List of business routes of the Interstate Highway System

The Interstate Highway System of the United States, in addition to being a network of freeways, also includes a number of business routes assigned by the American Association of State Highway and Transportation Officials. These routes connect a central or commercial district of a city or town with an Interstate bypass.
As the main purpose of these routes are to serve a certain downtown area, business Interstates are typically routed along surface roads. These routes do not have to meet Interstate Highway standards and are not considered part of the Interstate Highway System. AASHTO does, however, apply similar standards as to new U.S. Highways, requiring a new business Interstate to meet certain design standards.
Business Interstates are more commonly found in the western regions of the United States, as well as both across the Great Plains and in the state of Michigan. In contrast, Eastern states generally did not designate as many business Interstates. This geographic difference can be attributed to the way the Interstate Highway System was constructed in different parts of the country. In many eastern states, the new Interstates were often built to parallel the existing U.S. Highway network, rather than directly replacing those older routes. With the exception of mountainous areas, this left the majority of U.S. Highways intact, or co-signed routes with the Interstates. However, in the western states, the construction of the Interstate system more frequently involved directly overlaying the former U.S. Highway alignments. This sometimes left sections of the old U.S. Highways disconnected, especially in rural areas between cities and towns. To maintain access to those former highway segments, business Interstate designations were often applied as a way to guide drivers to key commercial districts and services. Although business Interstates are primarily found along primary Interstates, a few auxiliary Interstates also have their own business route designations.
Like auxiliary Interstates, business Interstates can be repeated from state to state along their route. The only difference is that business Interstates can also be repeated in several locations within the same state.

Terminology and guidelines

As the main purpose of these routes is to serve a certain downtown or urban area, business Interstates are typically routed along surface roads rather than limited-access freeway segments. This allows the business route to directly connect to and navigate a local street network, providing more accessible connections to the central business district. Business Interstates are often designated along existing city streets or arterials that already have a strong commercial and retail presence, rather than cutting through the urban core on a new alignment.
Since business Interstates are primarily routed along surface roads, they do not have to meet the strict design standards required for the mainline Interstate Highway System in order to qualify for that kind of designation. However, despite sharing the "Interstate" designation and similar shield design, business Interstates are not actually considered part of the mainline Interstate Highway System. They are treated more as auxiliary or supplemental routes that complement the overall Interstate network, rather than being fully integrated components.
The design standards for business Interstates are more akin to the guidelines used for new U.S. Highway routes. While they aim to maintain a baseline of safety and continuity, the standards are less stringent than the exacting specifications for lane widths, access control, grade separations, and other factors that define the main Interstate Highway network.

Business route classifications

Business Interstates are signed with green shields that visually resemble the standard Interstate Highway shield, but with a few key differences. The word BUSINESS is used instead of INTERSTATE, and, above the number, where the state name is sometimes included, the word LOOP or SPUR appears.
A business loop has both ends connected to its parent Interstate route. This allows the business loop to provide direct access to the central business district or other major destinations within a city or town. Many business loops were created to replace a decommissioned U.S. Route. One example includes all the I-5 business loops in northern California, which replaced US 99W through towns like Woodland, Arbuckle, or Williams. Sometimes, a business loop may either be cosigned with or supplement an active U.S. Route, such as I-70 Business Loop in Denver, Colorado, that follows US 40 and Colfax Avenue across the metropolitan area.
A business spur has one end connected to the parent Interstate route, while the other end dangles or terminates at a specific destination, often the downtown or central business district of a city or town. One example is Business Spur I-75 into Bay City, Michigan. Sometimes, a business spur originates from an Interstate's terminus and continues into the city/destination. An example of this is I-20 Business Spur into Florence, South Carolina, which originates from I-20's eastern terminus at I-95.
While most business Interstates are usually routed along surface roads, they are sometimes routed onto freeways. Some of these stretches of freeways were once designated as mainline Interstates themselves, such as I-85 Business Loop in Spartanburg, South Carolina, and I-80 Business Loop in Sacramento, California.