A lot of times when people see a modern car accident or push on a plastic bumper, they wonder why cars aren't built the way they used to be. Well, a lot of it has to do with human survivability. If you've ever had high school level physics you'll understand the following formula:
F = ((m2 - m1) x (V1-V2)/(t2 - t1))
Where the masses of the first and second vehicles are m1 and m2 and the start of the collision and end of the collision are t1 and t2, respectively. V1 and V2 are the relative speeds of the vehicles — V1 being starting speed and V2 normally being zero. "F" is what's important here, as it's what either turns the occupants into a gooey bag of skin or lets them walk away.
F, of course, is force. It is the result of one car hurtling into another. It's what acts on the human attached to the inside of the car by way of seatbelt. When cars crash into each other there are an incredible number of variables, not the least of which are angles, bumper heights, braking forces etc., but the ones that can be controlled are the ones which have the biggest impact in that equation.
When crashing, you want to minimize the amount of speed involved; slamming on the brakes is a great way to do that. Minimizing mass is going to be a tougher one, considering your car is pretty heavy to start out, and you both won't have a lot of time to jettison stuff while yelling "Oh shit, oh shit, oh shit." But what about that time thing? What if you could stretch out the amount of time it takes from the start of the accident to the end? This is what crumple zones do.
The cars in the above gallery are designed to survive an accident, not to protect their occupants. In some of the scenarios, the car looks not too much worse for the wear, but the drivers likely suffered significant injuries that would have been avoided in a modern car. The cars that turned into mangled messes probably did the same to their drivers. No car can ever be completely safe; even though the new ones don't look tough, you're way better off in 'em.
Engineers design crash zones to fail along a predictable path using varying materials and construction techniques. As a result, predictable bending, breaking, and stretching occurs. These failure events act like a giant shock absorber sitting in front of your dash, "soaking up" force. If you play around with the above equation, you can see that stretching out the time, even by fractions of seconds, has a remarkable effect on the forces transmitted to the passengers. Of course, this is an extremely simplified look at why cars turn into play-doh during an accident these days. But at least it gives you a fighting chance of explaining why the next time your folks crank up the "back in my day" speech.
Photo credit to Complete All