Like I said about a week ago, it should be evaluated on a state-by-state basis. At least, we don't want XX- to say the letters on every occasion. LA- is obvious in Louisiana since the signs say "LA" on them and the people call them that, but on a state with a less intuitive USPS abbreviation that isn't used on their signs, I don't think we'd want that.
Maybe we could answer it more generally. Maybe this is more a philosophical question, but I don't think I'd mind all that much if XX- just said the state name for every state, and maybe we could even make that the standard for all states. Does it really matter if people call it "Route 6" or "L A 48", when "Massachusetts six" or "Louisiana forty-eight" would still convey the message just as well?
My problem isn't so much with "Louisiana" as it is with the pause.
Why is there a pause between MD- and LA- and the number? Was that an accident, or was it expected? There should never be a pause there. You wouldn't want it to say "stay left for i ...... ten".
As for LA (or XX), well, maybe we should be more careful with those? Every time I see a state name abbreviated on a sign it is not done using the USPS abbreviations. Rather, they're abbreviated "Ala." or "Va." or maybe "Calif."; perhaps we should incorporate those instead?
Since you bring it up, how about "AFB" says "air force base" and "ANG" says "air national guard", for example? An exit I take a lot when I'm in Michigan is signed for "Selfridge ANG", or at least it was, and that was pronounced "Selfridge Ang", as if a word. I changed it to "A 'N' G", and now it says the letters, but it looks kinda gross.