That brings up an interesting idea.

I was thinking about the problem of determining which roads might be drawn on screen without having to going through all of them every frame. I came to the conclusion that it's over my head

But what if a desktop PC computed the route and when it sent it to GPSapp, it also sent along a list of all the roads within a given radius along the current trip's path. It could even build an index as to which ones branch off the current path.

That might make it easy enough for GPSapp to show surrounding roads when drawing the current path.