Interstate vs. Freeway

Last Thursday I wrote this post about my daily commute. While I was writing it, I couldn't help but think how different the post would read if it had been written a few years earlier, back when I was living in California. Back then I never used the word interstate, instead I was also driving on the freeway. The Santa Monica Freeway, the Long Beach Freeway the San Diego Freeway. That's just what they're called. Just substituting interstate in there to make Santa Monica Interstate, Long Beach Interstate, San Diego Interstate, well it just sounds wrong.

Of course, there has to be a reason for the difference in name, right? My thought is that California is a pretty big state, so any highway you're on might not go to any state. And if it only exists in California, how could it be called an interstate. So freeway it is, because there are no red lights, you're free to drive without stopping.

While on the other hand, Louisiana is a smaller state. So you'd assume that any highways you drive on there would have to go through other states. Which makes them interstate, except for the fact you're still able to drive freely on them. But I think it makes at least a little logical sense, don't you?

Except there's one small hiccup with this exact answer. See the Santa Monica Freeway, well it goes by another name as well. It's actually the very same Interstate 10 I use to drive into work everyday! So the Santa Monica Freeway is an interstate too!

I want to know what is the "correct" way to refer to those things you drive on. Do you use interstate or freeway or something else completely different?

~Shoshanah