I just finished teaching a unit on Newton's three laws of motion, which would be applicable here. The first law is commonly called the "law of inertia". Inertia is an objects tendency to remain at rest or stay in motion, whichever one it happens to be doing at the time, until acted upon by an outside force. What this means is a car travelling down the highway at 60 mph, if it crashes into a wall, everything in the car that is not tied down will continue to travel forward at 60 mph. The use of seat belts and shoulder harnesses have been very effective in this area in keeping vehicle occupants from crashing through their windshields. As far as everything else, think about all the loose items in the vehicle that could become instant missles. Loose change, tools, bottles, CDs, mirror jewelry and the like could all become life threatening projectiles. Another safety feature in recent vehicles has been the development of air crash bags, which deploy upon initial contact with a wall or another vehicle.