BEagle seems to have hit the nail on the top of the nail, but an alternative conspiracy theory exists:
1. Aircraft should not low-fly over built up areas.
2. Aircraft low-flying therefore go around the built up areas.
3. These "choke points" become more congested than other wide open spaces.
4. The symbols show a preferred direction of travel, to minimise aircraft bouncing into each other.
An airborne one-way system, if you like.