The thing is that these rules are not “unwritten” at all. They are part of the (written down) traffic code. To give an example, you have pedestrian zones where you are not allowed to bike, and you have regular roads where pedestrians need to keep to the sidewalk. Also, you have bicycle paths meant for cyclists only, and sidewalks meant for pedestrians only. When you walk on the road or on a cycling path you are violating the traffic code, not some unwritten rule.