1. A navigation system comprising: a processor; data storage that is operatively coupled to the processor; open sky data entities stored in the data storage and accessible by the processor, wherein each open sky data entity represents a visibility rating of open sky at a respective position; road-network data entities stored in the data storage and accessible by the processor, wherein each road-network data entity represents a road network at the respective position, and wherein the open sky data entities and the road-network data entities are associated based on position; and program instructions stored in the data storage and executable by the processor for receiving a route request including a starting location, a destination, a visibility preference, and responsive thereto, using the road-network data entities to generate a route based on the visibility preference and the visibility rating of open sky for the road-network data entities; wherein the route is generated by calculating a path between the starting location and a destination using the visibility preference to select road-network data entities.