pseudoprometheus opened the ticket and wrote:

Originally I was going to request that the middle of cities become 'no-fly zones' so that planes wouldn't fly over dense urban areas. However, since creating no-fly zones would involve creating a CPU-intensive path-finder, an alternative idea came to mind: instead of having the computer do the path-finding, have the player decide which paths the planes ought to travel. (Note: Manually determining a plane route would NOT be required. Rather, this is just for the sake of allowing players to make the simulation more realistic if they want to.)

One would do this by placing 'air buoys' on the map. In a way, it'd be a mix between buoys and waypoints. They could be placed anywhere (like buoys, except on land as well), and wouldn't need a pathfinder to work (like waypoints). After the player has placed the 'buoys' and updated the plane orders, then one could keep planes from unrealistically flying over the downtown areas of cities (while letting helicopters go in).

I would imagine that these would appear on the map as signs (so they wouldn't bulldoze or cost anything). For the sake of keeping these 'buoys' hidden from the player while leaving signs visible, we could add an option to the transparency options to toggle them.

Reported version: 0.6.3
Operating system: All

This issue was imported from FlySpray: https://bugs.openttd.org/task/2345