<P> Seatbelts restrain the passengers so they don't fly through the windshield, and are in the correct position for the airbag and also spread the loading of impact on the body . Seat belts also absorb passenger inertial energy by being designed to stretch during an impact, again to reduce the speed differential between the passenger's body and their vehicle interior . In short: a passenger whose body is decelerated more slowly due to the crumple zone (and other devices) over a longer time survives much more often than a passenger whose body indirectly impacts a hard, undamaged metal car body which has come to a halt nearly instantaneously . It is like the difference between slamming someone into a wall headfirst (fracturing their skull) and shoulder - first (bruising their flesh slightly) is that the arm, being softer, has tens of times longer to slow its speed, yielding a little at a time, than the hard skull, which isn't in contact with the wall until it has to deal with extremely high pressures . The stretching of seatbelts while restraining occupants during an impact, means that it is necessary to replace them if a vehicle is repaired and put back on the road after a collision . They should also be replaced if their condition has deteriorated e.g. through fraying or mechanical or belt mounting faults . In New Zealand it is officially mandatory to replace worn inertia reel type seatbelts only with' webbing grabber' type belts that have less play and are more effective on older cars . Newer cars have electronically fired pre-tension seatbelts that are timed to work with the airbag firing . Buying used seatbelts is not a good idea even in countries where it is legal to do so, because they may have already been stretched in an impact event and may not protect their new users as they should . </P> <P> The final impact after a passenger's body hits the car interior, airbag or seat belts is that of the internal organs hitting the ribcage or skull due to their inertia . The force of this impact is the way by which many car crashes cause disabling or life - threatening injury . Other ways are skeletal damage and blood loss, because of torn blood vessels, or damage caused by sharp fractured bone to organs and / or blood vessels . The sequence of energy - dissipating and speed - reducing technologies--crumple zone--seat belt--airbags--padded interior--are designed to work together as a system to reduce the force of the impact on the outside of the passenger (s)'s body and the final impact of organs inside the body . In a collision, slowing down the deceleration of the human body by even a few tenths of a second drastically reduces the force involved . Force is a simple equation: Force = mass X acceleration . Cutting the deceleration in half also cuts the force in half . Therefore, changing the deceleration time from . 2 seconds to . 8 seconds will result in a 75 percent reduction in total force . </P> <P> A misconception about crumple zones sometimes voiced is that they reduce safety for the occupants of the vehicle by allowing the body to collapse, therefore risking crushing the occupants . In fact, crumple zones are typically located in front of and behind the main body of the car (which forms a rigid' safety cell'), compacting within the space of the engine compartment or boot / trunk . Modern vehicles using what are commonly termed' crumple zones' provide far superior protection for their occupants in severe tests against other vehicles with crumple zones and solid static objects than older models or SUVs that use a separate chassis frame and have no crumple zones . </P> <P> They do tend to come off worse when involved in accidents with SUVs without crumple zones because most of the energy of the impact is absorbed by the vehicle with the crumple zone--however, even for the occupants of the' worse off' car, this will still often be an improvement--as the result of two vehicles without crumple zones colliding will usually be more hazardous to both vehicle's occupants than a collision that is at least partly buffered . </P>

Where are the crumple zones located in a car