The height of a sign is the vertical distance between the nearest edge of the roadway and the bottom of the sign. Signs in rural districts shall be mounted at a height of at least five feet. However, where parked vehicles or other obstructions are likely to interfere with the visibility of the sign, the height should be at least seven feet. Signs in business and residence districts shall be mounted at a height of at least seven feet. Route markers shall be considered as a single sign for purposes of this section. The snowmobile route marker assembly shall be mounted at a height of at least four feet.