Even better, the back of cars space for luggage could be used to place digital lockers that would have keys that would be told the participants as required.
Parked cars are everywhere. So using the luggage to provide node buffers to the network seems a great idea.
And the owner of the car would use a mobile app to know to which car parked should he put each parcel.
The computational system would be separate from the physical implementation of lockers on the back of each car, so it would give freedom for each car owner to adapt his car to have the digital lockers as he wished.
A mobile app would provide the driver with the required information and would be used to update tracking information to the network.
http://i46.tinypic.com/2exavz7.jpg