Here are some useful resources to help you to understand the physics behind safety features of a car...
Safety Features
http://www.pbs.org/wgbh/nova/escape/timecar.html
Crumple Zones
http://www.bbc.co.uk/schools/gcsebitesize/science/add_ocr_gateway/forces/crumplezonesrev3.shtml
The Car and the Wall
http://www.physicsclassroom.com/mmedia/newtlaws/cci.html
How seat belts work
http://auto.howstuffworks.com/seatbelt2.htm