California Highways: 
As defined in the state highway code, there is no distinction between "Interstates", "US Highways", and state highways -- they are all state highways.
Federal law is different. Federal Law (specifically 23 USC 103, which was significantly reworked by Intermodal Surface Transportation Efficiency Act (ISTEA) of 1991) defines two highway systems: the National Highway System (NHS) and the Interstate System, which is a component of the National Highway System. Note that this is a change from the original method of Interstate funding, where there were four Federal-Aid systems: Interstate, Primary, Secondary, and Urban. This series of pages discusses Interstate funding (and highway funding in general) in more detail on the page on Chargeable and Non-Chargeable Interstates, because that was the original distinction that governed Interstate construction funding.
The Interstate system (formally "The Dwight D. Eisenhower System of Interstate and Defense Highways") serves the 48 contiguous states, Alaska, Hawaii, Puerto Rico, and the District of Columbia. It carries more than 21 percent of the nation's traffic on only 1% of the nation's total road and street mileage. A good history of the interstate system can be found at the FHWA Route Finder. There are specific design requirements for Interstate highways, and all highways classified as Interstate (with the exception of those in Alaska and Puerto Rico) must meet those standards. The goals of the highways on the Interstate system are to:
The Interstate system has a maximum number of miles defined in law: it cannot exceed 43,000 miles (currently, it is at 42,795 miles). This mileage does not include mileage signed as interstate under 23 USC 103(4). Such mileage is colloquially called "non-chargeable" mileage. This distinction used to be important for Interstate Completion funding. Based on a review of 23 USC 103, it does not appear to impact funding today (were "today" means "after 2026").
As the original distinction between "chargeable" and "non-chargeable" Interstate mileage has gone away, and the old distinction of Federal-Aid-{Interstate, Primary, Urban, Secondary} (FAI, FAU, FAP, FAS) has gone away, it has been replaced with a system called Functional Classification. Functional Classification defines seven levels of highways:
The functional classification determines eligibility for Federal Funding programs. There are detailed definitions for each level of classification, and those classifications also depend on whether the highway is urban or rural. The functional classifications are defined by the FHWA in the Highway Performance Monitoring System. It appears the current guidance for functional classification dates to 2023. As an example, this is the definition for "Other Freeway or Expressway":
Roadways in this functional classification category look very similar to Interstates. While there can be regional differences in the use of the terms ‘freeway’ and ‘expressway’, for the purpose of functional classification the roads in this classification have directional travel lanes are usually separated by some type of physical barrier, and their access and egress points are limited to on- and off-ramp locations or a very limited number of at-grade intersections. Like Interstates, these roadways are designed and constructed to maximize their mobility function, and abutting land uses are not directly served by them.
It appears that the functional classification for "Interstate" depends only upon designation by the Secretary of Transportation, and does not continue the distinction between "chargeable" and "non-chargeable".
Under ISTEA, the Interstate Program includes completion funding for Interstate Construction, Interstate Substitute highway projects, and an Interstate Maintenance program to rehabilitate, restore, and resurface the Interstate system. Reconstruction is also eligible for funding if it does not add capacity, except for high occupancy vehicle (HOV) or auxiliary lanes. Federal Aid comes in multiple forms. The Surface Transportation Program can be used on Interstate, National Highway System, and all roads functionally classified by FHWA as other than local or rural minor collectors. The Congestion Mitigation and Air Quality Improvement Program is directed towards transportation projects which will contribute to Clean Air Act requirements in non-attainment areas for ozone and carbon monoxide. The Bridge Replacement and Rehabilitation Program is continued.
According to the FHWA, to mark the Interstate routes, the American Association of State Highway Officials (AASHO) asked its member states to submit suggestions. The States submitted dozens of ideas in several forms, ranging from a 55mm color transparency to a 4-foot square aluminum blank. The signs were tested, an a final version, a combination of submissions from Missouri and Texas, was selected. On September 19, 1967, the U.S. Patent and Trademark Office issued Trademark Registration 835,635 for the shield.
Archival information regarding submissions of proposed routes for both Interstate and US Highways can be found in the AASHTO Route Numbering Archive.
The following sections contain the more information on Interstates and California's Interstate mileage:
In 2006, Caltrans published a special series on their website commemorating the 50th anniversary of the creation of the Interstate Highway system. The following series of pages preserve that work, which was lost in the great Caltrans accessibility rework of 2020:
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© 1996-2024 Daniel P.
Faigin.
Maintained by: Daniel P. Faigin <webmaster@cahighways.org>.