As member demand grows in a neighborhood, eventually an additional vehicle will be needed to serve the users in that area. The carsharing organization may have the option of either adding a vehicle to an existing location or creating a new car location between existing stations. Each has advantages and disadvantages.
Adding the new vehicle to existing location - This has the big advantage to members in that they are already familiar with the location. In the event of a vehicle or reservation problem it may be possible for the on-call staff to more easily substitute another vehicle at the pod in order to get the member underway quickly. However, unless the reservation system allows reserving a class of vehicle rather than a specific vehicle/location, there may be member confusion on which vehicle to use. In addition, if the member is confused about which vehicle they've reserved, they may take the wrong vehicle since not all systems actually check the reservation when the member arrives at the car.
Establishing a new vehicle location - This gets the new vehicle closer to some members but necessitates "educating" the member where the stations are. (Longer-term members can be notorious for continuing to use the vehicles they're familiar with, even when new locations are opened that might be closer.) And establishing a new location probably requires establishing a relationship a new parking space provider - not always an easy task. While there is clear evidence that the further away from a station the member is, the lower the usage on average. It seems likely that if the difference is only a few blocks this "closeness effect" could be minimal.
City Carshare in SF and Philly Carshare seems to favor pod expansion; Flexcar favors the single-vehicle concept; Zipcar seems to have a mix of pod and single vehicle stations.
The 1997 "Pay as You Drive report" by Graham Lightfoot for the EU included statistical calculations showing that 3 vehicles in a location (or nearby) provides an optimum balance of availability compared to individual vehicle stations. Of course there are other considerations in siting cars - not the least of which is the availability of a parking space to rent. And other factors, such as accessibility, night-time safety, etc., also influence choices.