dnovotny

Yes, but one has to consider probability as well. I have a family friend that refuses to wear a seat belt because he's fearful of being unable to get out of the car (and consequently burn to death). So he's worried about a small risk while ignoring the much higher odds of being in an accident where the seat belt will be a lifesaver. I believe this is what you're missing with your post. Overall I'm confident you'll be safer with a roll bar in the car (as long as you're in the front seats) and I'm betting on this since I'm installing a roll bar into my '01 (which will probably become my daily driver). Consider what is the latest safety technology on European convertibles: in case of a rollover, a form of roll bar pops up behind the front seats. In a convertible there's no roof preventing your head from being caught above the roll bar. Yet this is a safety feature which has been added; the engineers most certainly considered this possibility. As you pointed out due to inertia one will be propelled head forward first. That makes somehow recoiling high enough and far enough to squeeze your head just above the head rest unlikely.

And if you're concerned about hitting the main hoop, put in a 5-point seat belt. That will prevent you from having enough vertical movement to hit the roof. The risk boils down to the amount of uncontrolled movement one will have with the seat belt on. I know its much more likely to have your roof collapse in. Also the roof structure on most vehicles is very suspect; parts of the structure can rip apart and consequently rip you apart. One advantage of the main hoop is its unlikely to come apart and deflect significantly (as long as its installed and reinforced appropriately). Obviously when you've added a roll bar you've created new risks and possibilities for injury. The better question is have you reduced your overall risk of injury. I'll leave that for you to decide.

Legal disclaimer: I'm not a chassis/vehicle design engineer so take my input as just opinion, not fact. However, I do have a Master's in EE and minor in Physics.