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.
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.
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.
This section contains the following background 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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