and Expressway System, and is part of the National Highway System, a network of highways that are considered essential to the country's economy, defense, and mobility by the Federal Highway Administration. Antelope Valley Freeway The southern portion of the freeway, from I-5 to the Avenue D exit near Lancaster, has been designated the Antelope Valley Freeway by the state legislature. The Antelope Valley Freeway begins in the Santa Susana Mountains at the Newhall Pass interchange by splitting from the Golden State Freeway (I-5). This is the busiest portion of the route with an annual average daily traffic (AADT) count of 169,000 vehicles per day. The freeway forms much of the eastern boundary of Santa Clarita along its route. Past Santa Clarita, the road continues northeast and crosses the Sierra Pelona Mountains and western San Gabriel Mountains via the canyon of the seasonal Santa Clara River. The ascent is mostly rugged and rural terrain, with only two small towns along the ascent, first Agua Dulce and later Acton. In Agua Dulce, the freeway forms the southern boundary of Vasquez Rocks Park, a county park. The highway crests the Sierra Pelona Mountains via Escondido Summit, at an elevation of , before descending and passing by Acton to the north. The highway then crests the San Gabriel Mountains via Soledad Pass, at an elevation of . The route of the highway through the mountains loosely parallels that of Metrolink Antelope Valley Line. After cresting both mountain passes, the highway descends into the Antelope Valley, a large valley within the Mojave Desert. The highway crosses Angeles Forest Highway and the California Aqueduct in the descent. SR 14 serves as the primary north–south thoroughfare for the communities of Palmdale and Lancaster. Between Palmdale Boulevard (County Route N2) and Avenue D in Lancaster, SR 14 runs concurrently with SR 138. Aerospace Highway From the Pearblossom Highway exit south of Palmdale to its northern terminus at US 395 near Inyokern, SR 14 has been designated the Aerospace Highway. Between Pearblossom Highway and Avenue S, there is a vista point overlooking Lake Palmdale, which features a historic plaque that honors aviation accomplishments including the Space Shuttle, breaking the sound barrier and the speed record. The freeway passes the Los Angeles–Kern county line at Avenue A, and continues to run north through Rosamond and Mojave. In Rosamond, the highway passes close to Edwards Air Force Base, which was often