In this situation my Garmin Dakota 20 is routing me up the oneway part
several roads around the block to get to the target just down the street. Could it be that we are incorrectly merging the oneway street with the non-oneway street, making the non-oneway section oneway? What are the criteria for merging ways? I think that the merging should be blocked if any of the routing attributes differ: oneway, toll, mkgmap:unpaved, access/motorcar/bicycle/foot, etc.
In http://www.openstreetmap.org/browse/node/101581030 the node is not part of a turn restriction relation. If it were, that would be a further reason not to merge the ways, I guess.
Yes, that would be a possible explanation. But as far as I remember, merging takes place only for the graphical representation of lines in the different levels. The routing graph should be not affected by this. @Markus: Are there any signs, that the destination road is a oneway? Regards, Johann