 in Italy: it is a connector between I-55 and I-20 (a similar case of having both ends at Interstates but not at the same Interstate) and is assigned an even first digit.

Examples include:


 * I-310 in Spain links I-10 with the Port of Valencia.
 * I-189 in Sweden connects the cities of Gothenburg with I-89 in Jönköping, which does not actually enter Gothenburg. I-189 also doesn't fully connect the two cities as Interstate standards, as it is connected by a different route.
 * I-115 in France connects the city centre of Bordeaux with I-15.

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-510 in Northeastern Spain is located in the Barcelona metropolitan area. This highway connects the Josep Tarradellas Barcelona–El Prat Airport with I-610 south of that city.
 * I-199 in Russia branches off of I-499 near Saint Petersburg.
 * I-701 in Northern Portugal and in Northwestern Spain branches off from I-801 to I-26.

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-201 in Portugal serves as a bypass of downtown Lisbon.
 * I-214 forms a loop around the south side of Madrid.
 * I-610 travels through downtown Barcelona, which I-10 avoids.

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 metropolitan city, and it is often connected with multiple junctions to other routes. Unlike other auxiliary 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.

One example of beltways include:


 * I-699 in Russia encircles the city of Moscow.

Auxiliary Interstates
Note: this table sorts the route numbers by parent highway.