I'm also guilty of saying "the" before the interstate number. And I am from CA! Only thing I can say is many of the interstates (freeways) in CA also go by names---The Golden State Fwy...Interstate 5. The San Diego Fwy---Interstate 10. So maybe because they have the names on the freeways or interstates is the reason we use "the"??? And it is difficult not to say freeways here if you have grown up with freeways all around you!