Castaic, formerly a small ranching community, serves as a major truck stop on I-5 today. As it sits at the bottom of the first major mountain grade north of Los Angeles, it is perfectly suited to this task.
In 1930, US 99 was built through Castaic with three lanes. In 1951, the town was bypassed by the four lane expressway version of US 99. In 1968, I-5 was completed through here and was built atop the old expressway.