Why doesnt' the BMW rollover system extend over the drivers head?

I think you'd be familiar with the two upside down parabolas that are directly behind the seats of the BMW roadsters. In case of a roll over wouldn't just a little bit of collision on the head be fatal? So why do they not extend over the head?Why doesnt' the BMW rollover system extend over the drivers head?Many car manufactures (including BMW, Audi, Jaguar, Mercedes-Benz, %26amp; Volvo) have adopted something called "ROPS", or "Roll-over Protection System" in their roadsters. BMW's RPS structures consist of two pyrotechnically charged roll hoops hidden behind the rear seats that will pop up in the case of a roll-over to protect the occupants. If the roof is up, the system will still work, in effect shattering the rear window at the same time.



http://www.youtube.com/watch?v=uu2LknXn-iE



http://www.youtube.com/watch?v=H70y5GN_PuY