Car2Go (a Daimler company) is tackling this problem with the concept of an operating area. You can pick up and drop of your car from a car2go designated spot or any legal parking spot within this area. Here is an example of their operating area in Austin
http://www.car2go.com/portal/austin/page/mybookings/mapEnlar...
They are also innovative on other fronts such as charging by the minute and a feature-rich touchscreen in each car.