The idea is to make the rear part of the vehicle a simple inflatable bag that is only deployed above a minimum speed. At lower speeds the bag is reeled in and stowed returning the vehicle to the length we are all accustomed to.

Note that the tail is not inflated by movement of vehicle as that would just be added and continued drag. A small compressor is required to inflate it and some sort of winding up mechanism is required to retract the tail.

The advantages are very low weight, no significant stability issues **, cheap and at least the illusion of some sort of padding between you and the idiot tailgater.

** Long tailed race cars have been tried at various times but once the tail is long enough to be effective it becomes susceptible to cross winds. The inflatable tail would just be blown aside.