Calculation of Decrease User Cost

Hi Olaf, Daniel,
I am working on the TOR for the 2.0 version and trying to figure out other issue we need to fix for the second version:
It seems that the benefits from "decrease user cost" are in many cases negative for asphalt roads.. I just tried several times. Could you remind me how that is calculate for roads that are already paved (therefore only one intervention is available)? in Issue #27 seems we were discussing this.
Additionally, if you think about other enhancement, just tag them in git hub as so. I saw that tag and was very useful to go directly to those issues. Thanks!