Generate a vectorizer object, to be found at the spline-presets. Load the b/w map into the object, set the "tolerance"-vaulue to 0 (or 1, depending on the case). Set the intermediate points to "None". Change the object to a spline by hitting C. Then put it under an Extrude-NURBS, and set the extrude-values to 0 (so that the object is plane). Convert it, and select all polygons. Extrude them with a variation of about 85-115%. That was it, now you're the new mayor :thumbsup:
and this, the map is a postworked map of the inner city in Vienna, I took it from Wien.at (http://www.wien.at)
Hope you like this little tutorial.