Transportation systems follow a similar philosophy as grids, in terms of making large-scale resources available to users on a shared basis. Jet aircraft may cost anywhere between $50 and $200 million. A private aircraft might provide excellent service to its owner on a coast-to-coast flight. The obvious shortcoming of this solution, however, is that the cost of the plane and the fuel it takes to fly it across the continent are out of reach for most people, and in any case, it probably does not represent the best use of capital for general-purpose transportation. The reason why millions of passengers can travel like this every year is because aircraft resources are shared, and any single user pays only for seats used, not for a complete jet and the infrastructure behind it.

Shared-resource models come with overheads: users need to make reservations and manage their time to predetermined schedules, and they must wait in line to get a seat. The actual route may not be optimized for point-to-point performance: passengers may have to transfer through a hub, and the departure and destination airports may not be convenient relative to the passenger’s travel plans, requiring additional hops or some driving.

Note that aircraft used for shared transportation are architected for this purpose. Aircraft designed for personal transportation are significantly smaller, and would not be very efficient as a shared resource.

Transportation systems are also heterogeneous, where sharing exists on a continuum. In an air-transportation system, users choose among a variety of dedicated resources, including general aviation and executive aircraft, time-shared aircraft, commuter aircraft, and the very large aircraft used in long-haul flights. Likewise, grids tend to gravitate toward heterogeneity in equipment availability during their lifetime, with nodes going through incremental upgrades. Grids tend to be deployed under diverse business models.

While the air-transportation system is an instructive instantiation of a grid, it is so embedded in the fabric of society that we scarcely consider it as such³. Computer systems will likely evolve in a similar way as aviation did sixty years ago, gradually gravitating toward an environment of networked, shared resources as technology and processes improve.