I am interested to discover the connection between the tags of highways and the instructions one receives using OSM for navigation. I experienced a situation yesterday when I was informed to take a slip road before reaching a roundabout; the approach and departure sections merely split (http://www.openstreetmap.org/#map=18/51.30122/-1.04263). Is it the tags alone (such as oneway=yes, perhaps, in this case) that dictate how navigation software interprets them? I should like to know for editing purposes.
Ok, Garmins has this same issue as did I a few years ago. If you have it set to shortest distance, technically, taking the slip road across to the other slip road than then re-entering is shorter. Not faster, not easier but it is shorter. That is why I asked.