addition: this is my workaround so far, please fill in the gap if you got better methods.
Say I got road. I uv-map the first polygons
then I split those polygons, so that they dont share UV-edges.
For the rest of the road, I use Unfold Strip from Loop
It will result in something like this, where the tiny thing below are the polygons with the correct UV-size.
Finally, I weld the vertices and then I stitch the two together. It is important that the Bias-value is set to 0, so that they size will match
So, quite some steps to reach the goal.
In my mind, being able to simply Unfold Strip of the selected area, ie not selecting the first two polyigons would have saved me many steps.
But apparently, Unfold Strip ignores my selection and unfolds all the connected polygons.
Also, I didnt find any settings for Unfold Strip. There seem to be only one function.
I would be happy if anyone could shed any new light on this uv-technique