Its a cultural preference on the West Coast (or at least in the Pacific NW). We will refer to "I-5" or "I-90," but we almost never use the word "interstate." We say highway or freeway, and generally there is a mental image behind it: one is 35-45mph (usually 45 though), traffic lights and two directions, the other is 60-70mph (usually 60 though), contains little relevant symbology affecting the flow of traffic, and is one direction only.
I refer you to Who Framed Roger Rabbit, when Judge Doom describes this strange new roadway - "they're calling it a 'freeway!" Well, they were in LA, so it makes sense to people like me... Judge Doom eventually got his commupance for his collaboration in the first freeway project, though