I switched the routing options of my etrex from fast route to short route. Now at the location http://www.openstreetmap.org/?lat=51.44248&lon=6.66183&zoom=17&layers=B000FTF the routing goes from "Moerser Hof" in the north to "Römerstraße" in the south over the service street of Edeka. The tag service=parking_aisle should be added but also it is tagged as permissive. You only can drive very slow and can't access all the time because of the barrier.

Well, my understanding of "permissive" is: it is owned privately but the owner allows access for the general public, e.g. the customers of your Edeka supermarket.
So I think what you got was what you asked for: the shortest legal route for your mode-of-transport. :-)

Later the routing goes at the location http://www.openstreetmap.org/?lat=51.38501&lon=6.69594&zoom=16&layers=B000FTF from "Am Stellwerk (bridge)" to "Dahlingschule" over the track grade3 which is only accessible for agricultural vehicles.

Is it necessary to add some restrictions to the line-style file?
By the way, which detailed effect does have the different roadclass and the roadspeed values on an etrex?

Sadly, out-of-the-box mkgmap seems to ignore the "vehicle" and "motor_vehicle" classes, although in many cases, they allow much simpler and cleaner tagging of access restrictions (such as the "all vehicle forbidden except for agricultural use" in your example).

In my style-file, (as far as i remember) i translated "vehicle=no/agricultural/private" to "motorcar=no" and "bicycle=no", "motor_vehicle=no/agricultural/private" to "motorcar=no".

This gives good results, but it would be cool if mkgmap could do this internally. :-)

-Martin