The immediate concern is the proliferation of drones and misuse of airspace in unrestricted zones, with potentially inaudinate number of fleets of drones deployed by countless agencies (never mind other competing organisations), buzzing around from site to site. Probably, not too far from swarms of drones, before much tighter regulations are scrambled to intercept and clear the skies.
So, rather than that, why not make the outdoor signage boards autonomous? This could be a better alternative solution. Autonomous outdoor display boards with an inbuilt robotic arm and fleet of micro-drones (possibly, tethered physically/electronically to the base robotic arm).
The boards are self-inspecting/maintaining with an inbuilt retractable robotic arm stored in a secure housing within the board. The robotic arm contains all the necessary equipment (camera, tool-bits, etc), as well as the fleet of micro-drones, for microscopic inspection and maintenance of the display and board generally. All surfaces are coated with self-cleaning properties.
There's also added onboard capacity to measure ad impresssions/views and calculate site revenues realtime.
Needless to say, the network of boards is connected back to the main base (wherever that might be), for realtime visual and data stream.
In this way, any transit inspection/maintenance would virtually be unnecessary, saving costs. There might be the rare human site visit for essential control base-monitoring or if a board needs external attention.