I found in my own amateur UX studies that there are drivers who hear "in a quarter mile, turn right" and don't know how far that is, even after having heard that prompt thousands of times. You'd think that after a while they'd build a mental model of the distance. Instead, they seem to just tune out the "in a quarter mile". This is one of the classes of users for which products need to be designed. (I have similar problems with people's heights.)

Also, as a passenger it's fairly easy to say "turn in five blocks" and track how many blocks are left, maybe even while holding a conversation. For drivers, that seems more difficult, probably due to cognitive overload, unless it's just a couple of blocks. And I've noticed that some drivers react to that kind of direction by scanning the view and quickly translating the number of blocks to a landmark.