"dangerous interference with traffic"
Indeed, it could lead to cars slowing down and who could want that in Autoland germany.
The Straftatbestand (ยง 315b StGB) exists for all alterations of traffic. After all, you could conceivably start to manipulate traffic lights [1], remove STOP-signs or erect low-visibility barriers.
[1] https://medium.com/asecuritysite-when-bob-met-alice/traffic-...
And this is what would be dangerous. But adding a crosswalk is not, if the law says otherwise it is a bad law.