I just did some small tests. This was just with a small route of about 1 mile or so though.How dependent is turn-by-turn on the server? Does it just use the server to calculate a route and then that whole route is pulled down and stored locally or is it constantly going back to the server during the route?
I turned off cellular data and Turn-By-Turn worked just as well. It will even re-route you if you go off path, I took a bunch of deliberate missed and wrong turns, and it rerouted pretty well, it also didn't just try to get me to just circle back to the old route, but will tell you a different way to get to the destination if it's now more optimal, I think. But you can also get it into situations where it wants to ask the server for better directions.
I can try with a 18 mile route tomorrow, if I remember.