There are both state and federal highways that are limited access. S&T has "limited access" grouped with Interstates. It apparently treats them the same. I travel from Columbus, GA, to Bay City, Michigan, a lot and nearly all mapping programs, including S&T take me north from Ohio using U.S. 23. It's limited access. If it was just interstates, it would route me through Detroit. I'd rather be set on fire for 15 minutes than to have to take I-75 through Detroit. I'm guessing that, for the most part, it means actually Interstates and limited access roads.
I've made the trip scores of times and I don't need maps or GPS. I use the GPS to check the progress of the trip and for entertainment.