Highways in the United States are split into at least four different types of systems, Interstate Highways, U.S. Highways, state highways, and county highways. Highways are generally organized by a route number or letter. These designations are generally displayed along the route by means of a highway shield. Each system has its own unique shield design that will allow quick identification to which system the route belongs. Below is a list of the different highway shields used throughout the United States.
The Interstate Highway System is a federally funded and administered (but state-maintained) system of freeways that forms the transportation backbone of the U.S., with millions of Americans relying on it for commutes and freight transport daily. Interstate highways are all constructed to precise standards, designed to maximize high-speed travel safety and efficiency. Interstate Highways also contain auxiliary routes, which are normally assigned a three-digit route number. All Interstate Highways are part of the National Highway System, a network of highways deemed essential to the defense, economy, and mobility of the country.
The U.S. Highway System (officially "United States Numbered Highways") is an older system consisting mostly of surface-level trunk roads, coordinated by the American Association of State Highway and Transportation Officials and maintained by state and local governments. U.S. highways have been relegated to regional and intrastate traffic, as they have been largely supplanted by the Interstate system for long-distance travel except in areas (especially in the west) where the Interstate system is absent or underdeveloped. This has led to the decommissioning and truncation of U.S. Highways that were formerly vital long-haul routes, such as U.S. Route 21 and U.S. Route 66.
Each state also has a state highway system. State highways are of varying standards and quality. Some state highways become so heavily traveled they are built to Interstate Highway standards. Others are so lightly traveled that they are roads of low quality.
Many state highway markers are designed to suggest the geographic shape of the state or some other state symbol such as its flag. Most of the others are generically rectangular or some other neutral shape. The default design for state highway markers is the circular highway shield, which is how state highways are designated on most maps. Several states still use the circular shield for road signage on their state highways.
The final administrative level in some states is the county highway. As the name suggests, this type of road is maintained by a county. In Louisiana, "parish roads" exist in place of county highways, as counties in that state are called parishes. County roads vary widely from well-traveled multilane highways to dirt roads into remote parts of the county.