It seems like it would be easier to use tiny robot zepplins instead. Much more CPU power would be required to control the robots, but that seems like it would be cheaper than setting up a nationwide network of tunnels.
Or in cities with footpaths, what about strapping a GPS and AI to a segway-like robot and sending that on its way?
Miniature Zepplins are great until the wind starts blowing. Most of the large cities are coastal.
Robots on segways are ok until someone decides they need another segway (or whatever it happens to be carrying).
Of course, I think the tunnels could suffer the same security problems, whether physical or virtual. Imagine hacking your neighbourhood tunnel to get free pizza!
Yes, wind might be an issue for miniture zepplins, though I wonder whether there might be ways to mitigate the effect of wind. It's not like they'd be flying very high.
Regarding robots on segways, the robots would not be on the segways; they'd be self-balancing robots of approximately human dimension with a rack for carrying parcels. You wouldn't be able to easily ride one, and stealing one could be made quite difficult. For instance, you could have the robot automatically phone home along with it's current GPS position when it detects itself being "kidnapped". Or you could program it to emit an extremely loud siren. Or fry its own circuits so that it's useless to the criminals. Or snap photographs of the perps and email them to a remote location.
Or in cities with footpaths, what about strapping a GPS and AI to a segway-like robot and sending that on its way?