Dunno, but from the top of my head: one-way streets, no-left-turn-allowed-here-signs, prolonged roadworks. Perhaps the map data doesn't even map up actual streets that well (does this road go over that one, or is it the other way around?).
My uncle from Germany who visited me last month showed me his Garmin GPS -- it reported speed limits instantaneously. Anyway, tagging speed limits with roads is one of the less difficult things in computing turn-by-turn directions, I would guess. It's just a matter of getting the data from whatever branch of gov't that has it -- (out of curiosity - if anyone knows, do tell, who exactly would be queried for such data? And what would be the logistics of it? Only acquirable for a certain fee?).
Most governments other than the US charge enormous amounts of money for map data, with very tight restrictions on licensing. Many consider it a state secret, and won't sell the data at all.
map data just shows where the roads are. Turn by Turn data shows which roads are one way, which side of the road the exit for that interstate is, and other such detail that you would need to know besides just the lines on the map.
are not correct. While 4th Ave is bi-directional, the segment between NW 51st St and NW 50th Pl is one way, north only (towards NW 51st St). I don't know if other GPS systems get this correct or not (I don't have a GPS system to check).
I'm assuming that's true. Using the data they had from a third-party for turn-by-turn would have cost them extra. Thus the decision to build their own.
