@t.jacobs : Good find ! I tryed what you said and it works !
You can even put buildings on these roads ! (but it's not a good idea because services can't reach them).

I think I might have found an easer solution for you ground level problem: Add a car lane to your road, so that it is visible, but in the lane options set "direction" to "avoid". It worked for a road I built. No car or service vehicle ever tried to use the road, not even snow ploughs.