Problem now:
You report incorrect turn restrictions in your app, someone will find it in the web editor and blind changes them mostly based on fuzzy or even no notes.

Idea
You mark a turn restriction map issue in your app. A simplified junction pops up with BIG red and green turn restrictions for your current view/street. You can toggle the arrows by tapping them allowing or disallowing turns.
Press save and forget. The changes will be kept locally in the client in order to avoid being routed the wrong repeatedly until the next map update.