Quote Originally Posted by mp5o
Actually, most modern seat belts are designed to keep you pinned to your seat. They even come with belt tensioner's that will take up slack(usually about 6") and lock within a fraction of a second upon impact.

Modern cars rely on crush zones(in conjunction with steel framework) to absorb the majority of the shock caused by an accident. The seat belt is designed to keep you stationary and the air bags are designed to fill the space between the dash or steering wheel and the passenger(s).
Inertia wheels lock the belt then in some newer cars & older high end ones are equipped with pretensioners that physically pull the belt tight by using the same sensors as airbags.

...BUT you're still not completely 100% stationary with an ordinary safety belt like you would be in a harness. You rollover or get hit in the side with a roll cage using a regular 3-point safety belt you will most likely hit the roll cage unless the seat wraps around your head. Look at the majority of the roll over crash test video's, majority of the dummy's hit the roof/pillar with a 3-point safety belt.