Delivery drones could someday hitch rides on public buses to dramatically extend their range in cities. Stanford's Intelligent Systems Laboratory and Autonomous Systems Lab modeled such a system to see if it even makes sense. According to their research paper, it does. In theory, anyway. Evan Ackerman writes in IEEE Spectrum:
The first thing to understand about this paper is that it's not trying to solve any of the practical problems surrounding the real-world deployment of a delivery network involving drones and buses, like how you'd get a drone to land on a moving bus, for instance. What the paper is about is how you'd get a potential network of drones and vehicles to operate efficiently, and how big of a difference that might be able to make in a package delivery context.
In a metropolitan area like San Francisco, the idea is that you'd have a bunch of package depots scattered around the city. You'd also have a bunch of drones, and every day, you'd need to figure out how to get all of those packages where they need to go in the minimum amount of time, using the existing bus routes and schedule to boost their range when necessary. And when I say "you," that's where this research comes in, because it's solving a big optimization problem that involves which drones make which deliveries in what order, when they should use buses, and for how long. It gets more complicated too, because there are conflicts that have to be resolved when buses can only carry a few drones at a time and you don't want the drones occupying the same space on the network at the same time. Oh, and don't forget that part of this planning process involves getting the drones back to their depots, too[…]
Using San Francisco (150 km2) as a model, the system is able to schedule drones (based loosely on the specs of the DJI Mavic 2) in such a way that hopping around on the bus system allows them to make deliveries anywhere in under an hour.