What? There is nothing about this post that makes an ounce of sense. Are we talking about cars or bikes here? If bikes, perhaps you could tell me how a helmet on my head protect others? If cars, lets address some issues with your theory.
1. Windshields are designed to keep you inside the car in a collision. It would be very, very difficult, if not impossible, to be ejected through a properly installed windshield with enough velocity to be considered a dangerous projectile in a collision. If the windshield wasn't installed properly, it is possible for a human body to blow the WHOLE windshield out of the car, but that would be mechanical failure.
2. To be ejected from a vehicle in an accident in it most often through a side window. This is almost always due to barrel rolling your vehicle. If you were to barrel roll your vehicle, it is usually safer to be ejected anyway, because as long as your car doesn't land on you, there is one impact with the ground, and it is over. Thats a much better option than being stuck inside that washing machine on spin cycle. Thats multiple impacts, as well as the stuff flying around inside the vehicle.
3. If you were to roll your vehicle down the highway, I'd be willing to bet that the 4k lbs. of out of control steel would be the main concern... not the 200 lbs. of soft squishy body that MIGHT be ejected from the vehicle. By that theory, we should make it illegal to travel with a jack, spare tire, any luggage, etc.
4. A motorcycle accident will always result in 2, maybe 3, projectiles (bike and bodies). If bodies are such a big danger to others in an accident, why are motorcycles even allowed on the road?
I will say that seat belts do keep you in place, so you can better control your vehicle. However, in this country, we have speed limits that are set for a safe traveling speed for the road. If you are obeying all laws, you aren't feeling enough inertia to be thrown out of your seat, anyway. If you need to make an emergency maneuver, you could be thrown from your seat, but at that point, you are pushing the limits of your vehicle's capabilities, so anything is possible anyway. You could swerve to miss a kid chasing a ball, roll your vehicle, and be thrown from the vehicle saving your life from the jack handle flying around the cab.
