One theory on that I've come up with is that out here in Cali, they call them "freeways". Where back East we always called them Interstates.

The word "interstate" always came first, and we'd often shorten "Interstate 95" to "I95".

Out here the use of the word "freeway" comes second. For example, "The 405 freeway". It gets shortened to "the 405".

Just my theory...and as you can tell, I've spent far too much time wondering about it myself.