 The southern portion of the freeway, from Interstate 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 (Interstate 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 the eastern boundary of Santa Clarita along its route. Past Santa Clarita, the road continues to the 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. Between the two towns the freeway forms the southern boundary of Vasquez Rocks Park, a county park. Just past Acton, the highway crests the San Gabriel mountains via Soledad Pass, at an elevation of 3,225 feet (983 m). The route of the highway through the mountains loosely parallels that of the main line of the Southern Pacific Railroad, which is also used for the Metrolink Antelope Valley Line.
