I wouldn't want a roll cage on a street car. I've been led to believe that a roll cage should only be used in a car where the driver is wearing a helmet,race car style restraints and protective gear. A typical 3 point seat belt allows a person's body to move around in the car to a degree. In an accident, a person's head could slam into the roll cage causing injuries.

A roll bar located behind the front seats in a car that doesn't have a back seat would probably be ok, but I'm not an expert on this subject.