Interstate Highways have the highest speed limits and the highest traffic. Interstates are numbered in a grid: even-numbered routes for east–west routes (with the lowest numbers along Mexico and the Gulf of Mexico), and odd-numbered routes are north–south routes (with the lowest numbers along the Pacific Ocean). Three-digit Interstates are, generally, either beltways or spurs of their parent Interstates (for example, Interstate 510 is a spur into the city of New Orleans, Louisiana, and is connected to Interstate 10).
U.S. Numbered Highways are the original interstate highways, dating back to 1926. U.S. Highways are also numbered in a grid: even numbered for east–west routes (with the lowest numbers along Canada) and odd numbered for north–south routes (with the lowest numbers along the Atlantic Ocean). Three-digit highways, also known as "child routes," are branches off their main one- or two-digit "parents" (for example, U.S. Route 202 is a branch of U.S. Route 2). However, US 101, rather than a "child" of US 1, is considered a "mainline" U.S. Route.
State highways are the next level in the hierarchy. Each state and territory has its own system for numbering highways, some more systematic than others. Each state also has its own design for its highway markers; the number in a circle is the default sign, but many choose a different design connected to the state, such as an outline of the state with the number inside. Many states also operate a system of county highways.
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