I guess it would probably go wrong easily when we're close to the waypoint due to positional errors. Maybe I shouldn't look at where the current position stands compared to the waypoint, but look at where the current 'vector' would cross the line segment that comes right after the turn.

The waypoints themselves are not chosen, they are the same ones that mapsonus gives. It seems like they pretty much give a new description whenever the street description changes. i.e. I've seen "Follow Penn Ave (Rte 8)" "Continue on Penn Ave". Still the same street, the description just changed slightly.