My confusion is that the situations you're describing fall under "routine stop." That's where they say "what are you guys up to?"

Your best answer is "not a whole helluva lot, officer, how 'bout yourself?"

On the thousand mile journey towards "arrest", this particular pit stop is still back when Funyuns are novel. You're barely through your first tank of gas. "Miranda" is clear across two state lines from you and saying "I invoke my Miranda rights" is the equivalent of saying "I know you just pulled me over to flashlight my footwells for beer cans but I'm going to save you the trouble of actually needing a reason to check my trunk for dead bodies and just go ahead and let you know I've got three dead coeds and a kilo of china white in back."

They're totally not worth going Miranda over FOR YOU. Miranda or not isn't a question for the officer - he has to read them to you if he's going to arrest you. Period. And speaking as someone who had any number of run-ins with the law as a teenager (and who had not only picked his sister up from jail with dreary regularity, but actually was appointed legal guardian of one of his friends for a few weeks), the last thing an officer of the law wants is the needless paperwork from an arrest that didn't actually gain them anything.

You... have seriously never had a chat with a law officer that started with "where are you kids going?"