Definition of Dynamic Ridesharing
A system that facilitates the ability of drivers and passengers to make one-time ride matches close to their departure time. The hallmarks of a dynamic service are flexibility and convenience.
A difficulty is in distinguishing "dynamic" services from "non-dynamic." It is easy to define a traditional car pool matching service as non-dynamic. In the pre-Internet era all such services were decidedly non-dynamic. "Match lists" based on common origins and destinations were mailed to users, and users were expected to contact one another for the purpose of creating a regular car pool (that kept the same schedule every work day).
Probably all such services have migrated to a web basis. Does this make them "dynamic"? Their purveyors often claim so. ("You can do that.") But given these services' emphasis on recurring, fixed-schedule travel, it is clear that any dynamic use is incidental to their purpose and design.
If we posit a goal of serving the daily commute market, then that places some requirements on the degree of flexibility and convenience necessary for a one-time-ride-match system (meaning that each ride match is a new arrangement, quite likely involving different matched partners each time). Daily commuters cannot be expected to spend more than a very few minutes arranging their commute. This probably precludes checking potential match partners' Facebook profiles, for example. It may even preclude a system that has a conventional web interface without other options such as via telephone keypad (IVR, or Interactive Voice Response) or cell phone text (SMS) message — it is difficult to imagine your typical harried commuter booting up their desktop computer in the morning (though there are no doubt many who do).
Systems such as the current implementations of GoLoco and Zimride, which primarily have a Facebook interface and do not have telephone access, may never be able to serve the daily commute market. From this perspective, these systems are not sufficiently "dynamic." Indeed, their current niche seems to be for longer, more expensive trips (such as college holiday rides home, or travel to conferences), where the user's time investment in logging on and checking user profiles is worthwhile.
Thus, our proposed definition of dynamic ridesharing is A system that facilitates the ability of drivers and passengers to make one-time ride matches close to their departure time, with sufficient convenience and flexibility to be used on a daily basis.