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.
Pennsylvania Route 97 (PA 97) is a state highway in the U.S. state of Pennsylvania. Known for most of its length as Baltimore Pike, the highway runs 9.363 miles (15.068 km) from the Maryland state line near Littlestown, where the highway continues as Maryland Route 97 (MD 97), northwest to U.S. Route 15 (US 15) near Gettysburg. PA 97 connects Gettysburg and Littlestown in southeastern Adams County. The highway also links those communities with Westminster and Baltimore. From PA 97's northern end, Baltimore Pike continues toward Gettysburg as State Route 2035 (SR 2035) through the Gettysburg Battlefield Historic District, where it provides access to the Gettysburg Museum and Visitor Center. Baltimore Pike was built as a turnpike in the early 19th century to connect Gettysburg, Littlestown, and Baltimore. The turnpike was a prominent linear feature during the 1863 Battle of Gettysburg despite not being the focus of a particular skirmish. Baltimore Pike was designated one of the original legislative routes in the early 1910s and became the northernmost part of US 140 in the late 1920s. The U.S. Highway was widened and resurfaced in the 1940s. When the US 140 designation was retired in the late 1970s, the highway became PA 97 to match the adjacent Maryland highway. With the creation of PA 97, the route had its northern terminus at the US 15 interchange while Baltimore Pike north of there became unnumbered.
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