While I work at a company that, among other things, builds a navigation system (with a twist of doing load balancing on streets; called NUNAV Navigation), I'm not really an expert in this topic. From working with GIS and GPS quite a bit, I know though, that a lot of things, while they are working very well, are not as exact as they look (e.g. bad GPS signal, especially when driving with low speed, map matching, etc.)
Maybe someone else can comment in more detail.